Initial Excited-State Structural Dynamics of 2′-Deoxyguanosine

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Initial Excited-State Structural Dynamics of 20-Deoxyguanosine Determined via UV Resonance Raman Spectroscopy Amira F. El-Yazbi, Alexandra Palech, and Glen R. Loppnow* Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada ABSTRACT: The resonance Raman spectra of 20 -deoxyguanosine, a DNA nucleoside, were measured in aqueous solution at wavelengths throughout its 260 nm absorption band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism with two electronic states yielded the initial excited-state structural dynamics in both states. The vibrational modes containing the N7dC8 stretching and C8H bending internal coordinates were found to exhibit significant initial structural dynamics upon photoexcitation to either state and are coincident with the photochemical reaction coordinate involving the formation of the 20 -deoxyguanosine cation radical.

’ INTRODUCTION DNA is the nucleic acid that stores the genetic information of an organism, and RNA is the nucleic acid that mediates translation of the genetic code into proteins. They are composed of two purine nucleosides, adenosine and guanosine (G), and three pyrimidine nucleosides, cytidine, thymidine, and uridine. Guanosine (Scheme 1) is present in both DNA and RNA. Nucleic acid damage caused by UV light can subsequently lead to cancer and other diseases. Photochemical damage arises from evolution on ground- and excited-state potential energy surfaces following absorption of a photon. Therefore, an understanding of the excited-state structural dynamics is essential for understanding the photochemical mechanisms of nucleic acids and their components. Absorption of UV light by nucleic acids can populate a lowlying guanine charge transfer (CT) throughout the extended stacked nucleobases of the DNA double helix,13 forming oxidized nucleobases such as the guanosine cation radical and apurinic sites in the DNA.48 In addition, UVB radiation can lead to reactive oxygen species (ROS).9 The energy from UVB radiation is absorbed by intermediate primary chromophores, such as porphyrins, flavins, quinones, and NADH,10 which in turn activate ground-state molecular oxygen. Although the actual ROS and the mechanisms whereby they are produced by nearUV radiation remain unclear,9 hydroxyl radicals are thought to be primarily responsible for DNA base oxidation.11 The most common products of this oxidation process are 8-oxo-7,8dihydro-20 -deoxyguanosine (8-oxo-dG), the formamidopyrimidine product, and the oxazolone product11 (Scheme 2). It must be re-emphasized that these oxidative damage products do not occur via direct irradiation of the DNA, but rather via irradiation of an intermediate chromophore that then generates the ROS that leads to oxidation damage. As 20 -deoxyguanosine (20 -dG) has the lowest ionization potential among all DNA components, it serves as the main site of electron loss in DNA with subsequent formation of the 20 -dG r 2011 American Chemical Society

Scheme 1

cation radical12 (20 -dG•+) via a low-lying CT state. The 20 -dG•+ undergoes nucleophilic addition of water at C8 to form 8-hydroxy-7,8-dihydroguanin-7-yl radical that either loses one electron to form 8-oxo-dG or gains one electron to form the formamidopyrimidine product.13 Among the three oxidative products, 8-oxo-dG is the most abundant form of oxidative DNA lesion and can pair with adenine during DNA replication, causing G:C to T:A transversion mutations.14,15 The presence of these lesions in DNA and RNA can cause mutagenesis, carcinogenesis, and aging and in large quantities can be lethal to cells.1619 A number of studies have been performed to examine the excitedstate dynamics of guanosine and guanine. The excited-state lifetime of 20 -dG has been characterized previously by several techniques to be less than 1 ps. Pecourt et al.21,22 applied transient absorption spectroscopy with subpicosecond resolution to determine the excited-state lifetime of guanosine to be 0.5 ps. A 0.9 ps excited-state lifetime was confirmed for GMP by Peon et al.23 using fluorescence Received: June 1, 2011 Revised: July 18, 2011 Published: August 12, 2011 10445

