Subscriber access provided by - Access paid by the | UCSB Libraries
Article
Modulation and Salt-induced Reverse-Modulation of the Excited State Proton Transfer Process of Lysozymized Pyranine : The Contrasting Scenario of the Ground State Acid-Base Equilibrium of the Photoacid Ishita Das, Sudipta Panja, and Mintu Halder J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
1
Modulation and Salt-induced Reverse-Modulation of the Excited State Proton Transfer
2
Process of Lysozymized Pyranine : The Contrasting Scenario of the Ground State Acid-
3
Base Equilibrium of the Photoacid
4
Ishita Das, Sudipta Panja, and Mintu Halder*
5
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
6
*Corresponding Author: E-mail:
[email protected] 7
Phone: +91-3222-283314, FAX: +91-3222-282252
8 9 10
Abstract
11
Here we report on the excited state behavior in terms of Excited State Proton Transfer
12
(ESPT) reaction as well as ground state acid-base property of Pyranine [8-hydroxypyranine-
13
1,3,6-trisulfonate (HPTS)] in presence of an enzymatic protein, human lysozyme (LYZ).
14
HPTS forms a 1:1 ground state complex with LYZ having the binding constant, KBH =
15
(1.4±0.05)×104 M-1 , and its acid-base equilibrium gets shifted toward the deprotonated
16
conjugate base (RO-), resulting in a downward shift in pKa. This suggests that the conjugate
17
base (RO-) is thermodynamically more favored over the protonated (ROH) species inside
18
lysozyme matrix, resulting in an increased population of the deprotonated form. But on the
19
other hand, for the release of the proton from the excited photoacid, interestingly the rate of
20
proton transfer gets slowed down due to ‘slow’ acceptor biological water molecules present
21
in the immediate vicinity of the fluorophore binding-region inside the protein. The observed
22
ESPT time constants, ~140 ps and ~750 ps, of protein-bound pyranine are slower than in bulk
23
aqueous media (~100 ps, single exponential). Molecular docking study predicts that the most
24
probable binding location of the fluorophore is in a region near to the active site of the
25
protein. Here we also report on the effect of external electrolyte (NaCl) on the reverse
26
modulation of ground state prototropy as well as ESPT process of the protein-bound
27
pyranine. It is found that there is a dominant role of electrostatic forces in the HPTS-LYZ
28
interaction process, since increase in ionic strength by the addition of NaCl dislodges the
29
fluorophore from the protein pocket to the bulk again. The study shows a considerable
30
different perspective of the perturbation offered by the model macromolecular host used
31
unlike the available literature reports on the concerned photoacid.
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
32
1. Introduction
33
Proton transfer reactions are fundamental and play a key role in various chemical and
34
biological processes 1-9. Intermolecular proton transfer in the excited state (ESPT) has drawn
35
great attention and has been extensively studied to extract information about the mechanism
36
and several parameters that control such reactions. Biological processes like DNA damage
37
due to disordering of base pair in presence of UV radiation, some enzymatic reactions,
38
reactions at photosynthetic centres etc. are induced by ESPT10,11. Molecules, having enhanced
39
acidity in the excited electronic state consequently undergo proton transfer upon
40
photoexcitation, are known as photoacids12. There are many photoacids including fluorescent
41
dyes and some drug molecules which are known to exhibit intermolecular or intramolecular
42
proton transfer. Such systems have been explored to gain information about their acid-base
43
reaction, ultrafast photodynamics in excited state, and effect of their confinement in different
44
chemical or biological environments12-14. Pyranine (8-hydroxypyranine-1,3,6-trisulfonate,
45
HPTS, Figure 1a) is also a photoacid and upon photoexcitation, its pKa downshifts from ~7.7
46
to ~0.6. ESPT reaction of HPTS has already been studied in several confined assemblies like
47
micelles, reverse micelles, liposomes, proteins, cyclodextrins, ionic liquids, and more9, 15-23.
48
Those studies reveal that proton-transfer rate of HPTS gets affected because of different
49
environmental effects offered by various host systems, and the excited state behavior of the
50
fluorophore has been exploited to probe the environment and local territory of host systems.
51
Hence considering the important role of proton-transfer process in several biological
52
processes and to probe the local environment that modulates proton-transfer processes, the
53
study of ESPT reactions in protein environments appears to be quite implicative. Besides
54
being a model photoacid, HPTS is also used as an intracellular pH indicator. Intracellular pH
55
plays many critical roles in cell, enzyme, and tissue activities, including proliferation and
56
apoptosis24,
57
contraction30, 31. Monitoring of pH alterations inside living cells is also important for studying
58
cellular internalization pathways, such as phagocytosis32, endocytosis33, and receptor ligand
59
internalization34.
25
, multidrug resistance26, ion transport27-29, endocytosis25, and muscle
60
HPTS has two distinct absorption maxima corresponding to the two different
61
protomers (Scheme 1), one centered at 404 nm for the protonated form and the other at 454
62
nm due to the deprotonated conjugate base. With increase of pH, absorbances at those two
63
wavelengths, decreases and increases, respectively, and this alteration is utilized (absorbance
64
ratiometric approach) to measure intracellular pH35. In this case the Henderson–Hasselbalch
ACS Paragon Plus Environment
Page 2 of 33
Page 3 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
36
65
equation
can be employed, which utilizes the ground-state pKa of the probe. If interaction
66
of the probe with any biomolecule affects the absorbance ratio of the two protomers, an
67
apparent pKa is obtained which will be different from the actual value. Hence, the knowledge
68
of the apparent pKa in presence of any biomolecule will be essential for the estimation of the
69
actual pH of the environment. In most of the available studies on ESPT process of HPTS in
70
several supramolecular assemblies, the fate of the ground state acid-base equilibrium of the
71
fluorophore was either not accounted or very little change was noticed.
72
In a recent study on ESPT reaction of HPTS with human serum albumin (HSA), it
73
was found that the ground state acid-base equilibrium of the photoacid shifts toward the
74
protonated form, and in the excited state the proton transfer occurs to biological water
75
molecules, gets slowed down inside the protein pocket23. It was suggested that, inside the
76
HSA pocket, HPTS binds in the subdomain IIIA which is known to bind ligands
77
predominantly by dipole-dipole, van der Waals, and hydrogen-bonding interactions. The
78
older literature reports suggest that the lower the pKa* (as well as the pKa), the faster the
79
ESPT rate would be37. In this work, we have chosen a model antibacterial enzymatic protein
80
human lysozyme (LYZ, Figure 1b), which is smaller than the serum protein and has only one
81
water exposed ligand binding catalytic active site. We intend to study how this protein affects
82
the ground state acid-base equilibrium as well as excited state proton transfer process of the
83
model photoacid pyranine.
84
Lysozyme is an enzyme found in various tissues and in some protecting secretions
85
like tears, saliva, mucus, milk etc. It can damage bacterial cell wall by cleaving the β-
86
glycosidic
linkage
between
N-acetylmuramic
acid
and
N-acetylglucosamine
in
38-40
87
peptidoglycan, and thereby protecting against bacterial infections
. It is a single chain
88
globular protein consisting of 130 amino acid residues. It has isoelectric point at pH ~11.35.
89
The active site of the protein has a crevice that separates the protein into two domains
90
connected by an α-helix. One domain consists mostly of β-sheet conformations (40-80 amino
91
acids), whereas the other domain (89-99 amino acids) is more α-helical in nature41. Lysozyme
92
has also been chosen as a model protein to study and understand folding and dynamics,
93
structure-function relationships, and ligand-protein interactions due to its small size, high
94
stability, natural abundance42,
95
endogenous and exogenous compounds including some important drug molecules44.
43
. Lysozyme is capable of binding reversibly a number of
96
Besides the studies involving confinement of the photoacids in different host
97
macromolecular systems, there are reports on ESPT process of HPTS in the presence of
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 33
98
electrolytes only or in presence of acetate base17, 45. Leiderman et al.45 showed that at a high
99
concentration of salt MgCl2 or NaCl (~5 M), proton-transfer rate gets slowed down, whereas
100
at concentration below 0.5 M of MgCl2 and 0.8 M of NaCl, there was no alteration in proton-
101
transfer rate. A recent report shows that ESPT rate of pyranine slows down in niosome
102
compared to bulk water, and in presence of NaCl (in concentration range 1 M to 4 M) a
103
further decrease in proton-transfer rate was observed22. However, there are a very limited
104
number of reports on the role of any external additive that could further affect ESPT process
105
of the confined photoacid. It is well known that ionic strength of medium is a parameter that
106
sometime can act as a modulator of protein-ligand complexation process affecting the binding
107
interaction46,
108
electrolyte, like NaCl. So, presence of NaCl may affect the HPTS-LYZ binding, which in
109
turn may further modify ESPT process of the photoacid in the complex.
47
. Ionic strength of meduim could be varied by adition of an external
110 (a)
(b)
111 112 113 114 115 116 117
Figure 1 (a) Molecular structure of pyranine, and (b) 3D representation of lysozyme
118 119 120 121
pKa ~ 7.7, pKa* ~ 0.6 +
H2O
+
H3O+
122 123 124
Scheme 1. Acid-base equilibrium of pyranine in water
125 126
Therefore, in the present study, we investigated the ground state prototropy and the
127
excited state proton-transfer behavior of HPTS in the presence of LYZ. We have also
128
examined the role of NaCl as an external addititive on the further modification of ground
129
state and excited state behavior of lysozymized HPTS. In order to find the most probable
130
binding location of HPTS in the protein, molecular docking study was performed. Circular
ACS Paragon Plus Environment
Page 5 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
131
dichroism spectral study was monitored to investigate if there is any conformational
132
alteration of LYZ upon binding the fluorophore.
