Ground- and Excited-State Interactions of a Psoralen Derivative with

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Ground and Excited State Interactions of a Psoralen Derivative with Human Telomeric G-Quadruplex DNA Sneha Paul, and Anunay Samanta J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b12475 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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The Journal of Physical Chemistry

Ground and Excited State Interactions of a Psoralen Derivative with Human Telomeric GQuadruplex DNA

Sneha Paul and Anunay Samanta* School of Chemistry, University of Hyderabad, Hyderabad-500046, India

ABSTRACT G-Quadruplex DNA has been a recent target for the anti-cancer agents and its binding interactions with small molecules, often used as anti-cancer drugs, have become an important area of research. Considering that psoralens have long been studied in the context of duplex DNA, but very little is known about its potential as a G-Quadruplex binder and its excited state interaction with the latter has not been explored, we have studied herein the binding of a planar water soluble psoralen derivative, 4′-aminomethyl-4,5′,8-trimethyl psoralen (AMT), with the 22mer human telomeric G-quadruplex forming sequence, AGGG(TTAGGG)3, labeled here as (hTel22), and investigate the consequences of photo-excitation of AMT by calorimetric and spectroscopic techniques. The results show an enthalpy driven 1:1 binding of AMT with hTel22 via end-stacking mode. Fluorescence quenching experiments on 6-fluorescein amidite labeled oligomers indicate that the binding site is nearer to the 3՛ end of hTel22 in the diagonal loop region. Femtosecond time-resolved transient absorption measurements indicate electron transfer from the guanine moiety of hTel22 to photoexcited AMT leading to the formation of a radical .

.

pair species (AMT -G +) which survives for 30 ps and is favored by a parallel/quasi-parallel orientation between the two.. The findings reveal psoralens as a prospective class of compounds for the development of anti-cancer therapeutics by targeting G-Quadruplex DNA. _________________________ *Corresponding author, Email: [email protected]

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INTRODUCTION Development of drugs and therapies to combat the deadly disease, ‘cancer’, is an active area of research for quite some time. In recent years, in addition to the canonical duplex DNA, noncanonical DNA motifs are also potential targets for chemotherapeutics. Among the latter, guanine rich (G-rich) oligonucleotides capable of folding into four-stranded structures, called GQuadruplex, in the presence of metal ions, have attracted enormous attention. 1-3 These structures comprise stacks of G-tetrads formed by association of four guanine molecules via cyclic Hoogsteen and Watson-Crick H-bonding.4 The presence of metal ions (typically K+ or Na+) in the central channel of a quadruplex is vital as they neutralize the negative electrostatic effect of the carbonyl groups of guanine and render stability to the assembly. 5,

6

G-Quadruplexes are

structurally diverse and various factors such as ionic strength, pH and molecular crowding of the medium7-11 as well as oligonucleotides sequence/length, and flanking nucleotides 12, 13 influence their conformation, which can broadly be classified into three groups: parallel, anti-parallel and (3+1) hybrid structures.4 The G-Quadruplex structures have been observed in several plant and animal cells. 14-18 In humans, they are present in the oncogene promoter region where they are involved in gene regulation at the transcription19 and translation levels.20 They are also found in the terminal region of human telomere 21 and inhibit the activity of the enzyme, telomerase, which is dormant in normal cells but show enhanced activity in carcinogenic cells leading to uncontrolled cell growth.22, 23 This is why there is an ongoing search for small molecules, which can interact and stabilize these structures and the interaction of several derivatives of acridines, 24, anthraquinones,26 triazenes,27,

28

25

perylenes,29 porphyrins,30 tetraphenylethene31 and various

natural products32-37 have been explored in recent years. 2 ACS Paragon Plus Environment

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Psoralens are furocoumarins which interact with duplex DNA by intercalation and are employed in PUVA therapy for cure of psoriasis and other skin diseases. 38,

39

Their role in

apoptosis and control of tumoral cell growth has also been realized. 40 They are particularly found to be effective against breast cancer and T-cell lymphoma.39, 40 Even though virtual screening approaches have shown the potential of psoralens as G-Quadruplex binders, 41 a comprehensive experimental study probing the interactions between the two is lacking. Most of the binding studies involving G-Quadruplex and small molecules focus on the ground state interactions between the two, an understanding of which is essential for the development of anti-cancer drugs. The literature, however, suggests that oxidation and reduction of nucleobases of DNA (either by direct photo-excitation or through photoinduced processes involving small molecules) plays an important role in DNA damage and/or repair. 42 Majima et al. studied the hole-trapping dynamics in riboflavin labeled G-Quadruplex and concluded that quadruplex structures of DNA can act as traps of the oxidative damage processes in the genome. 43 It is observed that photoinduced oxidation of the guanines in G-Quadruplexes by stress inducers and molecules such as porphyrins can be extremely efficient in inhibiting telomerase activity in tumor cells.44, 45 These studies thus highlight the need of probing the excited state interactions of small molecules serving as anti-cancer drugs and G-Quadruplexes and motivates us to undertake the present work on the psoralen derivative, 4′-aminomethyl-4,5′,8-trimethyl psoralen (AMT) and the 22-mer human telomeric G-quadruplex forming sequence, AGGG(TTAGGG) 3 (hTel22), which is known to adopt anti-parallel structure in Na+ rich conditions (Figure 1).4 The choice of AMT is governed by its water solubility, planarity, amino functionalization (which can enhance its interaction with DNA via H-bonding) and commercial usage as a drug in PUVA therapies. We have probed the ground as well as excited state interactions between the two using 3 ACS Paragon Plus Environment


calorimetric and steady state and time-resolved spectroscopic techniques. The binding constant (Kb) of AMT with hTel22 (~105 M-1) is found to be higher than that reported for G-C duplex DNA (~104 M-1).67 Additionally, a favorable geometry of AMT with respect to the G-Quartet plane is found to facilitate ultrafast electron transfer between them upon photo-excitation of .

.