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upconversion, and the primary decay channel was assigned to internal conversion. However, Gustavsson and co-workers2426 re-examined the fluorescence dynamics and found that the decays are more complex than a simple monoexponential decay. For GMP, these researchers obtained a biexponential behavior with a τ1 of 0.2 ps and a τ2 of 0.9 ps. The spectral analysis and angle calculation performed by the researchers showed that an important electronic relaxation took place after relaxation, but they had no hint about its nature.26 Canuel et al.27 studied the two-photon ionization of the gas-phase free nucleobases excited at 267 nm and probed at 400 nm. Their results showed a biexponential decay with a τ1 of 0.15 ps and a τ2 of 0.36 ps for guanine. The short time τ1 was ascribed to wave packet motion from the FranckCondon region of the lowest excited ππ* state (La) to its local minimum La,min, and the longer time, τ2, was ascribed to a barrier crossing for the La/nπ* internal conversion. These rapid electronic dynamics suggest that the initial excited-state structural dynamics are important in determining guanine photochemistry. A number of computational studies have been devoted recently to the study of guanine excited states, especially in the gas phase.20,2833 Chen et al.28 agree that the fast τ1 component is associated with the motion toward La,min. Alternatively, SerranoAndres et al.29 propose that the τ1 component is due to barrierless motion on La toward the La/S0 conical intersection, without any meaningful minimum on the La surface based on minimum CASPT2/CASSCF energy reaction paths. Recently, Karunakaran et al.34 interpreted the excited-state evolution using quantum chemical calculations at the time-dependent PBE0 level to involve the La and the highest excited ππ* (Lb) bright excited states, whereas the nπ* and πσ* dark excited states play a minor role. They found that the photoinduced evolution involves ultrafast Lb to La conversion (τba , 100 fs) and exhibits the presence of a wide planar plateau on La. However, these rates all represent electronic relaxation only, with little information about molecular structural changes occurring in the photochemically active excited electronic state. The intensities in a resonance Raman vibrational spectrum can yield the initial excited-state structural dynamics.3537 In resonance Raman spectroscopy, the intensities of the normal vibrational modes are greatly enhanced by excitation with wavelengths that closely match that of an electronic absorption band. The resonance Raman vibrational band intensity is directly proportional to the slope of the excited-state potential energy surface along that vibrational coordinate at the FranckCondon geometry. Resonance Raman studies of several DNA and RNA components at different excitation wavelengths have been previously

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performed.3851 However, the excited-state structural dynamics have been obtained for only a handful of nucleobases, including thymine,46 cytosine,47 5-fluorouracil,48 and 9-methyladenine,49 and there are no previous resonance Raman-derived initial excited-state structural dynamics of a nucleoside. Here, we report the excited-state structural dynamics of 20 -dG from resonance Raman spectroscopy at the FranckCondon geometry. It is fundamentally important to understand the structural and photochemical behavior of simple molecules like nucleobases to understand the photochemistry of complex molecules like DNA and RNA. We determined the initial excited-state structural dynamics of 20 -dG excited within the intense absorption bands centered at 252 and 280 nm.29,34 The observed initial excited-state structural dynamics of 20 -dG in both states, which ultimately lead to primarily nonradiative de-excitation, were coincident with the photochemical structural changes expected for the formation of the charge transfer state. The experimental initial excited-state structural dynamics measured here are discussed and compared to those predicted from computational studies of 20 -dG, as well as with the initial excited-state structural dynamics of other nucleobases.

