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Salt Promotes Protonation of Amine Groups at Air/Water Interface Woongmo Sung, Zaure Avazbaeva, and Doseok Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01198 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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The Journal of Physical Chemistry Letters

Salt Promotes Protonation of Amine Groups at Air/water Interface Woongmo Sung1, #, Zaure Avazbaeva1, and Doseok Kim*, 1 1

#

Department of Physics, Sogang University, Seoul 04107, Korea.

Present address: Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.

Corresponding Author * E-Mail address (corresponding author): [email protected] ABSTRACT. Interfacial water reorientation caused by charged Langmuir monolayers consisting of primary fatty amine (ODA) and cationic lipid having quaternary amine headgroup (DPTAP) were investigated by interface-selective vibrational sum-frequency generation spectroscopy. For DPTAP monolayer, initially large sum-frequency intensity from interfacial water OH band decreased steadily by increasing monovalent salt (NaCl, NaI) concentration due to counterion adsorption. On the other hand, ODA/water exhibited significantly smaller sum-frequency intensity than DPTAP/water implying only small portion of protonated amine group (-NH3+) initially existed. By increasing the ionic strength, however, SF intensity of water OH band was enhanced markedly up to ~1 mM, and then decreased in both NaCl and NaI solutions. By measuring the phase of the sum-frequency spectra, it was found that water dipoles under the ODA headgroup point downward indicating the surfaces were always positively charged. This

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demonstrated that increasing ionic strength facilitates protonation of primary amine headgroups. Simple model based on Poisson-Boltzmann (PB) theory explained this protonation behavior of primary amines.

TOC GRAPHICS

KEYWORDS Vibrational sum-frequency generation spectroscopy, Interfaces, Amine, Protonation, Water, Ionic strength. Amines are ubiquitous and play crucial roles in many biological systems.1-8 Peptide bond formed between the amino group and the carboxylic group of amino acids is essential in building protein structure.1 Negatively-charged DNA actively interacts with amine residues in proteins making condensed structures, the most well-known structure being chromosome in which DNA wraps around histone protein having lysine residues at the surface.2 In epigenetics, modification (methylation or acetylation) of these primary amine groups alters the interaction between the DNA and the histone protein impeding or promoting genetic expressions.5-8


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Amine groups are also abundant in cell membrane. For example, one of the most popular phospholipid DPPC has a choline group in which all of its protons are substituted by methyl and methylene groups to keep its positive charge (+e) intact under modulation of pH.9 Another lipid having choline headgroup (sphingomyelin) together with cholesterol are essential components of lipid raft, an ordered structure that plays critical functions in the cell membrane.10-15 Consisting of amphiphilic molecules, lipid bilayer in the cell membrane exposes hydrophilic headgroups to aqueous phase. This two-dimensional, crowded environment is where the headgroups interact with water molecules, ions, and other biological molecules in cytoplasm. To investigate structures dictated by these interactions at the interface, surface-sensitive nonlinear optical techniques such as vibrational sum-frequency generation (VSFG) and second harmonic generation (SHG) have been used in Langmuir monolayer systems mimicking biomembranes.16-23 Previous SHG study on primary amine monolayer found the shift of effective pKa from 10.5 (bulk) to 9.9 suggesting increase in surface acidity due to cationic molecules at air/water interface.24 In our previous VSFG study on n-octadecylamine (ODA) Langmuir monolayer, interfacial water OH band was very small for neutral pH at the subphase, and increased when pH is lowered below 4 concurrently destabilizing the monolayer.25 Sumfrequency (SF) signal of the OH band from the ODA monolayer system was investigated and found similar among 10-mM NaCl, NaBr, and NaI solutions.26 In contrast to fatty amine, monolayers consisting of molecules having quaternary amine group strongly reorient interfacial water molecules27 and always attract counterions, while showing clear preference for larger counterions.28-29 This difference in surface propensity of ions depending on their relative size and polarizability are also confirmed from the investigation on quaternary amine solution surface using phase-sensitive VSFG.30 Although numerous reports on primary and quaternary amine


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monolayers demonstrate differences in ion-monolayer interactions, no systematic comparison has been performed yet. In this study we compared the SF responses from reoriented interfacial water molecules under two different Langmuir monolayers consisting of primary and quaternary amine moieties exposed to aqueous subphases with varying ionic strengths.