133 134
2. Experimental section
135
2.1. Materials
136
Human lysozyme (>90%, recombinant, expressed in rice), HPTS were purchased
137
from Sigma-Aldrich. Other chemicals are of analytical grade (AR). Buffer solutions of 0.01
138
M phosphate buffer in the pH range 5.2 to 8.7 were used for pKa estimation. All other
139
experiments were carried out in 0.01 M phosphate buffer solution of pH 7.0. Ultrapure water
140
was used for all solution preparation. The pH of buffer solutions were measured with
141
EUTECH pH 510 ion pH-meter.
142 143
2.2. Instrumentation and methods
144
UV−Vis absorption spectra were recorded on a Shimadzu UV-2450 absorption
145
spectrophotometer against a solvent blank reference in the wavelength range of 350−550 nm.
146
The concentration of HPTS used was 1×10-5 M.
147
All steady-state fluorescence emission and steady-state emission anisotropy
148
measurements were taken on a Jobin Yvon-Spex Fluorolog-3 spectrofluorometer, using 1 cm
149
path length quartz cuvette. HPTS concentration kept at 2.5×10-6 M for all the fluorescence
150
measurements. Excitation of HPTS was done at wavelength of 380 nm. Emission spectra
151
were collected from 395 to 650 nm, keeping excitation slit at 1.5 nm and emission slit at 1
152
nm.
153
Time resolved fluorescence intensity decays were collected by using a time-correlated
154
single-photon counting picosecond spectrophotometer (LifeSpec II, Edinburgh Instruments,
155
U.K.). Samples were excited by a pulsed laser (EPL-375) centered at 375 nm, and emission
156
signals were collected at magic angle (54.7˚), using a photomultiplier tube (H10720-01
157
Photosensor module from Hammamatsu). The laser repetition rate was kept at 5 MHz, and
158
excitation peak power was ~70 mW48. The instrument response function (IRF) was ~220 ps.
159
The decay analyses were performed by using F 900 software from Edinburgh Instruments49.
160
The
161
r ( t ) = ( I ( t ) − G I ⊥ ( t )) ( I + 2 G I ⊥ ( t ))
162
at parallel ( I (t ) ) and perpendicular ( I ⊥ (t ) ) polarizations of the emission with respect to the
time-resolved
anisotropy
was
constructed
using
the
expression,
where G is the ratio between the fluorescence intensity
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 33
163
excitation beam. The value of G was measured at a gating window, in which the fluorescence
164
is almost completely depolarized (tail-matching technique). The quality of the fits was judged
165
in terms of the residual distribution and reduced χ2 values. All experiments were performed at
166
298K.
167
Time resolved area normalized emission spectra (TRANES) were constructed through
168
the following steps50. (a) Fluorescence intensity decays [(I (λ,t)] were collected at emission
169
wavelengths in the range of 420 nm to 580 nm, in 5 nm interval. Then, each emission
170
intensity decay was fitted to a multi-exponential function as: I (λ , t ) = ∑α i (λ ) exp(−t / τ i (λ )) .
n
i =1
171
(b) Time resolved emission spectra (TRES) were constructed at different times (tk) using the
172
equation: I (λ , tk ) = I ss (λ ). ∑ α i (λ ) exp(−tk / τ i (λ )) ∑ α i (λ )τ i (λ ) . Here, Iss(λ) is the steady-
n i =1
n i =1
173
state emission intensity at wavelength λ. (c) TRANES is then generated by area
174
normalization of each TRES.
175 176
2.3 Molecular docking
177
Crystal structure of Human lysozyme (PDB ID: 1IX0)51 was taken from protein data
178
bank for docking study. To get the software specific files, polar hydrogens were added and
179
water molecules were removed from the PDB file. The tri-negative protonated form of HPTS
180
was optimized by PM3 prescription using MOPAC 200252. That optimized structure of HPTS
181
was docked onto 3D crystal structure of LYZ using AutoDock 4.253, which uses Lamarckian
182
genetic algorithm to search optimum binding site of small molecules in protein. AutoDock
183
reports a docked energy that includes a solvation-free energy term and intermolecular
184
interaction energy term of the ligand. Blind docking of HPTS was run in LYZ crystal. For
185
performing docking analysis the grid centre was set at 60, 60, and 60 along X-, Y- and Z-
186
axes, respectively, with a grid spacing of 0.375 Å. The lowest energy conformation, out of 50
187
conformations (from 40 runs) was taken as the most preferred binding position of HPTS in
188
LYZ. Uncomplexed protein and HPTS docked conformation of lowest energy were used for
189
calculation of solvent accessible surface area (SASA) using the Discovery Studio Visualizer
190
4 from Accelrys Software Inc.54. The change in SASA for residue is estimated as ∆SASA =
191
SASALYZ - SASALYZ–HPTS.
192 193
ACS Paragon Plus Environment
Page 7 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
194
The Journal of Physical Chemistry
2.4. Circular Dichroism spectra
195
Circular dichroism (CD) spectra were recorded on a Jasco-810 automatic recording
196
spectropolarimeter at 298 K over a wavelength range of 200–250 nm with a scan speed 50
197
nm/min under constant nitrogen flushing. A quartz cell having path length 0.1 cm was used
198
and two successive scans were accumulated for each spectrum. Baseline was corrected with
199
buffer solution running under the same condition as blank and subtracted from the
200
experimental spectra. The concentration of LYZ was taken as 20 µM.
201 202
3. Results
203 204
3.1. Steady-state spectra in lysozyme, and in lysozyme with added NaCl
205
UV-Vis absorption study:
206
In aqueous buffer of pH 7, the UV-Vis spectrum of HPTS shows an intense absorbance
207
maximum at 403 nm due to the protonated form (ROH) and another weaker band at 454 nm
208
for its deprotonated form (RO- ). Addition of LYZ shifts the ground state prototropic
209
equilibrium of HPTS toward the deprotonated form, since the absorbance at 403 nm
210
decreases and that at 454 nm increases (Figure 2a). The increased population of RO- in
211
presence of LYZ suggests a preference for the deprotonated form in the ground state which
212
results in an altered pKa of HPTS. In order to estimate ground state pKa value of HPTS in the
213
absence and in presence of LYZ, absorbance monotored at 454 nm in different buffered
214
solutions were plotted as a function of pH of the medium (Figure 2b). The ground state pKa of
215
HPTS was estimated to be (7.30±0.04) in only aqueous media, that matches quite well with
216
literature reported value55 and a pKa value (6.90±0.06) was estimated in the presence of LYZ,
217
i.e., pKa of HPTS has been shifted by ~0.4 unit in the presence of the protein. Approximate
218
pKa* values of HPTS in the absence and in presence of LYZ were also estimated from the
219
Förster cycle56 and are tabulated in Table T1, Supporting information. The shift in pKa* of
220
HPTS in the presence of LYZ was found to be same as the shift in pKa (∆pKa*~0.4).
221
Absorbance spectra in different pH in the absence and presence of lysozyme are also
222
provided in Supporting information, Figure S1.
223
Ground state acid-base behavior of HPTS in the presence of lysozyme was studied
224
upon addition of salt (NaCl). It was found that with the maximum NaCl concentration used
225
here (0.2 M), the acid-base equilibrium of HPTS shifts slightly toward the deprotonated form
ACS Paragon Plus Environment
The Journal of Physical Chemistry
and a very small downward shift in pKa (by ~0.15 unit) was obtained (the measured pKa was
227
(7.15±0.07)). The addition of 0.2 M NaCl with the HPTS in the presence of LYZ changes the
228
pKa from (6.90±0.06) to (7.15±0.08), which matches the value obtained for HPTS-only in the
229
presence of the same concentration of NaCl. The pKa values and relative shift in pKa in
230
different systems along with pKa* (with respect to aqueous medium) are listed in Table T1,
231
Supporting information. The corresponding graphical plots for the determination of pKa are
232
shown in the Supporting information, Figure S2. The observed reversed behavior of ground
233
state acid-base equilibrium of HPTS upon salt addition to HPTS-LYZ system indicates that
234
the presence of salt most likely dislodges HPTS from lysozyme-pocket.
236
238 239
0Μ
0.01
[LYZ]
(A-A0)
237
(a)
0.00
−4
1.4 ×10 Μ
-0.01
-5
0.0
5.0x10
-4
1.0x10
[LYZ] (M)
0.06
−4
1.4 ×10 Μ
[LYZ] 0Μ
0.00 400
242
At 40 3n m
0.12
240 241
0.08
450
Absorbance at λ = 454 nm
0.18
At 45 4
235
nm
226
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 33
(b)
In absence of LYZ In presence of LYZ
0.06
0.04
0.02
∆pKa~0.4
0.00 500
5.5
6.0
Wavelength (nm)
6.5
7.0
7.5
8.0
8.5
9.0
pH
243
Figure 2. (a) Absorbance spectra of HPTS in absence and in presence of increasing lysozyme
244
concentration in aqueous phosphate buffer of pH 7. Inset shows change in absorbance (A-A0)
245
upon addition of lysozyme monitored at 403 nm and 454 nm. (b) Determination of pKa of
246
HPTS: Absorbance at 454 nm is plotted as a function of pH of the medium, in the absence
247
and in presence of lysozyme, [LYZ] =1.0 ×10-4 M.