’ EXPERIMENTAL METHODS 20 -Deoxyguanosine [2-amino-9-(20 -deoxy-9-β-D-ribofuranosyl)9H-purin-6-ol, 99% (Sigma, Oakville, ON)] and sodium sulfate [99% (Merck KgaA, Darmstadt, Germany)] were obtained commercially and used as supplied. Nanopure water from a Barnstead (Boston, MA) water filtration system was used to prepare the 20 -dG solutions. The laser system, harmonic generation system, sample cell, Raman scattering collection geometry, and spectrometer used in this work have been described previously.46 Briefly, typical UV laser powers were 614 mW at the sample, and excitation wavelengths of 244, 257, 266, 275, and 290 nm were used here. For each spectrum, the total accumulation time was 1530 min. All resonance Raman spectra were obtained using 0.64.0 mM 20 dG containing 0.21.0 M sodium sulfate as an internal standard. The addition of sodium sulfate as an internal intensity standard did not have any noticeable effect on the absorption spectrum or the resonance Raman spectra of 20 -dG. The resonance Raman spectrum of 3 mM 20 -dG containing 0.5 M sulfate in the overtone and combination band region was recorded with a UV Raman microscope (Renishaw, Chicago, IL).45 The methods used here for converting the resonance Raman intensities of 20 -dG into absolute cross sections and for selfabsorption correction effects have been described previously.4552 Sulfate cross sections were determined from the A term fit expression in eq 1 " #2 2 2 dσ R 3 υe þ υo ¼ Kυυ þ C ð1Þ dΩ ðυe 2  υo 2 Þ2 where (dσR)/(dΩ) is sulfate’s differential cross section, υe is the resonant electronic transition energy of sulfate, υ0 is the incident photon energy, υ (= υ0  982 cm1) is the scattered photon energy, K is a scaling constant, and C is the background-state contribution. Using this equation and the A term parameters K = 2.75  1010 Å2 molecule1 sr1, υe = 137000 cm1, and C = 2.00  1012 cm2, we obtained differential cross sections for sulfate of 3.54  1012, 2.71  1012, 2.28  1012, 1.93  1012, and 1.49  1012 Å2 molecule1 sr1 at 244, 257, 266, 275, and 290 nm, respectively. 10446

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Figure 1. Experimental (—) and simulated ( 3 3 3 ) absorption spectra of 20 -dG in water. The simulated absorption spectrum was calculated using eq 3 with the parameters listed in Table 1. Discrepancies observed at energies greater than 40000 cm1 are due to higher-energy electronic transitions that are not modeled in our simulation.

’ THEORY The resonance Raman excitation profiles and absorption spectrum were simulated with the time-dependent wave packet propagation formalism.3537 8πEs 3 EL e4 M 4 σR ¼ 9p6 c4

Z

∞

0

2   þ εi Þt=pGðtÞ 

σA ¼

4πEL e2 M 2 6p2 cn

Z

∞

0

Z  ∞  dE0 HðE0 Þ dt h f jiðtÞi exp½iðEL 0

ð2Þ Z dE0 HðE0 Þ

∞

∞

dt hijiðtÞi exp½iðEL

þ εi Þt=pGðtÞ ð3Þ where EL and ES are the energies of the incident and scattered photons, respectively, n is the refractive index, M is the transition length, εi is the energy of the initial vibrational state, |iæ and |fæ are the initial and final vibrational wave functions in the Raman process, respectively, H(E0) is a normalized Gaussian distribution of zerozero energies around an average energy, (E0), |i(t)æ is the initial ground-state vibrational wave function propagated on the excited-state potential surface, and G(t) is the homogeneous line width function. For molecules interacting with a bath, G(t) represents the dynamics of chromophoresolvent coupling within the Brownian oscillator model.3537 Significantly, the Æi|i(t)æ and Æf|i(t)æ overlaps are only sensitive to β/p, the excited-state slopes at the FranckCondon geometry along each normal mode. Thus, the resonance Raman intensities directly reflect the structural dynamics of the excited state. The implementation of these equations has been described in detail previously.5658

’ RESULTS The absorption spectrum and the UV resonance Raman spectra of 20 -dG are shown in Figures 1 and 2, respectively.