AMT. This leads to the formation of a radical pair, AMT -G +. These findings open up newer prospects for psoralens, which have long been known to interact with duplex DNA, but now its ground state as well as photoinduced interactions with various G-Quadruplexes can also be explored to design novel anti-cancer drugs.

(A)

(B)

Figure 1: (A) Schematic representation of anti-parallel folding topology of hTel22 in aqueous buffer containing Na+ ions (red circles). Guanines in syn conformation are denoted by light blue squares and dark blue squares represent guanines in anti conformation. (B) Structure of 4′-aminomethyl-4,5′,8-trimethyl psoralen (AMT). EXPERIMENTAL SECTION Materials and sample preparation 4′-aminomethyl-4,5′,8-trimethylpsoralen (AMT)

hydrochloride, NaCl, NaH 2PO4, Na2HPO4

were purchased from Sigma Aldrich and used without purification. The 22-mer unlabeled human telomeric sequence 5՛-AGGGTTAGGGTTAGGGTTAGGG-3՛ and 6-fluorescein amidite 4 ACS Paragon Plus Environment

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(FAM)

labeled

oligomers,

5՛-FAM-AGGGTTAGGGTTAGGGTTAGGG-3՛

and

5՛-

AGGGTTAGGGTTAGGGTTAGGG-FAM-3՛ (HPLC purified) were procured from reGene Biologics, Hyderabad, India and used as received. To prepare the G-Quadruplex DNA, the oligomers (labeled/unlabeled) were suspended in a 10 mM phosphate buffer (pH 7.2) containing 100 mM NaCl and the solutions were annealed by heating at 95 oC for 10 minutes and then allowed to cool to room temperature slowly. The solutions were stored at 4 oC for 24 hours prior to use. The concentrations of the formed G-Quadruplexes were estimated using the molar extinction co-efficient value of 228500 M-1cm-1strand-1 at 260 nm.46 The AMT stock solutions were prepared in the same buffer using Milli-Q water. Methods Steady state measurements Steady state absorption and fluorescence spectra were recorded on UV-vis spectrophotometer (Cary 100, Varian) and spectrofluorimeter (Fluorolog 3, Horiba Jobin Yvon), respectively. The UV-vis titration experiments were carried out for fixed AMT concentration (10 µM) and varying the hTel22 concentrations. For Job’s plot, the total concentration of AMT and hTel22 was fixed at 5 μM and the changes in absorbance were measured by varying mole fraction of each. Circular dichroism studies CD spectra were recorded using Aviv 420SF spectropolarimeter (Aviv Biomedical, Inc., Lakewood, NJ, USA) at 25 oC in the wavelength range of 220-320 nm at 1 nm interval with 1 nm bandwidth in a 1 cm pathlength cuvette having 2 mL sample volume. Each spectrum was an average of three scans collected at a speed of 50 nm/min and baseline corrected using blank 5 ACS Paragon Plus Environment


buffer. During the titration experiments, the spectra were recorded 5 minutes after each addition of AMT to a fixed amount of hTel22 for equilibration of the sample. For the CD melting experiments, 3 μM hTel22 was mixed with 20 μM AMT in aqueous buffer and the CD signal at 290 nm was monitored in the temperature range of 10-110 oC at intervals of 5 oC at a heating rate of 0.5 oC/min. Isothermal titration calorimetry ITC experiments were performed in a MicroCal VP-ITC instrument (MicroCal LLC, Northampton, MA, USA) at four temperatures 15, 20, 25 and 30 oC. All samples and buffer solutions were degassed prior to use. The titration was conducted by placing 20 μM hTel22 in a sample cell (1.44 mL) and injecting 1 mM AMT solution from the syringe (286.5 μL) in 30 aliquots with the first injection volume being 5 μL and the rest were 7 μL each with a resting time of 300 s between each injection. The reference cell was filled with the same buffer which was used for sample preparation. Blank experiments were also conducted at all temperatures by injecting same concentration of AMT into a buffer solution with identical titration parameters for correction of dilution effects. The corrected isotherms were then analyzed using single site binding model and non-linear least squares fitting algorithm available in the built-in the Microcal LLC ITC software to obtain the thermodynamic parameters of the binding event. Time-resolved emission and transient absorption studies Time-resolved fluorescence decay was recorded on a time–correlated single photon counting spectrometer (5000, Horiba Jobin Yvon IBH). Nano LED (330 nm, 1 MHz repetition rate, and 980 ps pulse width) and MCP photomultiplier (Hamamatsu R3809U-50) were used as an excitation source and detector, respectively. 6 ACS Paragon Plus Environment

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The transient absorption measurements were carried out on a femtosecond pump-probe setup which consisted of a mode-locked Ti:sapphire oscillator (Mai Tai, Spectra Physics) producing femtosecond pulses (fwhm < 100 fs, ~2.5 W at 80 MHz). These pulses centered at 800 nm were directed to a regenerative amplifier (Spitfire Ace, Spectra Physics), which was pumped by a frequency-doubled Nd:YLF laser (Empower, Spectra Physics) to produce amplified pulses. The major part of the output from the amplifier was passed through an optical parametric amplifier (TOPAS-Prime, Spectra Physics) to generate pump wavelength of 350 nm for excitation of the sample. The remaining amplified beam was directed through CaF 2 crystal for generation of white light used as the probe beam which was passed through an optical delay line (~4 ns). The instrumental resolution was ~80-90 fs. Additional details of the setup along with analysis of the data are reported elsewhere.47 RESULTS AND DISCUSSION Structural characterization: circular dichroism (CD) studies CD is an established tool for predicting the topology of G-Quadruplexes and investigating the influence of ligand binding on their structures.48, 49 The different polymorphs of G-Quadruplexes (parallel, anti-parallel and 3+1 hybrid) vary in the glycosdic bond angles of the guanine residues in the G-tetrads and the sugar phosphate backbone leading to alterations in their base-stacking geometries giving rise to distinct CD signals for each of them. 50 Figure 2A shows the CD spectra of hTel22 in aqueous buffer rich in Na+ ions in the presence of different quantities of AMT. The observed spectra with positive signals at 290 and 245 nm and a negative signal around 260 nm are indicative of an anti-parallel conformation. 49 It is evident that the addition of AMT does not perturb the structure of hTel22 much and the anti7 ACS Paragon Plus Environment


parallel conformation is maintained. The enhancement of CD signal at 290 and 245 nm with increasing amounts of ligand indicate stacking interactions between AMT and the outer G-tetrad of hTel22 leading to stabilization of the anti-parallel structure (as achiral molecule AMT does not exhibit any CD signal and cannot contribute to the observed enhancement of the signal). The CD melting studies (Figure 2B) which show an increase in melting temperature (T m) of hTel22 by 6 oC from 58 oC to 64 oC on addition of AMT, support this explanation.