248
Emission spectral study:
249
For a photoacid, the steady state emission spectra can provide a good qualitative
250
account of the ESPT process. In aqueous buffer of pH 7, fluorescence emission spectrum of
251
HPTS shows an intense band at 510 nm due to the deprotonated species (RO-*) in the excited
252
state and a very weak band at 437 nm from the photo-excited protonated species (ROH*)
253
(Figure 3). In aqueous phosphate buffer, the emission intensity of ROH* is very weak
254
compared to RO-* (IROH* /IRO-* is ~0.03, displayed in Figure 4b, Table 1) because of ESPT
255
reaction. Successive addition of LYZ to the aqueous buffered solution of HPTS increases the
256
emission intensity at ROH* with a concomitant decrease in intensity at the RO-*
ACS Paragon Plus Environment
Page 9 of 33
257
accompanying an iso-emissive point at 470 nm (Figure 3). In 2.4×10-4 M LYZ, when almost
258
saturation of such inter-conversion is observed, a ~3.5 fold increase in IROH* /IRO-* (~0.10,
259
Figure 4b) was obtained (the magnitude of IROH* /IRO-* as a function of lysozyme
260
concentration is provided in the Supporting information, Figure S3). The emission spectral
261
profile in the presence of LYZ indicates shifting of the equilibrium between the two
262
concerned protomeric species toward the protonated form. This could have resulted due to
263
increase in population of the ROH exhibiting an upshift of both pKa and pKa*. But, contrarily,
264
it was observed in the UV-Vis spectral study that the pKa as well as pKa* are downshifted in
265
presence of LYZ resulting in some increased population of the RO form. Hence, the most
266
probable cause for this observed feature of emission spectra could be due to a progressive
267
slowing down of ESPT reaction rate with increasing lysozyme concentration as compared to
268
that in the neat aqueous buffer9, 22. A clear and quantitative picture could be obtained only
269
from the time-resolved studies, discussed later.
-
270 6
1.0x10
0M
271 272 273 274
Fluorescence Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
[LYZ] −4
2.4×10 M 5
5.0x10
275 276 277 278
−4
2.4×10 M [LYZ] 0M 0.0 400
470 nm 450
500
550
600
Wavelength (nm)
279
Figure 3. Emission spectra of HPTS in the absence and presence of increasing lysozyme
280
concentration, in aqueous phosphate buffer of pH 7. λexc = 380 nm.
281
Then, to examine whether salt has any effect on further modulation of the excited
282
state proton transfer process of HPTS in the presence of lysozyme, NaCl was varied in the
283
concentration range of 0.02 M to 0.2 M. No change was noticed in ESPT behavior of the
284
fluorophore in the presence of salt only, as evident from steady state emission spectral feature
285
and in ESPT time constant (Supporting information, Figure S4, S5, Table T2). Interestingly,
286
upon addition of salt in the HPTS-LYZ complex the ratio (IROH* /IRO−*) is found to decrease
287
from 0.10 (without NaCl) to ~0.035 (with 0.2 M NaCl), which is similar to that observed in
288
neat aqueous buffer only (IROH* /IRO−*is ~0.03) (Figure 4a, 4b; Table 1). The reversal of
ACS Paragon Plus Environment
The Journal of Physical Chemistry
289
emission spectral profile (Figure 4a) in increasing salt concentration also passes through an
290
iso-emissive point at 470 nm. Such reversal in steady state emission profile of LYZ-bound
291
HPTS with increase of ionic strength is also an indication of removal of the fluorophore from
292
the protein environment to the bulk water, and this naturally shows a bulk-like faster proton-
293
transfer rate again compared to the retarded proton-switching process in its protein-bound
294
state.
296 297 298 299 300
6
1.05x10
0.12
(a)
5
7.00x10
5
3.50x10
0.00 400
(b) (ii)
In aqueous buffer only With LYZ With LYZ + 0.02 N NaCl With LYZ + 0.05 N NaCl With LYZ + 0.1 N NaCl With LYZ + 0.2 N NaCl
IROH* / IRO-*
295
Fluorescence Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 33
0.10
0.08
(iii)
0.06
(vii) 0.04
(i) (viii)
0.02 450
500
550
600
0.0
Wavelength (nm)
0.1
0.2
[NaCl] (M)
301 302
Figure 4. (a) Emission spectra of HPTS with 2.4×10-4 M lysozyme in increasing NaCl
303
concentration, in aqueous phosphate buffer of pH 7. (b) IROH* /IRO-* plotted against different
304
NaCl concentration: (i) in aqueous buffer only (0 M NaCl), (ii) in presence of 2.4×10-4 M
305
lysozyme (0 M NaCl), (iii)→(vii) in presence of 2.4×10-4 M lysozyme with increasing NaCl
306
(0.02 M to 0.2 M), and (viii) in 0.2 M NaCl only. λexc = 380 nm.
307 308
Estimation of binding constants:
309
For the estimation of the binding constant of HPTS-LYZ complex, the emission
310
intensity enhancement of ROH in presence of LYZ has been utilized. The binding constant
311
(KBH) of the complex has been estimated using the Benesi-Hildebrand equation (equation 1)57
312
(I∞ − I0 ) 1 = 1+ ........... (1) ( It − I0 ) K BH [ protein]n
313
Here, I0, It and I∞ are emission intensities of ROH (monitored at λem= 437 nm) in the absence
314
of LYZ, at any intermediate concentration of LYZ and at maximum LYZ concentration that
315
has been used here, respectively; and n is the number of binding sites. The plot of
ACS Paragon Plus Environment
(I∞ − I0 ) ( It − I0 )
Page 11 of 33
1 , using the Benesi-Hildebrand equation has been shown in Figure 5. The linear [ protein]n
316
vs.
317
nature of the plot (Figure 5) suggests a 1:1 stoichiometry (n = 1) of the complex. Binding
318
constant (KBH) estimated from the linear plot is found to be (1.4±0.05) ×104 M-1.
319
As observed from the fluorescence data that the presence of salt affects pyranine
320
binding to LYZ, the binding constants of HPTS-LYZ complex were determined in the
321
presence of different NaCl concentration also, using the above mentioned equation 1. The
322
estimated values of binding constants are listed in Table 2, which shows that the binding
323
constant decreases as salt concentration is increased. At 0.2 M NaCl, binding constant
324
reduces to ~2.5 fold from the initial value. This decrease in binding constant also suggests
325
that HPTS is getting dissociated from the complex and returns to the bulk water.
326
. 30
327 25
328
(I∞-I0) / (It-I0)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
20
329
HPTS-LYZ + 0 M NaCl HPTS-LYZ + 0.02 M NaCl HPTS-LYZ + 0.1 M NaCl HPTS-LYZ + 0.2 M NaCl
15
330
10 5
331
0
332
0.0
4
4.0x10
4
8.0x10
5
1.2x10
5
1.6x10
-1
333
1/[LYZ] (M )
334
Figure 5. Benesi-Hildebrand plot for HPTS-LYZ complexation in the absence and in
335
presence of NaCl.
336
Fluorescence Excitation spectra:
337
The fluorescence excitation spectrum collected at 510 nm in aqueous buffer
338
(Supporting information, Figure S6) is found to be similar in shape to that of the absorption
339
spectrum, having maximum intensity at 403 nm (for the protonated form) along with a
340
weaker band at 454 nm (for the deprotonated form). Addition of LYZ increases the band
341
intensity at 454 nm with a concomitant decrease in intensity at 403 nm. This also
342
substantiates the aforementioned ground state equilibrium shifting toward the deprotonated
343
conjugate base, as was revealed from absorption spectral study in the presence of LYZ.
344
Steady-state anisotropy:
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
345
Steady-state anisotropy gives a quantitative measure of local fluidity in the immediate
346
vicinity of a fluorophore. Steady-state anisotropy of HPTS was monitored upon addition of
347
lysozyme in the absence and in presence of salt and they are highlighted in Figure 6.
348
Anisotropy (r) of the probe was found to increase gradually with lysozyme concentration
349
followed by saturation. The protein scaffold renders a motional restriction to the bound
350
fluorophore due to local environmental rigidity compared to bulk aqueous buffer medium.
351
Steady-state anisotropy of the fluorophore was measured upon the addition of lysozyme in
352
the presence of added 0.2 M NaCl also. It was found that in the presence of 0.2 M NaCl, the
353
increase in anisotropy (r) of the probe as a function of lysozyme concentration was almost
354
negligible compared to that observed in the absence of added salt. Hence, steady-state
355
anisotropy data also supports that the presence of salt impairs the aforementioned HPTS-LYZ
356
binding (in the presence of 0.2 M NaCl in aqueous buffer, anisotropy (r) of HPTS was almost
357
same to that found in aqueous buffer only). Steady-state anisotropy measurements can also
358
provide the binding constant of protein-ligand complexation process. The detailed method
359
and the representative plot (Figure S7) for binding constant estimation has been discussed in
360
the Supporting information under the section titled “Binding constant estimation from steady-
361
state anisotropy”. The binding constant (Kan) evaluated from anisotropy data for HPTS-LYZ
362
complex, was found to be (1.5±0.07)×104 M-1, and (7.6±0.05)×103 M-1; in absence and in
363
presence of salt, respectively. These binding constant values are also in good agreement with
364
those obtained from the Benesi-Hildebrand method (Table 2).
365 366 367 368 369 370 371 372 373 374
ACS Paragon Plus Environment
Page 12 of 33
Page 13 of 33
0.04
With LYZ only With LYZ + 0.2 M NaCl
375 376 377 378
Anisotropy (r)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
0.03
0.02
379 0.01
380
0.0
5.0x10
-5
-4
1.0x10
-4
1.5x10
[LYZ] (M) 381
Figure 6. Variation of steady-state fluorescence anisotropy of HPTS with increasing
382
lysozyme concentration (in absence and in presence of NaCl) in aqueous buffer of pH 7;
383
emission collected at λem = 510 nm. λexc = 380 nm.