Figure 2. UV resonance Raman spectra of 20 -dG in water at excitation wavelengths within the 252 and 280 nm absorption bands. The sulfate internal standard Raman peak is denoted with an asterisk. The spectra have been scaled to the most intense peak and offset along the y-axis for the sake of clarity.

The absorption spectrum of 20 -dG shows two bands, at 252 nm (39682 cm1) and 280 nm (35714 cm1). These bands have been previously assigned as the Lb and La ππ* excited states, respectively. Seven bands for 20 -dG (1682, 1600, 1573, 1490, 1408, 1366, and 1321 cm1) are observed in the UVRR spectrum between 800 and 1800 cm1, with excitation at 244, 257, and 266 nm in the UVRR spectrum. Of these seven, 20 -dG exhibits three main, intense bands, a N7dC8 stretch + C8H bend + C8N9 stretch at 1490 cm1, a N3C4 stretch + C5N7 stretch + C4dC5 stretch at 1573 cm1, and a N7dC8 stretch + C8H bend at 1321 cm1. All of these bands have been previously assigned (Table 1).38,53,54 With excitation at 275 and 290 nm, three additional bands appear at 1270, 1178, and 1077 cm1. These additional three weak bands are resonantly enhanced by the low-energy electronic transition (La) appearing in the absorption band around 280 nm (Figure 2) and are not considered further. In addition, the 1682 cm1 band (Figure 1) increases in relative intensity as the Raman excitation wavelength increases, indicating that this mode may be enhanced by more than one electronic transition. Quantitative measurement of the resonance Raman cross section of each fundamental vibration as a function of excitation wavelength within the absorption band gives the resonance Raman excitation profiles. The experimental and simulated absorption spectra of 20 -dG are shown in Figure 1, while Figure 3 shows the experimental and simulated resonance Raman excitation profiles of 20 -dG that are modeled with eqs 3 and 2, respectively, and the parameters listed in Table 1. Figures 1 and 3 show the good agreement between experimental and calculated absorption spectra and resonance Raman excitation profiles. Two models were tested to fit the experimental data. One model better simulated the absorption band from 33000 to 43000 cm1, but the simulated excitation profiles were much broader than the experimentally observed ones. The second 10447

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Table 1. Harmonic Mode Parameters of 20 -dGa mode (cm1)

β/p(La )

assignmentb

c

β/p(Lb)

1321

ν(N7C8) [+19], be(C8H) [24]

1366

ν(N7C8) [26], ν(N1C6) [25], ν(N5N7) [+16]

204.90

478.10

1408

ν(C4N9) [33], ν(C5N7) [24]

140.80

253.44

620.87

1490

be(C8H) [+40], ν(C8N9) [32], ν(N7C8) [+21]

447.00

1028.10

1573

ν(N3C4) [30], ν(C4C5) [+24], ν(C5N7) [16]

314.60

707.85

1600

be(N1H) [+83], ν(C2N) [+15]

160.00

656.00

1682

ν(C6O) [+48], ν(C5C6) [21], be(N1H) [11], ν (C4C5) [+11], ν(N1C6) [10]

302.76

100.92

a Frequencies listed are the experimental frequencies reported here. The slopes, β/p, of the potential energy surfaces of the two excited states along that normal mode in cm1 were obtained by fitting eqs 2 and 3 with the following parameters: temperature T = 298 K, Brownian oscillator line shape k = Λ/ D = 0.1, Gaussian homogeneous line widths of La and Lb states ΓLa = 1050 cm1 and ΓLb = 1650 cm1, respectively, inhomogeneous line width Θ = 900 cm1, zerozero energies E0,La = 35750 cm1 and E0,Lb = 37600 cm1, transition lengths MLa = 0.56 Å and MLb = 0.83 Å, and the angle between the transition dipole moments of the two excited states = 108. The estimated errors in the parameters used in the calculation are as follows: zerozero energy (E0), (1%; transition length (M), (1%; homogeneous line width (Γ), (5%; inhomogeneous line width (Θ), (5%; slopes, (5%. b Mode assignments from refs 38, 53, and 54. Abbreviations: ν, stretch; be, bend. The plus (+) and minus () signs refer to the phase of the internal coordinate. Numbers in brackets represent the percentage potential energy distribution (PED) of the listed internal coordinate(s) in that normal mode. c Modes that are not enhanced by that particular state.