Figure 2: (A) CD spectra of hTel22 (5 μM) in aqueous buffer (pH=7.2) containing 100 mM NaCl in the absence (green) and increasing concentrations of AMT (red to pink). The CD spectrum of only AMT (300 μM) is shown in black. (B) CD melting curves of hTel22 (3 μM) alone and in presence of AMT (20 μM). The solid lines represent sigmoidal fits to the data. UV-vis absorption studies Figure 3A shows the steady state absorption spectrum of AMT and hTel22 in aqueous buffered solution containing 100 mM NaCl. The ground state interaction between AMT and hTel22 is evaluated by performing UV-Vis titration experiments with different amounts of hTel22 keeping the concentration of AMT constant. A hypochromicity of about 20% is observed at 330 nm and an isosbestic point around 355 nm (Figure 3B) is also seen which suggests that a two-state transition processes may possibly be involved in the interaction. 51 A separate titration experiment 8 ACS Paragon Plus Environment

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was performed by monitoring the absorbance of a set of samples with varying concentrations of AMT and hTel22 but keeping the total number of moles constant in each set from which Job’s plot was constructed, analysis of which yielded a 1:1 binding stoichiometry. This also suggests a single site for binding of AMT in hTel22. (A)

Figure 3: (A) UV-vis absorption spectra of AMT (solid line, 10 μM) and hTel22 (squares, 100 μM) in aqueous buffer (pH=7.2) containing 100 mM NaCl. (B) Change in the absorption profile of AMT (10 μM) with increasing concentrations of hTel22 (0-100 μM). (C) Job’s plot for binding of AMT to hTel22 by monitoring absorbance changes at 340 nm.

χAMT is the mole fraction of AMT.



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Isothermal titration calorimetric (ITC) studies ITC is an indispensable technique to understand the energetics of binding of small molecules to biomacromolecules. It directly measures the heat changes that occur during the binding event and provides valuable information about the binding affinity, stoichiometry, binding sites and nature of forces involved in binding. 52 The raw ITC profile for interaction of AMT with hTel22 at 288 K is shown in Figure 4A where the individual peaks correspond to each injection of AMT. The heat liberated on successive addition of AMT was obtained by integrating the area under the peaks and these values were then corrected by subtracting the heat of dilution obtained by titrating identical amounts of AMT into buffer alone. Figure 4B shows the plot of corrected heats of injection versus the molar ratio of AMT to hTel22. In order to obtain a clearer picture of the molecular forces driving these binding interactions, similar titrations and analysis were performed at three other temperatures (293 K, 298 K and 303 K), the ITC profiles of which have been provided in the supporting information (Figure S1). The thermodynamic parameters, binding constants and stoichiometries obtained from fitting the experimental data have been summarized in Table 1, for different temperatures.

(A)

(B)


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Figure 4: (A) Raw ITC titration profile of 0.02 mM hTel22 titrated against 1 mM AMT in aqueous buffer (pH=7.2) containing 100 mM NaCl at 288 K. (B) Binding isotherm obtained by integration of peak area and correction of heats of dilution as a function of molar ratio. The solid line is the fit to the experimental data. Table 1: Thermodynamic parameters for the binding of AMT to hTel22 at different temperatures as obtained from ITC experiments

T(K)

n

Kb x 105 (M-1)

ΔH (kcalmol-1)

TΔS (kcalmol-1)

ΔG= ΔH-TΔS (kcalmol-1)

288

1.11±0.02

1.04±0.32

-8.61±1.12

-2.27

-6.34

293

1.21±0.04

0.98±0.12

-9.28±1.05

-3.16

-6.12

298

1.06±0.06

0.92±0.08

-10.11±0.08

-3.91

-6.20

303

1.14±0.03

0.83±0.02

-11.05±0.06

-4.67

-6.38

The experimental data fits well to the “one set of sites” model of binding and a stoichiometry of 1:1 is obtained at all temperatures indicating that only one binding event occurs predominantly. This finding is in accordance with our results from Job’s plot analysis. At 288 K, a Kb value of the order of 105 suggests moderate binding between AMT and hTel22. This value decreases slightly with increasing temperatures, which is common for other small aromatic molecules which bind to G-Quadruplex DNA via stacking interactions. 53 The thermodynamic parameters reveal that the binding of AMT and hTel22 is characterized by negative ΔH and TΔS values and the reaction being enthalpically driven in the temperature range investigated here. The reaction becomes more exothermic with increasing temperature but the ΔG values are not affected much due to opposing enthalpic and entropic effects. A plot of ΔH versus TΔS gives an idea about the extent of the entropy-enthalpy compensation, where a slope of unity indicates complete compensation of entropy by enthalpy or vice-versa. Figure 5A shows 11 ACS Paragon Plus Environment


such a ΔH versus TΔS plot for AMT-hTel22 interaction yielding a slope of 1.2 confirming that the reaction is enthalpically favored.54-56 The negative value of ΔH suggests H-bonding and van-der Waals interactions between AMT and hTel22 as these are known to contribute majorly to the enthalpy of a system. 57 The unfavorable (negative) TΔS values arise due to loss in translational and rotational degrees of freedom upon binding to hTel22. According to literature, small molecules with a less extended aromatic surface are likely to interact with G-Quadruplex DNA via the end-stacking mode in enthalpically driven binding events.58, 59 Further, it is also known that groove binding of ligands leads to expulsion of groove bound water resulting in an increase of the entropy of the system and the binding being entropically driven.60,

61

However, in our case, an enthalpically driven

process with negative entropy terms eliminates the possibility of groove binding and substantiates the end-stacking mode of binding. It is also possible to obtain information on the mode of binding from the heat capacity changes (ΔCp) of a system, which is estimated from the variation of ΔH with temperature (equation 1).