Figure 1. Molecular structure of (a) 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) and (b) noctadecylamine (ODA) The regions colored by cyan, gray, and red indicate hydrophobic alkyl chain tail, charge neutral- and cationic headgroup, respectively.

As shown in Fig. 1(a), DPTAP (1,2-dipalmitoyl-3-trimethyl-ammonium propane chloride salt) has positively charged choline headgroup (-N(CH3)3+), of which the charge does not change with pH. In the fully packed Langmuir monolayer, its surface density is ~ 1/40 Å2. According to previous studies, water dipoles exhibit polar ordering under condensed DPTAP monolayer27, 29-30 due to strong static electric field normal to the interface. On the other hand, primary amine headgroup in the ODA (n-octadecylamine) molecule (Fig. 1(b)) alters its state from charge neutral (-NH2) to positive charge (-NH3+) depending on pH. In bulk water, pKa of primary fatty amine is ~ 10.5, and most of amine headgroups are expected to be protonated in neutral pH. When they form neat Langmuir monolayer, however, we found increase in the SF signal from


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the OH band only at acidic conditions (pH < 5)25 indicating clear depletion of protons at the interface as compared to bulk.

Figure 2. (a) SF intensity spectra from DPTAP monolayer/water interfaces (Purple: NaI solutions, Green: NaCl solutions). SF spectrum from air/water interface (blue) is plotted together for comparison. (b) SF intensity of water OH stretch band (3000 – 3550 cm-1) integrated from the above spectra. Integrated OH SF intensities from DPTAP/ water interface and from neat water surface are indicated as red and blue dashed lines, respectively.


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Figure 2 (a) shows SF spectra of DPTAP monolayers (surface area ~ 40 Å2/molecule) on NaCl (green) and NaI (violet) solutions. SF signal from 2800 to 3000 cm-1 corresponds to stretch modes of methyl and methylene moieties in alkyl chains of the DPTAP molecules.31 Two sharp peaks located at 2875 cm-1 and 2934 cm-1 from terminal methyl groups are pronounced while the contributions from methylene (~2850 and 2920 cm-1) vibrations are comparably small appearing as small bumps throughout different ionic strengths of NaCl and NaI, indicating the formation of well-ordered DPTAP molecules in condensed phase.32-33 In comparison to the air/neat water interface (blue squares), interfacial water dipoles under the DPTAP monolayer are highly ordered by positively charged quaternary amine headgroups leading to the markedly enhanced SF intensity of water OH stretch band (3000 – 3600 cm-1). Upon dissolution of monovalent sodium halides, this OH band decreased by screening effect of the counterions (Cl- and I-). In the NaCl solutions, the SF intensity of the OH band is attenuated noticeably for ionic strength larger than 10 mM whereas NaI is shown to reduce the SF intensity by one-third even at 10 µM, and the SF intensity is further reduced such that at 100 mM it is similar to that from air/water interface. Figure 2(b) shows the SF intensity integrated over the OH stretch band (3000 – 3550 cm-1) in Fig. 2(a). It is found that I- counterions screens the positively charged quaternary amine layer more than a hundred times efficiently than Cl-. In the previous study with grazing-angle X-ray, we found that the depth profile of I- anions is very localized and close to the headgroup. This ion specificity in the halide anion distribution matches well with the result from theoretical prediction

34-35

and MD simulation

36-37

on air/salt solution

or oil/salt solution interfaces where size and polarizability of ions play a crucial role.