384 385
3.2 Time-resolved fluorescence emission studies
386
Time resolved emission studies provide a quantitative picture of proton-transfer
387
process in the excited state. Fluorescence decays of ROH* were recorded at 435 nm and those
388
of the RO-* were recorded at 520 nm in a picoseconds TCSPC setup. Figures 7a, 7b show
389
emission decays of HPTS at 435 nm (for ROH* emission) and at 520 nm (for RO-* emission),
390
respectively, in different systems. The ESPT rate of HPTS could be tracked either by
391
monitoring the decay time of the excited protonated form (ROH*) at its emission maximum
392
~435 nm, or the rise-time of the conjugate base (RO-*) emission at ~520 nm. The time
393
constants of multi-exponential fits of emission decays at both the observation wavelengths
394
(435 nm and 520 nm) are presented in Table 3. In aqueous buffer, the decay fits with a tri-
395
exponential function with time constants ~100 ps, ~550 ps and ~3.25 ns at 435 nm. The
396
shortest time constant (100 ps) has a contribution ~92%, the other picosecond component has
397
~7%, and the nanosecond component contributes to ~1%. At 520 nm, emission decay was
398
fitted by a bi-exponential function with two time constants, ~100 ps corresponding to a rising
399
component and ~5.5 ns, a decay component. This rise-time (~100 ps) at 520 nm correlates
400
with the decay component of 100 ps observed at 435 nm. Several earlier reports indicate
401
nearly same value (~90 ps) of proton-transfer time constant of HPTS in water9. Hence, the
402
100 ps rise-time could be attributed to the ESPT time constant of HPTS in aqueous buffer. As
403
reported by Cohen et al.23, a ~260 ps component has been assigned to the lifetime of ROH* in
404
0.1 M aqueous phosphate buffer of pH 7.0. Here the ~550 ps component at the ROH* form
ACS Paragon Plus Environment
The Journal of Physical Chemistry
405
could be assigned to the lifetime of ROH*, and ~5.5 ns component to the lifetime of RO-*, in
406
0.01 M aqueous phosphate buffer. The difference is presumably due to different buffer
407
strengths, as the presence of phosphate ions are known to affect the lifetimes of the species
408
under observation23. In aqueous phosphate buffer, excited state equilibrium (due to geminate
409
recombination) between the protonated and deprotonated forms is not observed because of
410
the phosphate anions available in low concentration in the system, can act as scavengers of
411
protons, thus affecting the recombination process without any effect on the proton-transfer
412
rate2, 58. Here also geminate recombination was not observed as evident from Time Resolved
413
Area Normalized Emission Spectra (TRANES) (Figure 8) which shows that the protonated
414
form is practically non-existent after ~500 ps (lifetime value assigned for ROH).
415 10.0k
417
8.0k
418
(a)
IRF In aqueous buffer only With LYZ With LYZ + 0.02 M NaCl With LYZ + 0.05 M NaCl With LYZ + 0.1 M NaCl With LYZ + 0.2 M NaCl
6.0k
10.00k
(b)
IRF In aqueous buffer only With LYZ With LYZ + 0.2 M NaCl
7.50k
Counts
416
Counts
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 33
5.00k
419
4.0k
420
2.0k
421
0.0
2.50k
5
6
7
Time (ns)
8
9
5
6
7
8
9
10
Time (ns)
422 423
Figure 7. Fluorescence emission decay of HPTS in different conditions, in aqueous
424
phosphate buffer of pH 7. Decays collected at (a) 435 nm (for ROH*), and (b) at 520 nm (for
425
RO-*). λexc = 375 nm, [LYZ] =1.8×10-4 M.
426
In 1.8×10-4 M lysozyme, fluorescence decay at 435 nm and the rise obtained at 520
427
nm are found to be markedly slower compared to aqueous buffer. Emission decays of HPTS
428
are found to fit by three exponentials at both the observation wavelengths (435 nm and 520
429
nm). At 520 nm (due to RO-*), the transient is fitted by three exponential components
430
showing two different rise-times, and one decay time. Between the two time constants, the
431
shorter one comes at ~140 ps, and the other comes at ~780 ps. In pure aqueous buffer,
432
relative amplitude of the 100 ps rising component (emission monitored at 520 nm) was found
433
to be ~52%, with ~48% contribution from the 5.5 ns which is the lifetime of excited anionic
434
species. In presence of lysozyme, the tri-exponential fit of the decay collected at 520 nm,
ACS Paragon Plus Environment
Page 15 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
435
gives relative amplitudes of ~13% corresponding to the 140 ps component and ~9% for 780
436
ps component, and 78% of the RO-*. Both of the rise-times, in the presence of LYZ, are
437
slower compared to that in aqueous media suggesting a slower ESPT process of lysozymized
438
HPTS. In order to substantiate the findings of salt addition study from steady state emission
439
measurements and to get a quantitative picture in terms of proton-transfer time constants
440
upon salt addition to HPTS-LYZ system, further time resolved emission measurements were
441
attempted. The retarded decay (at 435 nm) and rise (at 520 nm) of HPTS, due to binding with
442
lysozyme, both turned faster with increasing salt concentration (Figure 7a, 7b). Here also the
443
emission transient at 520 nm is found to fit by tri-exponential decay function with two rise
444
components and one decay component. Now, the shorter 140 ps rise component, found in the
445
presence of LYZ, goes progressively faster with increasing NaCl concentration, and with 0.2
446
M NaCl it becomes ~99 ps which is close to that found in aqueous buffer media only, while
447
the other slower ~780 ps rise component survives with nearly same magnitude. In 0.2 M
448
NaCl, among the three components, the 99 ps component contributes to ~52%, the other
449
longer rising component contributes to ~2% and the decay component contributes to ~46%. If
450
we consider the relative weightage between the two growing components, it is found that in
451
the presence of lysozyme only, the 140 ps component has a contribution of ~58%, and the
452
other has ~42% weightage (Table 4). The addition of salt (increase of ionic strength) makes
453
the 140 ps time constant faster and its relative contribution also increases, while the relative
454
weightage of the longer component is found to reduce largely. In 0.2 M NaCl, relative
455
weightage of the shorter rising component is found to be ~95%, while the other one has a
456
very little contribution of only ~5%. TRANE spectra (Figure 8) also complements the
457
observed proton-transfer rates (or, formation rate of RO-*) in different systems as the intensity
458
ratio of ROH* and RO-* at any instant in different systems matches well the observed proton-
459
transfer time constant (Supporting information, T3). Hence, the results from time resolved
460
decays with increasing ionic strength also suggest that the probe comes out from the protein
461
pocket to bulk aqueous medium.
462 463 464 465 466 467 468
ACS Paragon Plus Environment
The Journal of Physical Chemistry
5
469 470 471 472 473 474
3x10
Fluorescence intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
-*
(a)
0 ps 10 ps 20 ps 50 ps 100 ps 200 ps 500 ps 1 ns 5 ns 10 ns 15 ns
[RO ] 0 ps *
[ROH] 5
2x10
15 ns
5
1x10
0 420
(b)
-*
[RO ]
5
2x10
0 ps *
[ROH] 15 ns 5
1x10
15 ns
15 ns
480
510
Wavelength (nm)
-*
(c)
[RO ]
0 ps
5
3x10
540
570
0 420
450
480
510
0 ps 10 ps 20 ps 50 ps 100 ps 200 ps 500 ps 1 ns 5 ns 10 ns 15 ns
*
[ROH] 15 ns 5
2x10
15 ns
5
1x10
0 ps
0 ps 450
0 ps 10 ps 20 ps 50 ps 100 ps 200 ps 500 ps 1 ns 5 ns 10 ns 15 ns
Page 16 of 33
0 ps
540
570
Wavelength (nm)
0 420
450
480
510
540
Wavelength (nm)
475
Figure 8. Time resolved area normalized emission spectra of HPTS in (a) aqueous buffer
476
only (b) with 1.8×10-4 M LYZ (c) with 1.8×10-4 M LYZ + 0.2 M NaCl. λexc = 375 nm.
477
In an ESPT reaction, where proton is transferred to water molecules, the rate will
478
strongly depend on dynamics of the acceptor water molecules. The water molecules confined
479
in a supramolecular assembly like protein are a bit immobilized in nature and, moreover, the
480
probe attached to protein also experiences some specific or non-specific interactions with the
481
nearby charge-containing amino acid side-chains23. An earlier work on ESPT of confined
482
HPTS in γ-cyclodextrin reports about two proton-transfer time constants ~140 ps and ~1.4 ns
483
which are due to the change in properties of water in the vicinity of cyclodextrin cavity21.
484
Two different proton-transfer time constants of HPTS (250 ps and 2.4 ns) were reported in
485
P123-CTAC aggregate due to two different locations of dye, one that is a buried location
486
inside the hydrophobic core (2.4 ns component assigned) and other is a water exposed
487
location (250 ps component)59. Cohen et al. have reported proton-transfer time constant of
488
HPTS spanning from 150 fs to 1.22 ns while bound to human serum albumin23, where the
489
slower 1.22 ns decay was assigned to proton transfer to biological water molecules in the
490
vicinity of the protein. Considering the molecular structure and electrical charge properties of
491
the probe, they suggested that it binds primarily in the subdomain IIIA cavity of site II of
492
HSA. Here the small protein lysozyme has only one, a bit water exposed, binding cavity
493
which is its catalytic site and binds small molecules60. We investigated a most probable
494
binding location of the probe in LYZ by molecular docking study. The factors those are most
495
likely to be responsible in modulating the proton-transfer process in LYZ only and also upon
496
salt addition to lysozymized pyranine, have been addressed in discussion part.