C p 

 ( H ) .......... ....(1) T

ΔCp values are indicative of the structural changes that occur on of ligand binding. 62 These values also provide an idea about the hydrophobic interactions; a negative value indicates burial of non-polar surfaces and expulsion of water.63,

64

Ha et al. proposed a semi-empirical

relationship between ΔCp and the driving force (ΔGhyd) of hydrophobic interactions65 given by equation 2.

Ghyd  80C p .......................(2)

The plot of ΔH versus temperature in Figure 5B yields a slope (ΔC p) of -0.08 kcal/mol from which the ΔGhyd turns out to be -6.4 kcal/mol. The small negative value of ΔC p suggests minor 12 ACS Paragon Plus Environment

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structural alterations of hTel22 upon ligand binding, which is in accordance with our CD results indicating that the anti-parallel structure of hTel22 is retained. The value also suggests moderate hydrophobic interactions owing to partial burial of the non-polar aromatic surface of AMT upon binding. Such a situation can arise when a ligand stacks on the G-tetrad plane as demonstrated earlier.53

(kcal/mol)

Figure 5: (A) Enthalpy-entropy compensation plot for association of AMT and hTel22 along with linear fit to the data points. (B) Variation of ΔH of the binding process with temperature. The slope of the linear fit to the data gives the value of ΔC p. Probable location of binding site: fluorescence quenching studies of labeled hTel22 To further investigate the preferred binding site of AMT in hTel22, we used oligomers, 5՛ FAMhTel22 and 3՛ FAM-hTel22, wherein 6-fluorescein amidite (FAM) dye is covalently attached to the 5՛ end and 3՛ end, respectively. The fluorescence of FAM is known to be quenched upon direct interaction with organic molecules44, 66 and hence, the strongest binding site is determined by titrating a known amount of labeled pre-formed G-Quadruplex (5 nM) with varying amounts of AMT. Figure 6 shows efficient fluorescence quenching of FAM with increasing concentrations of AMT suggesting binding near the diagonal loop region (Figure 6B), where the 13 ACS Paragon Plus Environment


labeling site is located. Further, it is observed that the fluorescence of 3՛ FAM-hTel22 is quenched at a lower concentration of AMT as compared to 5՛ FAM-hTel22 implying that the strongest binding site is closer to the 3՛ end of the G-Quadruplex. It should be noted that covalent attachment of FAM does not disturb the conformation of hTel22 as revealed by CD measurements (Figure S3).

(A)

(B)

Figure 6: (A) Fluorescence quenching of 3՛ (squares) and 5՛ (circles) FAM-labeled hTel22 (λex=490 nm, λem=520 nm) with increasing amounts of AMT in aqueous buffered solution (pH=7.2) containing 100 mM NaCl. F0 and F represent the fluorescence intensity of FAMlabeled hTel22 in the absence and presence of AMT, respectively. The solid lines are sigmoidal fits to the experimental data. (B) Schematic representation of the preferable binding site of AMT (yellow disc) in hTel22. Steady state emission studies To obtain an idea about the excited state interaction between AMT and hTel22 we monitored the steady state emission behavior of photo-excited AMT with increasing concentration of hTel22. Figure 7 depicts a drastic decrease (~80%) in emission intensity of AMT on additions of hTel22. This observation is not due to any change in absorbance at the excitation wavelength (355 nm) as the excitation is made at the isosbestic point (Figure 3B). The fluorescence quenching thus 14 ACS Paragon Plus Environment

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arises from some photoinduced process, which in most likelihood involves an electron transfer reaction between the two. Energy transfer can easily be ruled out due to lack of overlap between the absorption spectrum of hTel22 and emission spectrum of AMT. Moreover, photoinduced electron transfer between psoralens and duplex DNA is known. 67, 68

Figure 7: Steady state emission spectrum (λex=355 nm) of 10 μM AMT with increasing concentration of hTel22 in aqueous buffer (pH=7.2) containing 100 mM NaCl. Transient absorption (TA) studies In order to understand the nature of interaction between photo-excited AMT and hTel22, we have studied the TA spectrum of the system. Figure 8A shows the TA spectra upon 350 nm excitation of AMT at indicated delay times. The broad positive absorption with maximum around 390 nm can be attributed to S1-Sn excited state absorption68 and the weak negative signal at ~490 nm is clearly due to the stimulated emission of AMT as its fluorescence peak appears at 460 nm (Figure 7). The kinetics at 390 nm is best described by a rising component of 250 fs, which presumably represents the vibrational relaxation in the S 1 state and a decay time constant of greater than 1 ns. The latter is close to the radiative lifetime (1.5 ns) of AMT in aqueous solutions as estimated by time-resolved emission experiments (Figure S4). The other positive 15 ACS Paragon Plus Environment


signal at longer wavelengths (> 500 nm) does not seem to decay in the monitored time window of 1 ns. Considering that triplet-triplet absorption band of psoralens appears around 500 nm, 69 we assign this long-lived signal to the triplet state absorption of AMT. In the presence of hTel22, the broad positive absorption around 400 nm (Figure 8B) is found to decay absorption much faster with time constants 30 ps (87%) and 1 ns (13%) (Figure 9B). The negative signal due to stimulated emission is not discernible anymore and is masked by the positive absorption signal. Furthermore, the long-lived signal beyond 500 nm is replaced by a broad positive absorption band which decays almost completely in 500 ps. This absorption is .-

attributed to (AMT - G .+) based on literature67 and we conclude that this species is formed by photoinduced electron transfer from guanine of hTel22 to AMT. Even though this species also absorbs at 390 nm, as the singlet-singlet absorption of AMT interferes at this wavelength, we investigated the kinetics at 530 nm to extract the time constants of the electron transfer reaction. Biexponential decay with a 35 ps component (94%) and a ~1 ns (6%) along with a growth component of 1 ps is observed (Figure 9C). The decay time constants are similar to that obtained from the kinetic analysis at 390 nm with increased amplitude of the shorter time constant (Table 2). We therefore assign the 30 ps component to the decay time of the radical species and the 1 ns to the lifetime of free AMT present in the system. Further, the rising component is attributed to the formation time of the radical species. The measured decay and rise parameters are summarized in Table 2 from which a scheme has been proposed (Scheme 1). The observed time constant for the forward electron transfer (1 ps) is slightly faster than that observed in duplex DNA primarily due to lower oxidation potential of guanine in the G-quartets (H-bonded as well as stacked with other guanines) of G-Quadruplexes is lower when compared with isolated guanine or only two/three stacked guanine bases.43 16 ACS Paragon Plus Environment