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Figure 3. (a) SF intensity spectra of ODA monolayer/water interfaces (Purple: NaI solutions, Green: NaCl solutions). (b) SF intensity of water OH stretch band (3000 – 3550 cm-1) integrated from the above spectra. Integrated OH SF intensities from DPTAP/water and from ODA/water interfaces are indicated as red and black dashed lines, respectively.

In contrast to the DPTAP/water interface, we observed that the SF signal from the OH band from ODA monolayer changes quite differently with increasing ionic strength. Figure 3(a) shows SF spectra from ODA monolayer/salt solution (molecular area ~20 Å2) represented in the same


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manner as in Fig. 2. Although the surface density of the primary amine headgroup is doubled with respect to that of the quaternary amine in the DPTAP monolayer the SF intensity of the OH band is very small for ODA on pure water, not much increased from that of air/water interface. This result clearly indicates that the surface density of protonated amine group (-NH3+) is much smaller than that of the quaternary amine groups in the DPTAP monolayer. Upon dissolution of NaCl and NaI salts, surprisingly, SF intensity of the water OH band gets increased rapidly in moderate ionic strength (10 µM – 1 mM), peaked at 1 mM, and then decreased at higher salt concentrations. This change is unexpected and counter-intuitive as added counterions in solution always increase ionic strength to diminish the electric field from charged interface. It is recently proposed that SF and SH responses from electric double layer can be affected by interference effect over a probing depth even for constant surface charge density.38 The proposed ~ 30 % increases around 1 mM salt concentration cannot account for observed 5 ~ 7 fold increases in our case (Fig. 3(b)). So added salt must have resulted in the increase of surface charge density σ for the primary amine monolayer case. From pressure-area isotherm experiment, we also found increase in surface pressure for ODA monolayer by added salt whereas DPTAP monolayer behaved oppositely (see Fig. S3 in supporting info). According to the previous X-ray florescence study on ODA monolayer/CsI solution interface, it was found that I- counterions are enriched at the interface whereas Cs+ coions do not exist.39 From this one would postulate that the excess of I- counter anions at the interface brings increased σ (from charge neutral to more negatively charged surface). Second proposition is the enhanced surface proton concentration (leading to more protonation of amine headgroups) by added salts. For fatty acid monolayer, protons are more accessible to the interface due to


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negative surface potential, and it leads to “apparent pKa” (measured from bulk pH) shift of fatty acid molecules in monolayer from 5.1 of its bulk value to ~ 10.40 Likewise, in condensed fatty amine layer, amine groups “sense” less protons at the interface than bulk due to repulsive force between positively charged surface and proton leading apparent downward shift of pKa ~ 5.25 By increasing ionic strength, however, counterion screening effect kicks in, and it may help protons more accessible to the interface by reducing the repulsive force from the layer of headgroups (calculation shown in Fig. S1). We want to emphasize that this counterion screening results in “less depletion” of protons at the interface, not an excess of protons as compared to the bulk.

Figure 4. (a) Phase-sensitive SF spectra of DPTAP/water (red) and DPTAP/10 mM NaCl (green) interfaces. Negatively charged arachidic acid monolayer/water interface at pH 12 (blue) is plotted together for comparison


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(positive water Imχ(2) band for upward orientation of interfacial water dipoles41). (b) Phase-sensitive SF spectra of ODA/water (red), ODA/10 mM NaCl (green), and ODA/water at pH 2 (violet).

By measuring the phase-sensitive VSFG spectra, we confirmed that the second proposition is valid. Figure 4(a) shows Imχ(2) spectra from DPTAP monolayer/water interfaces. The spectrum from the arachidic acid (CH3(CH2)19COOH) monolayer on high pH water (in which carboxyl headgroups are mostly deprotonated (-COO-)) is shown for comparison. The strong negative water OH band in DPTAP indicates downward orientation of water dipoles underneath.31,