497 498
ACS Paragon Plus Environment
570
Page 17 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
499
The Journal of Physical Chemistry
3.3. Time resolved anisotropy decay
500
Time resolved anisotropy measurement reports about the rigidity of the environment
501
around a probe. Rotational relaxation time of free probe molecule and probe molecule
502
confined to any macromolecular rigid environment differs considerably and it gives some
503
idea about rotational restriction imposed on the probe after its interaction with the host
504
macromolecule. In the aqueous buffer only, fluorescence anisotropy decay of HPTS (Figure
505
9) fits with a single exponential function with a rotational relaxation time constant of ~154
506
ps, similar values in aqueous medium has already been reported in the literature9. In presence
507
of 1.8×10-4 M LYZ, the emission anisotropy of HPTS becomes bi-exponential, with
508
rotational time constants ~199 ps (α1rot ~13%) and ~2.17 ns (α2rot ~87%). The longer
509
nanosecond component could be assigned to be due to highly restricted rotational motion of
510
HPTS inside the protein matrix, and the other picosecond component is probably due to some
511
contribution coming from loosely protein-bound HPTS.
512
In order to find how salt affects rotational relaxation time of the probe in the complex,
513
time resolved anisotropy measurements were monitored upon salt addition. Anisotropy decay
514
shows bi-exponential nature upto 0.05 M NaCl (Table 5). The contribution of the shorter
515
component increases with a concomitant decrease in the longer component indicating the
516
contribution of larger population of free HPTS in aqueous phase. The average rotational time
517
constant decreases with increasing salt concentration and at some higher NaCl concentration
518
anisotropy decay shows single-exponential behavior. A rotational time constant ~190 ps is
519
found in the presence of 0.2 M NaCl. This decrease in rotational time constant in the presence
520
of salt suggests that HPTS gets untied from the environmental rigidity of the protein. Hence,
521
transient anisotropy measurement in the presence of salt also suggests that HPTS gets
522
dislodged from the protein and returns to the bulk water, and this has already been
523
corroborated by steady state data.
524 525 526 527 528 529
ACS Paragon Plus Environment
The Journal of Physical Chemistry
530
0.25
531
0.20
532 533 534
Anisotropy, r (t)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
In aqueous buffer only With LYZ With LYZ + 0.02 M NaCl With LYZ + 0.05 M NaCl With LYZ + 0.1 M NaCl With LYZ + 0.2 M NaCl
0.15
0.10
0.05
535 0.00 5
6
7
8
9
536
Time (ns)
537
Figure 9. Emission anisotropy decay of HPTS collected at 520 nm in different systems in
538
aqueous buffer of pH 7. λexc = 375 nm. [LYZ] =1.8×10-4 M.
539
3.4. Molecular docking study
540
In order to find a preferred location of HPTS (ligand) in the HPTS-LYZ complex, docking of
541
the fluorophore into 3D crystal structure of LYZ was modelled by molecular docking
542
simulation using Auto Dock 4.2. Several literatures are available regarding binding of
543
different ligands and antigens to the catalytic active site of LYZ61-63. The energetically
544
favorable docked pose of HPTS to LYZ is depicted in Figure 10. The ligand was found to be
545
docked near the substrate binding region of lysozyme and the amino acid residues in close
546
vicinity of docked HPTS (within 4 Å) are: Trp 34, Glu 35, Asn 44, Asp 53, Gln 58, Trp 109,
547
Val 110, Ala 111, Asn 114, and Arg 115. Considering the chemical nature of the amino acid
548
residues in close proximity of the ligand, these could interact with the ligand via different
549
modes as follows. The amino acid residues Asp 53, Asn 114, Arg 115, and Val 110 make
550
hydrogen-bond with the sulfonate groups of HPTS. Presence of some polar amino acid
551
residues like Gln 35, Asn 114,44 and Trp 34,109 can provide van der Waals interaction. The
552
Arg 115, having a positively charged side chain is expected to interact via electrostatic mode
553
with the negatively charged sulfonate groups of HPTS. The π-π interaction may also take
554
place between the aromatic ring of Trp 35, 109 and the pyrene moiety of HPTS. The Val 110
555
and Ala 111 can provide some hydrophobic interaction also. So overall, a combination of H-
556
bonding, van der Waals forces, π-π and electrostatic interactions are playing role to host the
557
ligand. Upon NaCl addition, mainly electrostatic interaction becomes feeble with some
558
possible weakening of van der Waals forces, π-π interaction, and hydrogen-bonding as well.
559
While hosting a ligand, some amino acid residues of the protein get involved in binding the
560
ligand through different kind of interaction modes as discussed and this results in a decrease
ACS Paragon Plus Environment
Page 18 of 33
Page 19 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
561
in solvent accessible surface area (SASA) of the interacting amino acid residues54. Here we
562
calculated the change in SASA values of the interacting nearby amino acid residues and the
563
data are tabulated in Table 6. There are noticeable changes in SASA values of participating
564
amino acid residues, suggesting substantial binding of the probe with LYZ.
565 566 567 568 569 570
Pyranine
571 572
Figure 10. Interacting amino acid residues around HPTS (within 4 Å) in binding region of
573
lysozyme.
574
3.5. Circular dichroism spectral study
575
Circular dichroism spectral study of a protein provides information about its
576
secondary structure. Any structural changes in the protein due to ligand binding could be
577
monitored from CD spectral behavior. CD spectra of LYZ shows two negative peaks at 208
578
nm and 222 nm characteristics of n→π* transition in amide bond of α-helix64.
579
Estimated α-helix content of LYZ was found to be 21.7% (The details regarding
580
corresponding equations for α-helix content calculation and the CD spectra (Figure S8) are
581
provided in Supporting information under the section titled “Circular dichroism spectra”).
582
Upon binding of HPTS, α-helix content slightly decreases to 19.97% at 1:1 protein-ligand
583
ratio. This decrease in α-helix content suggests an alteration of lysozyme secondary structure
584
upon HPTS binding. On adding NaCl (1.6 M) in the HPTS-LYZ complex, α-helix content
585
was found to be increased again to 20.9%, which indicates that the protein again recovers its
586
secondary structure to some extent.
587 588
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
589 590 591
4. Discussion The experimental results show that the ground state as well as the excited state behavior of HPTS is markedly modified upon binding with lysozyme.
592
As observed from UV-Vis absorption study, the deprotonated tetra-negative anion has
593
more preference over the protonated tri-negative anion for lysozyme causing a downshifted
594
ground state pKa of the protein-bound HPTS. If we consider the overall charge of the protein
595
(LYZ) at the experimental pH condition (pH 7) it has net positive charge (as isoelectric point
596
of LYZ is ~11.35) and that is why most likely it prefers the deprotonated anionic protomer
597
presumably on the electrostatic ground. The effect of salt on HPTS-LYZ complex in the
598
ground state as well as in excited state also suggest about a major role of electrostatic
599
interaction in the complexation process. While attempting for a local pH estimation using
600
HPTS (the intracellular pH indicator) utilizing the Henderson–Hasselbalch equation, the pKa
601
value corresponding to aqueous solution is utilized. But, interestingly it was found that the
602
presence of lysozyme alters the absorbance ratio of those two protomers of HPTS and
603
consequently gives a shifted pKa. In that case, using the same pKa value obtained for aqueous
604
medium will end up in erroneous pH estimation. Hence, to probe the actual pH of the
605
environment where the probe interacts with some other biomolecule (here LYZ), the shifted
606
pKa (as stated earlier) of the probe should be plugged in.
607
Strikingly, although there is a preference of the deprotonated form of HPTS over its
608
protonated one in lysozyme, resulting a downshifted pKa as well as pKa*, the excited state
609
proton-transfer becomes slower than in bulk water (only 100 ps). To have a scenario about
610
the slowing down of proton-transfer of the probe in LYZ matrix, we have focused on the
611
factors that control the kinetics of proton-transfer in the excited state. An ESPT process in
612
which a proton is transferred to a suitable acceptor molecule (in aqueous solution, a proton is
613
donated to water molecule) depends on dielectric constant, solvation dynamics, and
614
availability of sufficient water molecules to solvate the ejected proton and the conjugate base.
615
The dielectric constant determination for a macromolecule like protein is a complicated task
616
due to the presence of many charged side groups and simulation studies have shown that it
617
depends also on the model used 65
65
. For hen egg white lysozyme in water, static dielectric
618
constant was reported
to be ~30. Hen egg white lysozyme, consisting of 129 amino acid
619
residues, has ~59% sequence homology with human lysozyme66. So, a comparable value of
620
the static dielectric constant for human lysozyme is also expected. Considering a nearly
621
similar value it is evident that there is a reduction in dielectric constant in lysozyme pocket
ACS Paragon Plus Environment
Page 20 of 33
Page 21 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
622
and hence the solvation energy of the ejected proton as well as the population of the
623
conjugate base will be less as compared to that in bulk water. The other very important factor
624
is solvation dynamics. For a polar reaction, the activation barrier of its polar transition state
625
depends on solvation and any slow component of solvation will retard the reaction due to
626
insufficient or incomplete solvation67. In many natural macromolecular systems, water is
627
confined in such environment where its free movement is restricted and its three-dimensional
628
H-bonded network gets disrupted due to the interaction with the macromolecule. Such
629
confined water molecules are different in their dynamical nature and show some ultraslow
630
component of solvation compared to bulk. For water in bulk, solvation occurs in ~1 ps67. A
631
slow component like 530 ps was detected in eosin covalently attached to lysozyme68, whereas
632
a 38 ps component was reported for tryptophan located near the surface of a protein subtilisin
633
Carlsberg69. These suggest that water molecules available in the protein cavity or on the
634
surface are more constrained in nature and their dynamical property also differs depending on
635
the location in the protein. Here in our system, between the two ESPT time constants
636
observed in presence of LYZ, the slower component of 780 ps could be attributed to proton-
637
transfer occurring to water molecules that are deep-seated (tightly-bound) inside the protein
638
scaffold and showing relatively slower dynamics; while the other 140 ps component could be
639
ascribed to water molecules that are loosely associated to the protein and shows nearly bulk-
640
like behavior. This has also been suggested by time resolved anisotropy measurements that
641
some of the population of HPTS remains there as loosely attached to the protein (having a
642
rotational time constant ~199 ps), and the rest of the population is tightly bound to the protein
643
(rotational time constant ~2.17 ns). The other governing factors of ESPT process is the
644
requirement of sufficient number of water molecules to solvate the ejected proton and the
645
conjugate base. In the vicinity of the active site of protein, the water molecules present are
646
smaller in number, and those can also contribute to slowing down the ESPT process in
647
lysozyme.