Figure 3. Experimental (symbols) and calculated (—) resonance Raman excitation profiles of 20 -dG. Excitation profiles were calculated with eq 2 by using the parameters listed in Table 1. Excitation profiles have been offset along the ordinate for the sake of clarity.

model better fit the experimental resonance Raman excitation cross sections and had more bands of the absorption band than the first model. Because it is known that there are three transitions in the ∼260 nm absorption band,29,34 the second model was deemed more appropriate, and these are the simulations shown in Figures 1 and 3. The absorption spectrum shows two electronic transitions, with E0 values of 39700 (252 nm) and 35750 cm1 (280 nm), while the third electronic transition (nπ*) is a state with a low oscillator strength and should not contribute any resonance enhancement. The deviations that are observed between the simulated and experimental absorption spectra in Figure 1 at energies greater than 40000 cm1 are attributed to higher electronic transitions that were not included in this model. Similar deviations have been observed in related studies of other nucleobases.4552 The different relative Raman intensities of the vibrational bands shown in the UVRR spectra

(Figure 2) are directly reflected in the different experimental Raman cross sections (Figure 3) and excited-state slopes obtained (Table 1). To better constrain the parameter set that accurately describes the absorption spectra and fundamental resonance Raman excitation profiles, the overtones and combination bands in the 18002900 cm1 region were also measured at 257 nm. Although some overtones are expected at frequencies of >3000 cm1, these were obscured by the broad OH stretching vibrations of water. The overtone and combination band intensities are much more sensitive to the excited-state slopes (β/p), and this measurement provides an additional constraint on the excited-state parameters. The experimental and simulated cross sections for all of the observed overtones and combination bands of 20 -dG are listed in Table 2, showing a reasonably good match within experimental error. 10448

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Table 2. Experimental and Calculated Differential Resonance Raman Cross Sections for the Overtone and Combination Bands of 20 -dGa energy (cm1)

dσ/dΩexpt

dσ/dΩcalc

(1011 Å2 molecule1 sr1)

(1011 Å2 molecule1 sr1)

2682 2810

1321 + 1366 1321 + 1490

(0.60 ( 0.063)  102 (0.18 ( 0.019)  103

0.32  102 0.13  103

2973

1408 + 1573, 1366 + 1600, 2  1490

(0.20 ( 0.021)  103

0.18  103

1490 + 1573, 1490 + 1600, 1366 + 1682, 1408 + 1682

(0.21 ( 0.021)  10

0.23  103

3073 a

assignment

3

Excitation wavelength of 257 nm. Subscripts expt and calc represent experimental and calculated, respectively.