41

Upon addition of NaCl, the water OH band remains negative, and is shown to decrease appreciably because counterion adsorption shields the electric field and disturbs the polar orientation of water dipoles. On the other hand, the Imχ(2) spectrum of ODA/water is small and negative, and becomes more negative at 10 mM NaCl. This negative OH band at 10 mM resembles the spectral profile of the ODA monolayer at pH 2 consisting mostly of protonated amine headgroups (-NH3+).25 It demonstrates that increasing ionic strength of the solution facilitates protonation of the amine groups in ODA monolayer, which increases the electric field to orient the water molecules underneath. The SF signal of water OH band is maximized at 1 mM ionic strength due to abrupt increase in protonated amines as seen from the steep increase of ionization fraction (red arrow in Fig. 5), and further increase of the ionic strength increases the adsorption of counterions to screen the electric field. To semi-quantitatively explain the ionic strength dependence of the amine protonation, the model based on acid-base equilibrium at interface and Poisson-Boltzmann (PB) theory was used. At fatty amine/water interface, the equation for acid-base is given as,


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+

[NH 3 ]+[H 2 O]

[NH 2 ]+[H 3O + ]z =0 , eψ 0

Ka =

− [NH 2 ][H 3O+ ]z =0 , [H 3O + ]z =0 = CH + e kBT , + [NH 3 ]

(1)

where Ka, [NH2], [NH3+], [H3O+]z=0, CH+, and ψ0 are reaction constant, deprotonated amine concentration, protonated amine concentration, interfacial proton concentration, bulk proton concentration, and surface electric potential, respectively. By rearranging terms, eq. 1 becomes,

pH b − pK a = log10

(1 − x ) eψ 0 − , 2.3k BT x

(2)

where pHb = - log10 CH+ and x are bulk pH and ionization fraction (x=[NH3+]/([NH3+]+[NH2])), respectively. In case of monovalent salt in water, ψ0 is determined by Grahame equation of charge neutrality condition 42,

σ = 8C0ε 0ε r k BT ⋅ sinh(

eψ 0 e )=x , 2 k BT A

(3)

where σ, C0, and A are surface charge density, total bulk ionic strength (including salt ions and protons), and surface area of ionizable moieties. By substituting eq. 3 into eq. 2, relation between total salt concentration and ionization fraction can be obtained as below 40,

Ctot = Csalt + CH + = Csalt + 10− pHb =(

134 x ), (1 − x) A sinh((log10 − pH b + pK a ) / 0.87) x

(4)

where Ctot, Csalt, CH+, pHb, A, x, and pKa are total ionic strength, ionic strength of monovalent salt and proton (in M), bulk pH, surface area of ionizable site (in Å2), ionization fraction, and pKa of the ionizable moiety. In this calculation, we chose dielectric constant εr and pKa of primary amine group the same as bulk values (εr ~ 80 and pKa ~ 10.5) for simplicity. However,


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experimental and simulation studies suggest significantly reduced dielectric constant for interfacial water.43-47 There also have been suggestions that pKa of ionizable moiety changes sensitively following the environment and the surrounding medium.47,48 To check for these possibilities, we calculated x in different dielectric constants (30 – 80) and pKa values (10.5 ± 1.0), and found that reducing εr by 30 yields x = 0.49 (Fig. S2(a), pKa = 10.5) and reducing pKa by 9.5 yields x = 0.34 (Fig. S2(b), εr = 80) at 1 mM salt concentration (x = 0.6 for εr = 80 and pKa = 10.5). These ionization (protonation) fraction of ODA monolayer is enough to reorient water molecules underneath to account for the observed spectra in Fig. 3.

Figure 5. Calculated ionization (-NH3+) fraction of the ODA monolayer at surface area of 20 Å2/molecule versus added monovalent salt concentration. In order to estimate surface titration behavior of the ODA monolayer, calculation is performed at different bulk pHb ranging from 3.5 to 8.0. The numbers above the black arrows indicate ionization fraction in the absence of monovalent salts. The slope of ionization fraction gets maximized around 1 mM ionic strength at pHb 5.7 as indicated by the red arrow.