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Figure 8: Transient absorption spectra of (A) free AMT (0.1 mM) and (B) AMT in presence of hTel22 (1 mM) in aqueous buffer (pH=7.2) containing 100 mM NaCl at λ ex=350 nm. 2.0

(A)

1.5

2.5

1 .5 1 .0 0 .5 0 .0 0

1.0

4

8

12

1.5 1.0 0.5

0.5 0.0

(B)

2.0 A(mOD)

(mOD)

0.0 0

300

600 Time (ps)

0

900

300

600 Time (ps)

900

0 .6

0.6

0 .4

A(mOD)

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


(C)

0.4

0 .2 0 .0 0

10

20

30

0.2 0.0 0

300

600 Time (ps)

900

Figure 9: Transient absorption decay profiles of free AMT at 390 nm (A) and AMT bound to hTel22 monitored at 390 nm (B) and 530 nm (C). The insets in (A) and (C) show the kinetics at shorter timescales. The solid lines are the fits to the experimental data. 17 ACS Paragon Plus Environment


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Table 2: Rise and decay parameters of free and hTel22-bound AMT monitored at different wavelengths.

System

Monitoring wavelength

τ1 (ps)

a1 *

τ2 (ps)

a2 *

τrise (ps)

(nm) AMT

390

>1000

0.25±0.06

390

32±0.8

0.87±0.05

1200

0.13±0.02

0.51±0.04

530

35±0.2

0.94±0.05

1200

0.06±0.005

1.1±0.02

AMT+hTel22

*ai’s represent the amplitude associated with the decay components.

The electron transfer reaction observed here is thermodynamically feasible with a driving force of -0.51 eV for the charge separation and -3.14 eV for the charge recombination process. However, it is well known that the relative orientation of the drug molecule with respect to the host also an important factor in the electron transfer processes in DNA. Wasielewiski and coworkers recently demonstrated the importance of π-stacked arrangement of the donor and acceptor in photoinduced electron transfer process and show that the extent of π-stacking plays an important role in determining the efficiency of the electron transfer reaction. 70 Further, Manet and co-workers showed that although 1:1 and 1:2 complexes are formed between doxorubicin and G-Quadruplex DNA, only the former participates in ultrafast electron transfer due to favorable quasi-parallel geometry.71 In the present work, the ITC results show that AMT binds to hTel22 via the end stacking mode of binding that provides appropriate parallel/quasi-parallel geometry for electron transfer to occur efficiently. The observation of ultrafast photoinduced electron transfer in AMT-hTel22 leading to oxidation of guanine brightens the potential of psoralens to be employed and further developed as anti-cancer drugs.


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Scheme 1: Schematic representation of the photoinduced processes occurring in free (A) and bound (B) AMT along with their time constants.

CONCLUSIONS While psoralens have long been studied in the context of duplex DNA, very little is known about its potential as a G-Quadruplex binder and its excited state interaction with the same. It is found that water soluble and planar psoralen derivative, AMT interacts with hTel22 in an 1:1 stoichiometric ratio with a binding constant of the order of ~10 5 M-1, which is enthalpically driven. The binding event does not disturb the anti-parallel conformation of hTel22, but renders stability to it by enhanced stacking interactions. These observations are indicative of an endstacking mode of ligand binding. Fluorescence quenching experiments on FAM-labeled hTel22 reveal that the binding site of AMT is nearer to the 3՛ end in the diagonal loop region of hTel22. Upon photo-excitation of AMT, one observes ultrafast electron transfer (1 ps) from the guanine .-

moiety of hTel22 leading to the formation of a radical species (AMT - G .+), which survives for about 30 ps. The fast electron transfer process in photo-excited AMT-hTel22 suggests that AMT and the guanine moiety of hTel22 are in a favorable parallel/quasi-parallel geometry with each 19 ACS Paragon Plus Environment


other, substantiating our conclusion of end-stacking mode of interactions. The study highlights psoralens as a prospective class of compounds for the development of anti-cancer therapeutics by targeting G-Quadruplex DNA. ASSOCIATED CONTENT Supplementary Data ITC profiles at different temperatures, CD spectra of labeled oligomers, time-resolved fluorescence decay of AMT and calculation for free energy changes of electron transfer reaction. ACKNOWLEDGEMENT This work is supported by Research Grant No: EMR/2015/000582 and J.C. Bose Fellowship (to AS) from the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), India. We also acknowledge UGC-UPE for financial support to our University. The transient absorption measurements were carried out using the femtosecond laser facility of the School. SP thanks University Grants Commission for the BSR Fellowship. The authors thank Debparna Datta and Prof. M. J. Swamy for their help with the ITC experiments.