’ DISCUSSION Structural Dynamics. Here, we present a discussion of the initial excited-state structural dynamics on the La and Lb excited states. In Table 1, the resonance Raman-derived slopes of each normal mode on both the La and Lb potential energy surfaces are listed. The normal mode descriptions are from recent calculations53,54 and experiment.38 In resonance Raman spectroscopy, the most intense modes undergo the most initial structural changes in the molecular excited state (Table 1). For the La state, the highest initial excited-state slopes occur along the 1490, 1573, and 1682 cm1 modes. Accordingly, the N7dC8, C8N9, N3C4, C4dC5, C5dN7, C6dO, C5C6, and N1C6 bonds and the bond angles localized at C8 and N1 show the largest structural changes. This shows that the dynamics of the La state are mostly localized at C4C6 and C8, with some delocalization over the entire nucleobase (Scheme 1). These dynamics are consistent with the formation of the three damage products. As illustrated in Scheme 2, the 8-oxo-dG, formamidopyrimidine, and oxazolone derivatives have additional ketone, aldehyde, and amine groups, respectively, at the C8 position. The formamidopyrimidine product also shows cleavage of the C8N9 bond. Finally, the oxazolone derivative shows changes in structure at the C6 position as a result of the dissociation of the pyrimidine ring of the 20 -dG. All of these initial excited-state dynamics are consistent with these oxidative products. For the Lb state, the 1490 and 1573 cm1 modes have the highest initial excited-state slopes, indicating a similarity in excited-state structural dynamics between the two states, but the 1321 and 1600 cm1 modes in the Lb state also have high slopes. The 1321 and 1490 cm1 modes (Table 1) indicate that the structural dynamics are localized on or near the pyrrole ring (Scheme 1), where much of the nucleobasespecific damage also occurs (Scheme 2), while the 1600 cm1 mode correlates to the vibrations of the C2N bond and N1H bond angle, which aligns with the formation of the oxazolone derivative. The 1573 cm1 mode is delocalized over the connection between the pyrrole and pyrimidine rings. Therefore, an important conclusion of this work is that most of the initial excited-state dynamics of 20 -dG lie along the same modes implicated in 20 -dG photochemical and oxidative damage. Photochemical and Nonphotochemical Oxidative Dynamics. Although purine nucleobases have lower quantum yields for their photochemistry than the pyrimidine nucleobases, two main dimeric photoadducts have been reported for adenine.55 For guanine, the only known photochemical damage leads to an apurinic site via excitation to the CT state followed by cleavage of the glycosidic bond.48 Nonphotochemical damage includes oxidation of the pyrrole ring and formation of the decomposition products formamidopyrimidine and oxazolone11 (Scheme 2). The excited-state structural dynamics provide information about the internal coordinates that undergo distortions when

the 20 -dG is excited. If the excited-state structural dynamics of 20 dG are coincident with the photochemical reaction coordinate, we would expect to see resonance Raman intensity in modes that correlate with the formation of the CT state. For apurination, a change in the N9C10 bond length should be observed. The N9C10 mode is not observed in the resonance Raman spectrum. Among all the nucleobases, guanine has the lowest oxidation potential. It is the main site of electron loss in DNA and forms the guanine radical cation just before undergoing oxidative damage.12 Ab initio and DFT calculations13 reveal that the exocyclic amino group goes from partial sp3 pyramidalization in guanine to a planar structure in the guanine cation radical; conjugation of the NH2 group to the bicyclic π-electrons of the cation radical causes the planarization. Also, removing an electron from guanine decreases the number of πantibonding electrons. These changes in electronic structure shorten the N1C2, N3C4, C5N7, C6dO, and C8N9 bonds and lengthen the C2dN3, C4dC5, C5C6, N7dC8, and N1H bonds.58 These predicted structural changes upon oxidation in the CT state agree well with the measured initial excited-state structural dynamics of 20 -dG derived from the UVRR spectra. For the oxidative products, the 8-oxo-dG (Scheme 2) results from addition of the hydroxyl radical at C8 followed by oxidation, forming the ketone at the C8 position.11 Thus, structural changes at C8 are expected. The oxazolone product is formed by the addition of the hydroxyl radical to the C4 position of the 20 -dG, removal of a water molecule, oxidation, and rearrangement.11 For the oxazolone product, N7 in 20 -dG is replaced with O, which would affect both N7C8 and C5N7 stretching vibrations of 20 -dG. Also, two amine groups replace the H at C8. This in turn affects both the N7C8 and C4N9 stretches and the C8H bend. In addition, the sixmembered ring of the purine structure has decomposed leaving only the C4N3 and C5dO bonds, so one would expect to find a variety of distortion in the C6dO, C5C6, and C4C5 stretches and the C2N and N1H bends. The third oxidation product, the formamidopyrimidine, results from addition of the hydroxyl radical to C8 of 20 -dG followed by reduction.11 It involves the dissociation of the C8N9 bond with the formation of an aldehyde group at the C8 position. Formation of this product should affect the N7C8, C4N9, and C8N9 stretches and the C8H bend. For all of these oxidative products, both a change in the bond order of the N7C8 bond from double to single and a distortion of the C8H bond angle are common features. Therefore, if the initial excited-state structural dynamics lie along this nonphotochemical oxidation reaction coordinate, it is reasonable to expect that excitedstate distortions should involve a lengthening of the N7C8 bond, and a greater pyramidalization of N7 as the hybridization changes from sp2 to sp3. On the other hand, if the initial excited-state structural dynamics lie along the photochemical reaction coordinate, shortening of the N1C2, N3C4, C5N7, C6dO, and C8N9 bonds and lengthening of the C2dN3, C4dC5, C5C6, N7dC8, 10449