648
Next we discuss the most likely causes for further modification in ESPT process of
649
LYZ-bound pyranine upon NaCl addition. First we consider how the factors, which proton-
650
transfer depends on, are expected to get affected and influence the proton-transfer rate in the
651
presence of NaCl only. Presence of small ions in water decreases the dielectric constant of the
652
medium, and solvation energy of proton and the conjugate base will be reduced to some
653
extent, making proton-transfer process slower. In high ionic strength, solvation becomes slow
654
and the number of free water molecules also gets reduced. These three factors should retard
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
655
the ESPT process in presence of NaCl. But, as mentioned previously, the steady state
656
emission and time resolved data show that the presence of NaCl in our studied concentration
657
range (upto 0.2 M) does not affect the proton-transfer rate directly by slowing it down.
658
Instead, the presence of salt makes the retarded ESPT process faster again in the complex.
659
Importantly, the concentration range of NaCl that we used here does not salt out LYZ 70 and
660
hence protein salting-out from the medium should not be the cause for the observed proton
661
transfer rate alterations. It is well known that electrostatic interaction between two interacting
662
partners (like protein and ligand) can be screened with increased ionic strength46, 47. Hence, it
663
is most probable that the increased ionic strength of medium results in weakening of the
664
binding interaction between lysozyme and pyranine, i.e. in other words, electrostatic
665
interaction has a dominant role in the complexation process. As previously mentioned, by
666
considering charges on the two interacting partners it is quite likely that the electrostatic
667
interaction is the major mode of binding. Therefore, as observed from the steady-state and
668
excited-state spectroscopic data, the salt addition throws HPTS from interaction zone of LYZ
669
to bulk water. As a consequence, the 140 ps time constant, assigned to be due to the loosely
670
associated water molecules in the protein, disappears and consequently the 99 ps time
671
constant reappears and this is very similar to the behavior of the probe in bulk water.
672
Whereas, the other longer component (780 ps), assigned to be due to the deep-seated water
673
molecules inside the protein matrix, survives with very negligible contribution.
674
5. Conclusion
675
The present work shows that the ground state and the excited state behavior of the photoacid
676
pyraine gets markedly modified while bound in lysozyme cavity compared to bulk aqueous
677
media. In spite of the preference for the conjugate base of HPTS inside the protein, the ‘slow’
678
biological water associated with the protein matrix retards the proton-transfer process in the
679
excited state causing a slowing down of the proton transfer rate. HPTS forms a 1:1 complex
680
with the protein in the ground state with a binding constant (KBH) of (1.4±0.5)×104 M-1.
681
Transient emission anisotropy measurements confirm the said complexation as rotational
682
time constant of HPTS increases in the presence of the protein. The docking simulation study
683
shows a possible binding region near the active site of the protein, and there is a little
684
conformational change in the protein upon ligand binding as observed in the circular
685
dichroism spectra. Since there is a major dominance of electrostatic forces of interaction
686
between HPTS and lysozyme, salt addition interrupts the fluorophore-protein complex,
687
ultimately throwing the fluorophore away from lysozyme cavity to the bulk. As a
ACS Paragon Plus Environment
Page 22 of 33
Page 23 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
688
consequence, binding constant of the complex gets reduced and rotational time constant
689
decreases again. Importantly, the shifted prototropic equilibrium of HPTS toward the
690
conjugate base due to interaction with lysozyme has been found to be back-shifted toward the
691
protonated species in the presence of salt and consequently the excited state proton transfer
692
rate has been observed to follow bulk-like feature ultimately. The study shows a considerable
693
different perspective of the perturbation offered by the model bio-macromolecular host with
694
substantial water exposure at the interaction site unlike the available literature reports
695
involving bio-macromolecular pockets and some other macromolecular cavity associated
696
with much less water exposure and different electrical nature on the concerned photoacid. It
697
is indeed worth noting that modulation of the prototropy in the presence of different
698
biomacromolecular host can lead to distinctive behavioral pattern that need to be explored in
699
detail to understand the intricacies of physico-chemical processes.
700 701
Supporting information
702
Absorption spectra with varying pH, in absence and in presence of LYZ; pKa estimation in
703
presence of 0.2 M NaCl and in presence of LYZ + 0.2 M NaCl, IROH* /IRO−* with increasing
704
LYZ concentration, excitation spectra of HPTS in absence and in presence of LYZ, intensity
705
ratio of ROH and RO- obtained from time resolved area normalized emission spectra at
706
different times, steady state emission spectra, time resolved emission decay with decay
707
parameters at maximum NaCl concentration used (0.2 M), binding constant evaluation from
708
steady-state anisotropy measurements, and Circular dichroism spectral study in detail are
709
presented in supporting information.
710 711
Acknowledgement
712
MH thanks DST SERB-India (Fund no. SB/S1/PC-041/2013) and IIT Kharagpur for financial
713
supports. ID thanks IIT Kharagpur and SP thanks UGC-India for their fellowship. We would
714
like to thank the anonymous reviewers for their critical comments and suggestions.
715 716 717
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
718
References
719 720
1.
Agmon, N. Elementary steps in excited-state proton transfer. J. Phys. Chem. A 2005, 109, 13-
721
35.
722
2.
723
A 2000, 104, 6689-6698.
724
3.
Kuhlbrandt, W. Bacteriorhodopsin - the movie. Nature 2000, 406, 569-570.
725
4.
Rini, M.; Magnes, B. Z.; Pines, E.; Nibbering, E. T. J. Real-time observation of bimodal
726
proton transfer in acid-base pairs in water. Science 2003, 301, 349-352.
727
5.
728
transfer rates in diffusion-controlled reactions. Chem. Phys. Lett. 1997, 281, 413-420.
729
6.
730
(HPTS). J. Phys. Chem. A 2007, 111, 230-237.
731
7.
732
1955-1976.
733
8.
734
surfaces. J. Am. Chem. Soc. 1984, 106, 265-273.
735
9.
736
transfer of pyranine in a gamma-cyclodextrin cavity. Chem. Phys. Lett. 2005, 412, 228-234.
737
10.
738
absence of the DNA backbone. Nature 2000, 408, 949-951.
739
11.
740
bacterial reaction centers. FEBS Lett. 2003, 555, 45-50.
741
12.
742
intermolecular reactions. J. Photochem. Photobiol., A 1993, 75, 1-20.
743
13.
744
reactions. J. Photochem. Photobiol., A 1993, 75, 21-48.
745
14.
746
cyclodextrins and human serum albumin protein. Langmuir 2012, 28, 6746-6759.
747
15.
748
cell membranes. J. Phys. Chem. B 2009, 113, 10210-10221.
749
16.
750
cell membranes. Langmuir 2008, 24, 3690-3698.
751
17.
752
from pyranine to acetate in gamma-cyclodextrin and hydroxypropyl gamma-cyclodextrin. J. Phys.
753
Chem. A 2006, 110, 13646-13652.
Genosar, L.; Cohen, B.; Huppert, D. Ultrafast direct photoacid-base reaction. J. Phys. Chem.
Pines, E.; Manes, B. Z.; Lang, M. J.; Fleming, G. R. Direct measurement of intrinsic proton
Spry, D. B.; Goun, A.; Fayer, M. D. Deprotonation dynamics and stokes shift of pyranine
Douhal, A. Ultrafast guest dynamics in cyclodextrin nanocavities. Chem. Rev. 2004, 104,
Politi, M. J.; Fendler, J. H. Laser pH-jump initiated proton transfer on charged micellar
Mondal, S. K.; Sahu, K.; Sen, P.; Roy, D.; Ghosh, S.; Bhattacharyya, K. Excited state proton
Nir, E.; Kleinermanns, K.; de Vries, M. S. Pairing of isolated nucleic-acid bases in the
Paddock, M. L.; Feher, G.; Okamura, M. Y. Proton transfer pathways and mechanism in
Arnaut, L. G.; Formosinho, S. J. Excited-state proton-transfer reactions I. Fundamentals and
Formosinho, S. J.; Arnaut, L. G. Excited-state proton-transfer reactions II. Intramolecular
Martin, C.; Gil, M.; Cohen, B.; Douhal, A. Ultrafast photodynamics of drugs in nanocavities:
Spry, D. B.; Fayer, M. D. Proton transfer and proton concentrations in protonated nafion fuel
Moilanen, D. E.; Spry, D. B.; Fayer, M. D. Water dynamics and proton transfer in nafion fuel
Mondal, S. K.; Sahu, K.; Ghosh, S.; Sen, P.; Bhattacharyya, K. Excited-state proton transfer
ACS Paragon Plus Environment
Page 24 of 33
Page 25 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
754
18.