REFERENCES 1. Balasubramanian, S.; Neidle, S., G-quadruplex nucleic acids as therapeutic targets. Curr Opin Chem Biol. 2009, 13, 345-353. 2. Bidzinska, J.; Cimino-Reale, G.; Zaffaroni, N.; Folini, M., G-quadruplex structures in the human genome as novel therapeutic targets. Molecules 2013, 18, 12368-12395. 3. Collie, G. W.; Parkinson, G. N., The application of DNA and RNA G-Quadruplexes to therapeutic medicines. Chem. Soc. Rev. 2011, 40, 5867-5892. 4. Burge, S.; Parkinson, G. N.; Hazel, P.; Todd, A. K.; Neidle, S., Quadruplex DNA: sequence, topology and structure. Nucleic Acids Res. 2006, 34, 5402-5415. 5. Campbell, N. H.; Neidle, S., G-quadruplexes and metal ions. Met Ions Life Sci. 2012, 10, 119-134. 6. Zaccaria, F.; Paragi, G.; Guerra, C. F., The role of alkali metal cations in the stabilization of guanine quadruplexes: why K+ is the best. Phys. Chem. Chem. Phys. 2016, 18, 20895-20904. 20 ACS Paragon Plus Environment

Page 20 of 24

Page 21 of 24 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


7. Heddi, B.; Phan, A. T., Structure of human telomeric DNA in crowded solution. J. Am. Chem. Soc. 2011, 133, 9824-9833. 8. Lane, A. N.; Chaires, J. B.; Gray, R. D.; Trent, J. O., Stability and kinetics of G-quadruplex structures. Nucleic Acids Res. 2008, 36, 5482-5515. 9. You, J.; Li, H.; Lu, X.-M.; Li, W.; Wang, P.-Y.; Dou, S.-X.; XiJul, X.-G., Effects of monovalent cations on folding kinetics of G-quadruplexes. Biosci. Rep. 2017, 37, 1-12. 10. Nakano, S.-i.; Sugimoto, N., Model studies of the effects of intracellular crowding on nucleic acid interactions. Mol. BioSyst. 2017, 13, 32-41. 11. Fujii, T.; Podbevšek, P.; Plavec, J.; Sugimoto, N., Effects of metal ions and cosolutes on Gquadruplex topology. J. Inorg. Biochem. 2017, 166, 190-198. 12. Gaynutdinov, T. I.; Neumann, R. D.; Panyutin, I. G., Structural polymorphism of intramolecular quadruplex of human telomeric DNA: effect of cations, quadruplex-binding drugs and flanking sequences. Nucleic Acids Res. 2008, 36, 4079-4087. 13. Kumar, N.; Sahoo, B.; Varun, K. A. S.; Maiti, S.; Maiti, S., Effect of loop length variation on quadruplex-Watson Crick duplex competition. Nucleic Acids Res. 2008, 36, 4433-4442. 14. Biffi, G.; Tannahill, D.; McCafferty, J.; Balasubramanian, S., Quantitative visualization of DNA Gquadruplex structures in human cells. Nat. Chem. 2013, 5, 182-186. 15. Lipps, H. J.; Rhodes, D., G-quadruplex structures: in vivo evidence and function. Trends Cell Biol. 2009, 19, 414-422. 16. Maizels, N.; Gray, L. T., The G4 genome. PLoS Genet. 2013, 9, e1003468. 17. Ribeyre, C.; Lopes, J.; Boulé, J. B.; Piazza, A.; Guedin, A.; Zakian, V. A.; L.Mergny, J., The yeast Pif1 helicase prevents genomic instability caused by G-quadruplex-forming CEB1 sequences in vivo. PLoS Genet. 2009, 5, e1000475. 18. Yadav, V.; Hemansi; Kim, N.; Tuteja, N.; Yadav, P., G Quadruplex in plants: A ubiquitous regulatory element and its biological relevance. Front Plant Sci. 2017, 8, 1163. 19. Siddiqui-Jain, A.; Grand, C. L.; Bearss, D. J.; Hurley, L. H., Direct evidence for a G-quadruplex in a promoter region and its targeting with a small molecule to repress c-MYC transcription. Proc. Natl. Acad. Sci. 2002, 99, 11593-11598. 20. Arora, A.; Dutkiewicz, M.; Scaria, V.; Hariharan, M.; Maiti, S.; Kurreck., J., Inhibition of translation in living eukaryotic cells by an RNA G-Quadruplex motif. RNA 2008, 14, 1290-1296. 21. Moyzis, R. K.; Buckingham, J. M.; Cram, L. S.; Dani, M.; Deaven, L. L.; Jones, M. D.; Meyne, J.; Ratliff, R. L.; Wu, J. R., A highly conserved repetitive DNA sequence, (TTAGGG) n, present at the telomeres of human chromosomes. Proc. Natl. Acad. Sci. U.S.A. 1988, 85, 6622−6626 22. Kim, N. W.; Piatyszek, M. A.; Prowse, K. R.; Harley, C. B.; D.West, M.; Ho, P. L.; Coviello, G. M.; Wright, W. E.; Weinrich, S. L.; Shay, J. W., Specific association of human telomerase activity with immortal cells and cancer. Science 1994, 266, 2011-2015. 23. Mergny, J. L.; Riou, J. F.; Mailliet, P.; Teulade-Fichou, M. P.; Mailliet, P.; Gilson, E., Natural and pharmacological regulation of telomerase. Nucleic Acids Res. 2002, 30, 839-865. 24. Basu, A.; Kumar, G. S., Calorimetric investigation on the interaction of proflavine with human telomeric G-quadruplex DNA. J. Chem. Thermodynamics 2016, 98, 208-213. 25. Kumar, V.; Sengupta, A.; Gavvala, K.; Koninti, R. K.; Hazra, P., Spectroscopic and thermodynamic insights into the interaction between Proflavine and human telomeric G-Quadruplex DNA. J. Phys. Chem. 2014, 118, 11090−11099. 26. D. Sun, D.; Thompson, B.; Cathers, B. E.; Salazar, M.; Kerwin, S. M.; Trent, J. O.; Jenkins, T. C.; Neidle, S.; Hurley, L. H., Inhibition of human telomerase by a G-quadruplex-interactive compound. J. Med. Chem. 1997, 40, 2113-2116. 21 ACS Paragon Plus Environment