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The Journal of Physical Chemistry A and N1H bonds are expected to support the formation of the 20 dG•+ in the CT state. Also, lengthening of the N9C10 glycosidic bond is expected to support the formation of apurinic sites. Upon comparison with our experimental results, Table 1 shows that the vibrations at 1321, 1490, and 1573 cm1 are localized near the 20 dG•+ formation site at the photochemical CT reaction coordinate. None of the N9C10 stretching and bending modes appear in the resonance Raman spectrum for 20 -dG. It should be noted that these vibrations also coincide with the nonphotochemical base oxidation reaction site. Thus, the UV resonance Raman spectra demonstrate that the initial excited-state structural dynamics for 20 -dG lie coincidently with both the photochemical and oxidation reaction coordinates, and that depurination occurs later on the excited-state potential energy surface. Comparison with Other Nucleobases. To compare the initial excited-state dynamics of the nucleosides, we can compare the structural changes that a molecule experiences in the excited state. From previous studies of the pyrimidine nucleobases, the dynamics in thymine46 and 5-fluorouacil48 lie primarily along the C5dC6 bond lengthening coordinate, while in uracil,47 most of the structural dynamics occur along a C5 and C6 pyramidalization coordinate. For 9-methyladenine,49 the structural changes in the excited state primarily involve lengthening of the pyrrole ring bonds together with the stretching of the N3C4 and C5C6 bonds. All these initial excited-state dynamics lie coincidently with the respective photochemical reaction coordinates. For 20 -dG, the dynamics of the La state are mostly localized at C4C6 and C8, with some delocalization over the entire nucleobase, while for the Lb state, the structural dynamics are localized on or near the pyrrole ring. These initial excited-state dynamics are photochemically relevant and consistent with the formation of the 20 -dG•+. Because UV resonance Raman spectroscopy probes the early structural events occurring on the excited-state potential energy surface, it appears that the initial excited-state structural dynamics of all nucleobases lie along reaction coordinates that eventually lead to photochemistry.

’ CONCLUSION Self-consistent analysis of the resonance Raman spectra of 20 deoxyguanosine and its absorption spectrum provide insight into its initial excited-state structural dynamics in both the 1La and 1Lb states. The results presented here demonstrate that the structural dynamics of 20 -deoxyguanosine lie primarily along the 1490, 1573, and 1682 cm1 modes for the La excited state and 1321, 1490, 1573, and 1600 cm1 modes for the Lb excited state. The vibrations assigned to these modes exhibit significant intensity in the resonance Raman spectra and lie coincidently with both the photochemical and oxidative reaction coordinates. These results are significant for developing a molecular mechanism for UVinduced nucleic acid damage, with its important physiological consequences and implications for the origins of life. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: (780) 492-9704. Fax: (780) 492-8231.

’ ACKNOWLEDGMENT We thank the Natural Science and Engineering Research Council of Canada (NSERC) Discovery Grants-in-Aid program for partial support of this research.

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