Mondal, T.; Das, A. K.; Sasmal, D. K.; Bhattacharyya, K. Excited state proton transfer in
755
ionic liquid mixed micelles. J. Phys. Chem. B 2010, 114, 13136-13142.
756
19.
757
proton transfer of pyranine in an ionic liquid microemulsion. J. Chem. Phys. 2010, 132, No. 194505,
758
DOI : 10.1063/1.3428669.
759
20.
760
microenvironment of a binding site. Eur. J. Biochem. 1982, 121, 637-642.
761
21.
762
antenna effect of the gamma-cyclodextrin outer surface, measured by excited state proton transfer. J.
763
Phys. Chem. B 2006, 110, 26354-26364.
764
22.
765
ultrafast proton transfer in niosome. J. Phys. Chem. B 2012, 116, 8105-8112.
766
23.
767
reaction dynamics within the human serum albumin protein. J. Phys. Chem. B 2011, 115, 7637-7647.
768
24.
769
monitor apoptosis and its suppression by bcl-2 over-expression in hybridoma cell culture. J. Immunol.
770
Methods 1998, 221, 43-57.
771
25.
772
by several agents is preceded by intracellular acidification. Proc. Natl. Acad. Sci. U.S.A. 1996, 93,
773
654-658.
774
26.
775
Proc. Natl. Acad. Sci. USA 1994, 91, 4101-4101.
776
27.
777
evaluation of the use of endoplasmic reticulum-targeted "cameleons". Endocrinology 2004, 145,
778
4540-4549.
779
28.
780
intracellular pH of Caco-2 cells. Int. J. Pharm. 2007, 338, 104-109.
781
29.
782
PK1 cells - recovery from an acid load via basolateral Na+/H+ exchange. J. Membr. Biol. 1987, 97, 63-
783
78.
784
30.
785
pH buffering power in smooth muscle. Pflugers Arch. 1998, 435, 575-577.
786
31.
787
different intensifies in mammalian single muscle fibres. J. Physiol. 1998, 512, 831-840.
788
32.
789
engulfment of apoptotic cells by macrophages using pHrodo succinimidyl ester. J. Immunol. Methods
790
2009, 342, 71-77.
Sen Mojumdar, S.; Mondal, T.; Das, A. K.; Dey, S.; Bhattacharyya, K. Ultrafast and ultraslow
Gutman, M.; Huppert, D.; Nachliel, E. Kinetic studies of proton transfer in the
Gepshtein, R.; Leiderman, P.; Huppert, D.; Project, E.; Nachliel, E.; Gutman, M. Proton
Mondal, T.; Ghosh, S.; Das, A. K.; Mandal, A. K.; Bhattacharyya, K. Salt effect on the
Cohen, B.; Alvarez, C. M.; Carmona, N. A.; Organero, J. A.; Douhal, A. Proton-transfer
Ishaque, A.; Al-Rubeai, M. Use of intracellular pH and annexin-V flow cytometric assays to
Gottlieb, R. A.; Nordberg, J.; Skowronski, E.; Babior, B. M. Apoptosis induced in Jurkat cells
Simon, S.; Roy, D.; Schindler, M. Intracellular pH and the control of multidrug-resistance. Varadi, A.; Rutter, G. A. Ca2+-induced Ca2+ release in pancreatic islet beta-cells: critical
Liang, E.; Liu, P. C.; Dinh, S. Use of a pH-sensitive fluorescent probe for measuring
Montrose, M. H.; Friedrich, T.; Murer, H. Measurements of intracellular pH in single LLC-
Bullock, A. J.; Duquette, R. A.; Buttell, N.; Wray, S. Developmental changes in intracellular
Chin, E. R.; Allen, D. G. The contribution of pH-dependent mechanisms to fatigue at
Miksa, M.; Kornura, H.; Wu, R. Q.; Shah, K. G.; Wang, P. A novel method to determine the
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
791
33.
Lakadamyali, M.; Rust, M. J.; Babcock, H. P.; Zhuang, X. W. Visualizing infection of
792
individual influenza viruses. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 9280-9285.
793
34.
794
Michael, N. P.; Milligan, G.; Game, S. A pH-sensitive fluor, CypHer 5, used to monitor agonist-
795
induced G protein-coupled receptor internalization in live cells. Biotechniques 2002, 33, 1152-1157.
796
35.
797
2709-2728.
798
36.
799
Chem. Educ. 2001, 78, 1499-1503.
800
37.
801
ultrafast pH jump. Chem. Phys. Lett. 1979, 64, 522-527.
802
38.
803
yesterday. Mol. Cell. Biochem. 1984, 63, 165-189.
804
39.
805
Structure of hen egg-white lysozyme - a 3-dimensional Fourier synthesis at 2Å resolution. Nature
806
1965, 206, 757-761.
807
40.
808
J. M.; Zhang, L.; et al. Characterization of bioactive recombinant human lysozyme expressed in milk
809
of cloned transgenic cattle. PLoS ONE 2011, 6, e17593.
810
41.
811
distortion of an N-acetylmuramic acid residue bound in site D. J. Mol. Biol. 1991, 220, 401-424.
812
42.
813
partly folded protein by heteronuclear NMR-spectroscopy - assignment and secondary structure-
814
analysis of hen egg-white lysozyme in trifluoroethanol. Biochemistry 1995, 34, 13219-13232.
815
43.
816
probing the process of unfolding. Biophys. J . 2007, 92, 2523-2535.
817
44.
818
X.; He, Q. The targeting of 14-succinate triptolide-lysozyme conjugate to proximal renal tubular
819
epithelial cells. Biomaterials 2009, 30, 1372-1381.
820
45.
821
the excited-state proton transfer and geminate recombination. J. Phys. Chem. A 2006, 110, 5573-5584.
822
46.
823
in the pH-dependent interaction of bovine serum albumin with cochineal red A: a combined view
824
from spectroscopy and docking simulations. J. Agric. Food. Chem. 2013, 61, 4606-4613.
825
47.
826
on binding of tartrazine with two homologous serum albumins: quantification by use of the Debye-
Adie, E. J.; Kalinka, S.; Smith, L.; Francis, M. J.; Marenghi, A.; Cooper, M. E.; Briggs, M.;
Han, J. Y.; Burgess, K. Fluorescent indicators for intracellular pH. Chem. Rev. 2010, 110,
Po, H. N.; Senozan, N. M. The Henderson-Hasselbalch equation: its history and limitations. J.
Smith, K. K.; Kaufmann, K. J.; Huppert, D.; Gutman, M. Picosecond proton ejection-
Jolles, P.; Jolles, J. What’s new in lysozyme research - always a model system, today as
Blake, C. C. F.; Koenig, D. F.; Mair, G. A.; North, A. C. T.; Phillips, D. C.; Sarma, V. R.
Yang, B.; Wang, J. W.; Tang, B.; Liu, Y. F.; Guo, C. D.; Yang, P. H.; Yu, T. A.; Li, R.; Zhao,
Strynadka, N. C. J.; James, M. N. G. Lysozyme revisited - crystallographic evidence for
Buck, M.; Schwalbe, H.; Dobson, C. M. Characterization of conformational preferences in a
Ghosh, A.; Brinda, K. V.; Vishveshwara, S. Dynamics of lysozyme structure network:
Zhang, Z. R.; Zheng, Q.; Han, J.; Gao, G. P.; Liu, J.; Gong, T.; Gu, Z. W.; Huang, Y.; Sun,
Leiderman, P.; Gepshtein, R.; Uritski, A.; Genosar, L.; Huppert, D. Effect of electrolytes on
Bolel, P.; Mahapatra, N.; Datta, S.; Halder, M. Modulation of accessibility of subdomain IB
Bolel, P.; Datta, S.; Mahapatra, N.; Halder, M. Spectroscopic investigation of the effect of salt
ACS Paragon Plus Environment
Page 26 of 33
Page 27 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
827
Hückel limiting law and observation of enthalpy-entropy compensation. J. Phys. Chem. B 2012, 116,
828
10195-10204.
829
48.
LifeSpec-II User guide, rev. 2, copyright © 2012 Edinburgh Instruments Ltd. UK
830
49.
Maity, B.; Chatterjee, A.; Ahmed, S. A.; Seth, D. Interaction of the nonsteroidal anti-
831
inflammatory drug indomethacin with micelles and its release. J. Phys. Chem. B 2015, 119, 3776-
832
3785.
833
50.
834
spectroscopy (TRANES): a novel method for confirming emission from two excited states. J. Phys.
835
Chem. A 2001, 105, 1767-1771.
836
51.
837
conformational stability of a protein. Protein Eng. 2003, 16, 5-9.
838
52.
839
Comput. Chem. 2011, 32, 174-182.
840
53.
841
A. J. Automated docking using a Lamarckian genetic algorithm and an empirical binding free energy
842
function. J. Comput. Chem. 1998, 19, 1639-1662.
843
54.
844
serum proteins toward individual response to specific ligands: HSA-pocket resembles flexible
845
biological slide-wrench unlike BSA. J. Phys. Chem. B 2014, 118, 6071-6085.
846
55.
847
internal aqueous hydrogen-ion concentration in phospholipid-vesicles. Biochemistry 1981, 20, 1534-
848
1538.
849
56.
850
physical-chemistry experiment that explores acid-base properties in the excited-state. J. Chem. Educ.
851
1992, 69, 247-249.
852
57.
853
of human serum albumin on the modulation of pKa of warfarin and structurally similar acidic guests: a
854
possible implication on biological activity. J. Photochem. Photobiol., B 2014, 130, 76-85.
855
58.
856
recombination processes. Chem. Phys. Lett. 1992, 192, 77-81.
857
59.