27. Wang, C.; Carter-Cooper, B.; Du, Y.; Zhou, J.; Saeed, M. A.; Liu, J.; Guo, M.; Roembke, B.; Mikek, C.; Lewis, E. A.; Lapidus, R. G.; Sintim, H. O., Alkyne-substituted diminazene as G-quadruplex binders with anticancer activities. Eur. J. Med. Chem. 2016, 118, 266-275. 28. Zhou, J.; Le, V.; Kalia, D.; Nakayama, S.; Mikek, C.; Lewis, E. A.; Sintim, H. O., Diminazene or berenil, a classic duplex minor groove binder, binds to G-quadruplexes with low nanomolar dissociation constants and the amidine groups are also critical for G-quadruplex binding. Mol. BioSyst. 2014, 10, 2724-2734. 29. Fedoroff, O. Y.; Salazar, M.; Han, H.; Chemeris, V. V.; Kerwin, S. M.; Hurley, L. H., NMR-based model of a telomerase-inhibiting compound bound to G-quadruplex DNA. Biochemistry 1998, 37, 1236712374. 30. Huang, X.-X.; Zhu, L.-N.; Wu, B.; Huo, Y.-F.; Duan, N.-N.; Kong, D.-M., Two cationic porphyrin isomers showing different multimeric G-quadruplex recognition specificity against monomeric Gquadruplexes. Nucleic Acids Res. 2014, 42, 8719-8731. 31. Zhang, Q.; Liu, Y.-C.; Kong, D.-M.; Guo, D.-S., Tetraphenylethene derivatives with different numbers of positively charged side arms have different multimeric G-Quadruplex recognition specificity. Chem. Eur. J. 2015, 21, 13253 –13260. 32. Arora, A.; Balasubramanian, C.; Kumar, N.; Agrawal, S.; Ojha, R. P.; Maiti, S., Binding of berberine to human telomeric quadruplex – spectroscopic, calorimetric and molecular modeling studies. FEBS J. 2008, 275, 3971-3983. 33. Ghosh, S.; Kar, A.; Chowdhury, S.; Dasgupta, D., Ellipticine binds to a human telomere sequence: an additional mode of action as a putative anticancer agent? Biochemistry 2013, 52, 4127-4137. 34. Guittat, L.; Cian, A. D.; Rosu, F.; Gabelica, V.; Pauw, E. D.; Delfourne, E.; Mergny, J. L., Ascididemin and meridine stabilise G-quadruplexes and inhibit telomerase in vitro. Biochim. Biophys. Acta 2005, 1724, 375–384. 35. Tawani, A.; Amanullah, A.; Mishra, A.; Kuma, A., Evidences for Piperine inhibiting cancer by targeting human G-quadruplex DNA sequences. Sci. Rep. 2016, 6, 39239. 36. Zhang, W. L. M.; Zhang, J.-l.; Li, H.-q.; Zhang, X.-c.; Sun, Q.; Qiu, C.-m., Interactions of daidzin with intramolecular G-quadruplex. FEBS Letters 2006, 580, 4905–4910. 37. Bhattacharjee, S.; Chakraborty, S.; Sengupta, P. K.; Bhowmik, S., Exploring the interactions of the dietary plant flavonoids fisetin and naringenin with G-Quadruplex and duplex DNA, showing contrasting binding behavior: spectroscopic and molecular modeling approaches. J. Phys. Chem. B 2016, 120, 89428952. 38. Cimino, G. D.; Gamper, H. B.; Isaacs, S. T.; Hearst, J. E., Psoralens as photoactive probes of nucleic acid structure and function: organic chemistry, photochemistry, and biochemistry. Ann. Rev. Biochem. 1985, 54, 1151-1193. 39. Kitamura, N.; Kohtani, S.; Nakagaki, R., Molecular aspects of furocoumarin reactions: photophysics, photochemistry, photobiology, and structural analysis. J. Photochem. Photobiol. C 2005, 6, 168-185. 40. Panno, M. L.; Giordano, F., Effects of psoralens as anti-tumoral agents in breast cancer cells. World J. Clin. Oncol. 2014, 5, 348-358. 41. Alcaro, S.; Musetti, C.; Distinto, S.; Casatti, M.; Zagotto, G.; Artese, A.; Parrotta, L.; Moraca, F.; Costa, G.; Ortuso, F.; Maccioni, E.; Sissi, C., Identification and characterization of new DNA G-Quadruplex binders selected by a combination of ligand and structure-based virtual screening approaches. J. Med. Chem. 2013, 56, 843-855. 42. Charge transfer in DNA: from mechanism to application; Wagenknecht, H.-A., Eds; Wiley-VCH: Weinheim, 2006. 22 ACS Paragon Plus Environment