Figure 5 shows ionization fraction versus added monovalent salt concentration. The values indicated above the arrows are ionization fraction when Csalt equals to zero, corresponding to surface titration of the primary amine group at zero-salt conditions. It is noteworthy that


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ionization fraction increase drastically between pHb 5.7 and 3.5 in the absence of monovalent salt. This result is consistent with our previous report which showed increase of OH signal below pHb ~ 4 and significant dissolution of ODA molecules into bulk phase

25

as well as the previous

pressure-area isotherm experiment.49 At pHb 5.7 (green line), ionization fraction of primary amine group steeply rises from x~0.18 (at 10-5 M) to x~0.89 (at 10-2 M), and this trend looks qualitatively well-matched with the increased OH SF intensities shown in Fig. 3(b). Since the slope of the ionization fraction curve at pHb 5.7 becomes more flat after 10 mM, there would be only small increases in protonated amine density whereas adsorption of counterion (Cl- and I-) and reduction of electric double layer depth become more crucial giving rise to subsequent decrease of the OH signal in Fig. 3.

To explain the phenomenon qualitatively, charged ODA headgroups initially repel protons to make the interfacial region deplete of protons at equilibrium condition. When monovalent salt is added, counterions start to screen charged amine headgroups allowing more protons access to interfacial region. For example, without added salt, calculated proton concentration at the interfacial region (< 1nm) is 10-11-10-12 M at pH 7 (Fig. S1(a) in supporting information). On the other hand, proton concentration at the interfacial region shows large increase (10-8-10-9 M) upon addition of 10 mM monovalent salt (Fig. S1(b) in supporting information).

Contrary to the behavior shown in Fig. 2, the ODA/water interface in Fig. 3 did not show appreciable difference between I- and Cl-. As surface propensity of I- over Cl- is firmly grounded both experimentally30,

50-52

and theoretically,34 we anticipate that I- is more inclined to the

interface as compared to Cl- for ODA/water case as well. The enrichment of counterion may bring two conflicting effects: 1) screening positive surface charges resulting in disturbance of


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water orientation as in the case of DPTAP/water interface, and 2) increasing surface accessibility of protons resulting in more protonation of amine headgroups. The former will attenuate the OH SF intensity whereas the latter will increase it, and altogether similar trend between I- and Cl- as in Fig. 3 may come out. Further investigations by MD simulations and X-ray spectroscopy are needed to quantitatively assess the surface excess of anions to resolve this issue. In summary, by monitoring the orientations of interfacial water dipoles, we found very distinct behaviors of quaternary and primary amine groups located at the air/water interface. Quaternary amine groups are always positively charged, under which counter anions adsorb to screen the electric field. The adsorption of larger, and more polarizable I- counterion is much more effective as compared to Cl-. On the other hand, primary amine headgroups are initially nearly chargeneutral even at pH much lower than its pKa due to the effect of nearby amine groups in the compact monolayer. Addition of salt in the solution promotes protonation of these amine groups to reorient water molecules giving rise to strong SF, subsequently counterion adsorption screens the electric field. The model based on PB theory and acid-base equilibrium at interface can explain the observed protonation behaviour satisfactorily. Our result suggests that, in physiological condition (pH 7-8, ~100 mM of ionic strength), large number of amine groups are expected to be protonated while conterions screen surface electric field efficiently (as shown in Fig. 5). Our finding would be valuable to correctly asses the charge status of biological membranes having amine or other moieties (e.g. carboxyl), in the practical use of adherent like polylysine to facilitate adhesion of cells and proteins used in cell culture, and to characterize the subtle change in the interaction between macro-biomolecules as in epigenetic modifications.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Description of VSFG setup; sample preparation; calculated counter- and coion depth profile; dielectric constant and pKa dependences in ionization fraction; pressure-area isotherm. (Supporting info.pdf) AUTHOR INFORMATION Corresponding Author *

E-mail:[email protected]

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by Sogang University research grant #201610088, and by the KRF grant No. 2017R1D1A1B03031150.

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