858
proton transfer of pyranine in a supramolecular assembly: PEO-PPO-PEO triblock copolymer and
859
CTAC. J. Phys. Chem. B 2007, 111, 13504-13510.
860
60.
861
alkanolamine forms of sanguinarine to lysozyme: spectroscopic analysis, thermodynamics, and
862
molecular modelling Studies. J. Phys. Chem. B 2014, 118, 13077-13091.
Koti, A. S. R.; Krishna, M. M. G.; Periasamy, N. Time-resolved area-normalized emission
Takano, K.; Yamagata, Y.; Yutani, K. Buried water molecules contribute to the
Allouche, A. R. Gabedit- A graphical user interface for computational chemistry softwares. J.
Morris, G. M.; Goodsell, D. S.; Halliday, R. S.; Huey, R.; Hart, W. E.; Belew, R. K.; Olson,
Datta, S.; Halder, M. Detailed scrutiny of the anion receptor pocket in subdomain IIA of
Clement, N. R.; Gould, J. M. Pyranine (8-Hydroxy-1,3,6-Pyrenetrisulfonate) as a probe of
Marciniak, B.; Kozubek, H.; Paszyc, S. Estimation of pKa* in the first excited singlet-state - a
Datta, S.; Halder, M. Effect of encapsulation in the anion receptor pocket of sub-domain IIA
Goldberg, S. Y.; Pines, E.; Huppert, D. Proton scavenging in photoacid geminate
Ghosh, S.; Dey, S.; Mandal, U.; Adhikari, A.; Mondal, S. K.; Bhattacharyya, K. Ultrafast
Jash, C.; Payghan, P. V.; Ghoshal, N.; Kumar, G. S. Binding of the iminium and
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
863
61.
Vocadlo, D. J.; Davies, G. J.; Laine, R.; Withers, S. G. Catalysis by hen egg-white lysozyme
864
proceeds via a covalent intermediate. Nature 2001, 412, 835-838.
865
62.
866
human lysozyme revealed by affinity labeling. Biochemistry 1996, 35, 13562-13567.
867
63.
868
directed mutagenesis of the catalytic residues Asp-52 and Glu-35 of chicken egg-white lysozyme.
869
Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 133-137.
870
64.
871
Proc. Natl. Acad. Sci. USA 1971, 68, 517-522.
872
65.
873
inhibitor and lysozyme calculated from molecular dynamics simulations. J. Phys. Chem. 1993, 97,
874
2009-2014.
875
66.
876
lysozyme: importance of ligand-induced perturbation of the secondary structure on the mode of
877
binding of exogenous ligand and possible consequences. J. Photochem. Photobiol., B 2016, 161, 253-
878
265.
879
67.
880
Chem. Res. 2003, 36, 95-101.
881
68.
882
environments studied by photon echo spectroscopy. J. Phys. Chem. B 1999, 103, 7995-8005.
883
69.
884
macromolecular hydration. J. Phys. Chem. B 2002, 106, 12376-12395.
885
70.
886
Methods Enzymol. 2014, 541, 85-94.
Muraki, M.; Harata, K.; Sugita, N.; Sato, K. Origin of carbohydrate recognition specificity of
Malcolm, B. A.; Rosenberg, S.; Corey, M. J.; Allen, J. S.; Debaetselier, A.; Kirsch, J. F. Site-
Halper, J. P.; Latovitzki, N.; Bernstein, H.; Beychok, S. Optical activity of human lysozyme.
Smith, P. E.; Brunne, R. M.; Mark, A. E.; Vangunsteren, W. F. Dielectric properties of trypsin
Panja, S.; Halder, M. Exploration of electrostatic interaction in the hydrophobic pocket of
Bhattacharyya, K. Solvation dynamics and proton transfer in supramolecular assemblies. Acc.
Jordanides, X. J.; Lang, M. J.; Song, X. Y.; Fleming, G. R. Solvation dynamics in protein
Pal, S. K.; Peon, J.; Bagchi, B.; Zewail, A. H. Biological water: femtosecond dynamics of
Duong-Ly, K. C.; Gabelli, S. B. Salting out of proteins using ammonium sulfate precipitation.
887 888 889 890 891 892 893 894 895 896 897 898
ACS Paragon Plus Environment
Page 28 of 33
Page 29 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
899
The Journal of Physical Chemistry
Table 1. IROH* /IRO-* of HPTS at different conditions in aqueous phosphate buffer of pH 7
900 System In aqueous buffer only With 2.4×10-4 M LYZ only With 2.4×10-4 M LYZ + 0.02 M NaCl With 2.4×10-4 M LYZ + 0.05 M NaCl With 2.4×10-4 M LYZ + 0.08 M NaCl With 2.4×10-4 M LYZ + 0.10 M NaCl With 2.4×10-4 M LYZ + 0.15 M NaCl With 0.02 M NaCl only
IROH* /IRO−* 0.033 0.102 0.064 0.052 0.044 0.041 0.038 0.031
901 902 903 904 905 906 907
Table 2. Binding constant of HPTS-LYZ complex in aqueous buffer of pH 7 and at different
908
NaCl concentration
[NaCl] (M) 0
Binding constant, KBH (M-1) (From Benesi-Hildebrand plot) (1.4±0.4)×104
Binding constant, Kan (M-1) (From Steady-state anisotropy data) (1.5±0.07)×104
0.05
(7.6±0.6)×103
--
0.1
(6.7±0.5)×103
--
0.2
3
(6.2±0.6)×10
(7.6±0.05)×103
909 910 911 912 913 914
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 30 of 33
915
Table 3. Picosecond decay parameters# of ROH* emission (at 435 nm) and RO-* emission (at
916
520 nm) in different systems, at pH 7. λexc=375 nm
917
Observation System
τ1 (ns)
τ2 (ns)
τ2 (ns)
α1
α2
α3
437
0.100
0.550
3.25
0.92
0.073
0.007
520
0.100
5.46
-
(-)0.52
0.48
-
437
0.139
0.736
3.64
0.82
0.12
0.06
520
0.139
0.780
5.77
(-)0.13
(-)0.09
0.78
437
0.134
0.840
3.63
0.86
0.093
0.047
520
0.134
0.737
5.66
(-)0.13
(-)0.10
0.77
437
0.110
0.820
3.56
0.89
0.078
0.032
520
0.110
0.770
5.57
(-)0.20
(-)0.06
0.74
437
0.103
0.785
3.54
0.91
0.064
0.026
520
0.103
0.744
5.51
(-)0.44
(-)0.02
0.54
437
0.099
0.763
3.51
0.90
0.063
0.017
520
0.099
0.760
5.48
(-)0.52
(-)0.02
0.46
wavelength (nm)
In aqueous buffer only With 1.8×10-4 M LYZ only With 1.8×10-4 M LYZ + 0.02 M NaCl
With 1.8×10-4 M LYZ + 0.05 M NaCl With 1.8×10-4 M LYZ + 0.1 M NaCl With 1.8×10-4 M LYZ + 0.2 M NaCl
918
#
th
Error in measuring/fitting lifetime parameters is ~4%. αi is the normalized weightage of i component.
919 920 921 922 923 924 925 926 927 928
ACS Paragon Plus Environment
Page 31 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
929
Table 4. Relative amplitudes of the two rise-time components# at RO-* emission (at 520 nm)
930
in the presence of LYZ at different NaCl concentration
931 932 933 934 935
System
τ1 (ns)
τ2 (ns)
α1
α2
936
With 1.8×10-4 M LYZ + 0 M NaCl
0.139
0.780
0.58
0.41
With 1.8×10-4 M LYZ + 0.05 M NaCl
0.110
0.770
0.77
0.23
With 1.8×10-4 M LYZ + 0.1 M NaCl
0.103
0.744
0.96
0.04
With 1.8×10-4 M LYZ + 0.2 M NaCl
0.099
0.760
0.95
0.05
937 938 939 940 941
#
Error in measuring/fitting lifetime parameters is ~4%.
942 943
Table 5. Anisotropy decay parameters of HPTS collected at 520 nm in different systems.
944
λexc= 375 nm
945 946
τ1rot (ns)
(α1rot)
τ2rot (ns)
In aqueous buffer only
0.154
(1.00)
--
With 1.8×10-4 M LYZ only
0.199
(0.13)
With 1.8×10-4 M LYZ + 0.02 M NaCl
0.205
With 1.8×10 M LYZ + 0.05 M NaCl
System
(ns)
--
0.154
2.17
(0.87)
1.90
(0.66)
1.60
(0.33)
0.663
0.188
(0.75)
1.79
(0.25)
0.580
With 1.8×10-4 M LYZ + 0.1 M NaCl
0.290
(1.00)
--
--
0.290
With 1.8×10-4 M LYZ + 0.2 M NaCl
0.191
(1.00)
--
--
0.191
-4
947
(α2rot)
αirot
th
is the normalized weightage of i component.
948 949
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
950
Table 6. Change in solvent accessible surface area (∆SASA) in Å2 of the concerned residues
951
(residues with % side chain solvent accessibility >10) present around the docked HPTS in
952
LYZ
953 954
Residues
∆SASA (Å)
Trp 34
32.4
Glu 35
39.1
Asn 44
21.23
Asp 53
24.13
Gln 58
13.22
Trp 109
9.05
Val 110
23.99
Ala 111
22.01
960
Asn 114
20.45
961
Arg 115
36.58
955 956 957 958 959
962 963 964 965 966 967 968 969 970 971 972 973 974 975
ACS Paragon Plus Environment
Page 32 of 33
Page 33 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
976
TOC Graphic:
977
Detailed scenario of the acid-base behavior of HPTS in the presence of human lysozyme
978 979 980 981 982 983 984
ACS Paragon Plus Environment