Page 22 of 24

Page 23 of 24 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


43. Choi, J.; Park, J.; Tanaka, A.; Park, M. J.; Jang, Y. J.; Fujitsuka, M.; Kim, S. K.; Majima, T., Hole trapping of G-quartets in a G-Quadruplex. Angew. Chem. Int. Ed. 2013, 52, 1134-1138. 44. Beniaminov, A. D.; Novikov, R. A.; Mamaeva, O. K.; Mitkevich, V. A.; Smirnov, I. P.; Livshits, M. A.; Shchyolkina, A. K.; Kaluzhny, D. N., Light-induced oxidation of the telomeric G4 DNA in complex with Zn(II) tetracarboxymethyl porphyrin. Nucleic Acids Res. 2016, 44, 10031-10041. 45. Szalai, V. A.; Singer, M. J.; Thorp, H. H., Site-specific probing of oxidative reactivity and telomerase function using 7,8-dihydro-8-oxoguanine in telomeric DNA. J. Am. Chem. Soc. 2002, 124, 1625-1631. 46. Nicoludis, J. M.; Barrett, S. P.; Mergny, J.-L.; Yatsunyk, L. A., Interaction of human telomeric DNA with N-methyl mesoporphyrin IX. Nucleic Acids Res. 2012, 40, 5432-5447. 47. Sekhar, M. C.; Santhosh, K.; Kumar, J. P.; Mondal, N.; Soumya, S.; Samanta, A., CdTe quantum dots in ionic liquid: stability and hole scavenging in the presence of a sulfide salt. J. Phys. Chem. C 2014, 118, 18481-18487. 48. Paramasivan, S.; Rujan, I.; Bolton, P. H., Circular dichroism of Quadruplex DNAs: applications to structure, cation effects and ligand binding. Methods 2007, 43, 324-331. 49. Villar-Guerra, R. d.; Gray, R. D.; Chaires, J. B., Characterization of quadruplex DNA structure by circular dichroism. Curr. Protoc. Nucleic Acid Chem. 2017, 68, 17.8.1-17.8.16. 50. Gray, D. M.; Wen, J. D.; Gray, C. W.; Repges, R.; Repges, C.; Raabe, G.; Fleischhauer, J., Measured and calculated CD spectra of G-quartets stacked with the same or opposite polarities. Chirality 2008, 20, 431-440. 51. Zhu, L.-N.; Wu, B.; Kong, D.-M., Specific recognition and stabilization of monomeric and multimeric G-quadruplexes by cationic porphyrin TMPipEOPP under molecular crowding conditions. Nucleic Acids Res. 2013, 41, 4324-4335. 52. Doyle, M. L., Characterization of binding interactions by isothermal titration calorimetry. Curr. Opin. Biotechnol. 1997, 8, 31-35. 53. Bhadra, K.; Kumar, G. S., Interaction of berberine, palmatine, coralyne, and sanguinarine to Quadruplex DNA: A comparative spectroscopic and calorimetric study. Biochim. Biophys. Acta 2011, 1810, 485-496. 54. Anbazhagan, V.; Sankhala, R. S.; Singh, B. P.; Swamy, M. J., Isothermal titration calorimetric studies on the interaction of the major bovine seminal plasma protein, PDC-109 with phospholipid membranes. PLos One 2011, 6, e25993. 55. Brummell, D. A.; Sharma, V. P.; Anand, N. N.; Bilous, D.; Dubuc, G.; Michniewicz, J.; MacKenzie, C. R.; Sadowska, J.; Sigurskjold, B. W.; Sinnott, B.; Young, N. A.; Bundle, D. R.; Narang, S. A., Probing the combining site of an anticarbohydrate antibody by saturation-mutagenesis: role of the heavy-chain CDR3 residues. Biochemistry 1993, 32, 1180-1187. 56. Sigurskjold, B. W.; Bundle, D. R., Thermodynamics of oligosaccharide binding to a monoclonal antibody specific for a salmonella O-antigen point to hydrophobic interactions in the binding site. J. Biol. Chem. 1992, 267, 8371-8376. 57. Pagano, B.; Mattia, C. A.; Giancola, C., Applications of isothermal titration calorimetry in biophysical studies of G-quadruplexes. Int. J. Mol. Sci. 2009, 10, 2935-2957. 58. Funke, A.; Weisz, K., Comprehensive thermodynamic profiling for the binding of a G-Quadruplex selective indoloquinoline. J. Phys. Chem. B 2017, 121, 5735-5743. 59. White, E. W.; Tanious, F.; Ismail, M. A.; Reszka, A. P.; Neidle, S.; Boykin, D. W.; Wilson, W. D., Structure-specific recognition of quadruplex DNA by organic cations: influence of shape, substituents and charge. Biophys. Chem. 2007, 126, 140-153.



60. Martino, L.; Virno, A.; Pagano, B.; Virgilo, A.; Di, S. M.; Galeone, A.; Giancola, C.; Bifulco, G.; Mayol, L.; Randazzo, A., Structural and thermodynamic studies of the interaction of distamycin A with the parallel quadruplex structure [d(TGGGGT)]4. J. Am. Chem. Soc. 2007, 129, 16048-16056. 61. Pagano, B.; Virno, A.; Mattia, C. A.; Mayol, L.; Randazzo, A.; Giancola, C., Targeting DNA Quadruplexes with distamycin A and its derivatives: an ITC and NMR study. Biochimie 2008, 90, 12211232. 62. Sturtevant, J. M., Heat capacity and entropy changes in processes involving proteins. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 2236-2240. 63. Baldwin, R. L., Temperature dependence of the hydrophobic interaction in protein folding. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 8069-8072. 64. Spolar, R. S.; Ha, J. H.; Record, M. T., Hydrophobic effect in protein folding and other noncovalent processes involving proteins. Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 8382-8385. 65. Ha, J. H.; Polar, R. S.; Record, M. T., Role of the hydrophobic effect in stability of site-specific protein-DNA complex. J. Mol. Biol. 1989, 209, 801-816. 66. Kaluzhny, D. N.; Mamaeva, O. K.; Beniaminov, A. D.; Shchyolkina, A. K.; Livshits, M. A., The thermodynamics of binding of low molecular weight ligands at extreme tetrads of telomeric GQuadruplexes Biophysics 2016, 61, 34-39. 67. Fröbel, S.; Levi, L.; Ulamec, S. M.; Gilch, P., Photoinduced electron transfer between psoralens and DNA: influence of DNA sequence and substitution. ChemPhysChem 2016, 17, 1377-1386. 68. Fröbel, S.; Reiffers, A.; Ziegenbein, C. T.; Gilch, P., DNA intercalated psoralen undergoes efficient photoinduced electron transfer. J. Phys. Chem. Lett. 2015, 6, 1260-1264. 69. Bensasson, R. V.; Chalvet, O.; Land, E. J.; Ronfard-Haret, J. C., Triplet, radical anion and radical cation spectra of furocoumarins. Photochem. Photobiol. 1984, 39, 287-291. 70. Mishra, A. K.; Harris, M. A.; Young, R. M.; Wasielewski, M. R.; Lewis, F. D., Dynamics of charge injection and charge recombination in DNA mini-hairpins. J. Phys. Chem. B 2017, 121, 7042-7047. 71. Changenet-Barret, P.; Gustavsson, T.; Markovitsi, D.; Manet, I., Ultrafast electron transfer in complexes of doxorubicin with human telomeric G-Quadruplexes and GC duplexes probed by femtosecond fluorescence spectroscopy. ChemPhysChem 2016, 17, 1264-1272.

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Ground- and Excited-State Interactions of a Psoralen Derivative with

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