Article pubs.acs.org/JPCA
Ab Initio Studies on the Photophysics of Uric Acid and Its Monohydrates: Role of the Water Molecule Shohei Yamazaki,*,†,‡ Shu-hei Urashima,§ Hiroyuki Saigusa,§ and Tetsuya Taketsugu‡ †
Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan ‡ Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan § Graduate School of Bio- and Nanosystem Sciences, Yokohama City University, Yokohama 236-0027, Japan S Supporting Information *
ABSTRACT: The photophysical behavior of three lowest-energy tautomers of uric acid and seven most stable isomers of uric acid monohydrate is comprehensively studied by ab initio calculations. Ground-state energies are calculated with the CCSD(T) method, while excitation and ionization energies as well as excited-state potential energy profiles of photoinduced processes are calculated with the CC2 method. For the 1ππ* state, it is found that the excitation energy of the monohydrate cluster is significantly lower than that of isolated uric acid when the water molecule is hydrogen-bonded at a specific carbonyl group. The calculated excited-state potential energy profiles suggest that some monohydrate isomers can undergo a migration of the water molecule from one site to another site in the 1 ππ* state with a small energy barrier. It is also found for both uric acid and its monohydrate that nonradiative decay via the NH bond dissociation in the 1πσ* state is likely to occur at higher excitation energies. On the basis of the computational results, possible mechanisms for the absence of specific isomers of uric acid monohydrate from the resonant two-photon ionization spectrum are discussed.
1. INTRODUCTION Photophysical behavior of nucleic acid bases is critical in the photostability and photodamage of DNA and RNA because these bases strongly absorb UV light. In particular, the excitedstate lifetime of canonical nucleobases is extremely short owing to intrinsic mechanisms of ultrafast nonradiative deactivation to the ground (S0) state, which provides a high degree of photostability.1−4 This fact also seems to support the hypothesis that the canonical form of the DNA/RNA bases has been selected for its photostability under hard UV radiation in the early stage of molecular evolution.5 For a deeper understanding of the photophysics of nucleobases, it is of great importance to elucidate the excitedstate behavior of base analogues and to make clear how it differs from that of natural DNA and RNA bases. The purine bases including 2-aminopurine,6−11 xanthine,12,13 and hypoxanthine14−17 are typical examples whose excited states have been extensively studied in experimental and theoretical works. However, the understanding about the base analogues is still limited compared to that for the natural bases. The present paper focuses on the theoretical study of the photophysics of uric acid, which is another typical purine base, and that of its monohydrated clusters. Uric acid is the final product of purine metabolism in humans, generated by oxidation of xanthine. The present study is motivated by the unexpected observation in the one-color resonant two-photon ionization (R2PI) spectra of uric acid and its monohydrates in a supersonic jet, recently reported by Asami et al.18 In the case of © 2014 American Chemical Society
isolated uric acid, the observed R2PI spectrum was assigned to the most stable tautomer (triketo form), while contribution from the next two lowest-energy tautomers (monoenol or diketo forms) was ruled out with the help of IR−UV doubleresonance spectroscopy. The molecular structure of the three lowest-energy tautomers is shown in Figure 1, where the triketo
Figure 1. Molecular structure of keto, enol(C2-OH), and enol(C8OH) tautomers of uric acid. The values in parentheses show the CCSD(T)/aug-cc-pVTZ energies (in kJ/mol) of S0 equilibrium structures relative to the keto tautomer, corrected with the CC2/ aug-cc-pVDZ zero-point vibrational energy. Received: December 4, 2013 Revised: January 10, 2014 Published: January 21, 2014 1132
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and monoenol (diketo) forms are labeled as keto and enol, respectively. For monohydrates, Asami et al. concluded that the R2PI spectrum consists of signals from the two most stable isomers of the keto tautomer, which form hydrogen bonds with the water molecule at different atomic sites of uric acid. However, quantum chemical calculations of the ground-state energies by the same authors predicted that at least five lowestenergy isomers, either of the keto or enol monohydrate, may be found within 5 kJ/mol in the S0 state. These experimental and computational results suggest that the R2PI signal from the third or less stable isomers is missing because of a specific photophysical mechanism even if these isomers have significant population in the S0 state. Several possibilities can be considered as the mechanism for the absence of the R2PI spectrum of a specific monohydrate isomer. First, the ionization potential (IP) of uric acid may be considerably raised by hydrogen bonding with water at a specific atomic site. Second, in one-color R2PI, which employs the same UV frequency for excitation and subsequent ionization, resonant ionization by the two photons would be prevented if the excitation energy of the monohydrate is substantially lower than that of bare uric acid. Third, onephoton resonance may be suppressed when the Franck− Condon (FC) factor for excitation is decreased by the formation of monohydrate. Another possibility for the failure to observe the R2PI spectrum of a specific species is that its excited-state lifetime is extremely short. The DNA/RNA base guanine is a prototype molecule for this type of R2PI missing. Mons et al.19 reported that the R2PI spectrum of guanine does not exhibit signal from the biologically relevant (keto−amino) tautomer but consists of the components from less stable tautomers. Ab initio calculations on the excited-state potential energy profiles of guanine tautomers revealed that the keto−amino form exhibits particularly efficient nonradiative deactivation.20−24 On the other hand, the R2PI spectrum of xanthine, an intermediate of purine metabolism formed by deamination of guanine, was assigned to the lowest-energy tautomer.12 This finding is consistent with subsequent ab initio calculations, suggesting a less efficient decay mechanism compared to the keto−amino tautomer of guanine.13 In the present work, we perform ab initio quantum chemical calculations for the neutrals and cations of uric acid and its monohydrates to reveal their photophysical properties. To this end, ionization and excitation energies as well as potential energy profiles of related photophysical processes are calculated for the three lowest-energy uric acid tautomers, that is, keto, enol(C2-OH), and enol(C8-OH) forms shown in Figure 1, and their monohydrated clusters. Note that the enol(C2-OH) and enol(C8-OH) tautomers have the OH group at the C2 and C8 positions, respectively. A particularly important purpose of this work is to elucidate the effect of monohydration on the photophysics of uric acid and its dependence on the hydration site. Possible mechanisms for the absence of R2PI signal from some energetically stable monohydrates are discussed on the basis of the computational results. In the excited-state potential energy calculations for nonradiative decay pathways, we consider dissociation of the NH bond in the 1πσ* state. This process is well-known as a mechanism of ultrafast deactivation of the excited state for the DNA/RNA bases guanine23,24 and adenine25−27 as well as many other organic molecules and their solvated clusters.25,28
For monohydrates, we also consider photoinduced isomerization, that is, migration of the water molecule from one site to another site in the excited state. Recent experimental and theoretical studies have revealed that the R2PI spectrum of a trihydrated cluster of 7-azaindole can be strongly suppressed by photoinduced rearrangement of the hydrogen bond network, presumably because of the lowering of the FC factor.29,30 If such a hydrogen bond rearrangement occurs via water migration in a specific monohydrate of uric acid, its R2PI signal may not be detected.
2. THEORETICAL METHODS The CC2 (second-order approximate coupled cluster) method31 was employed for electronic structure calculations of uric acid and its monohydrates, using the spin-restricted and unrestricted Hartree−Fock (RHF and UHF) wave functions as reference for neutral species and their cations, respectively, and applying the resolution of the identity (RI) approximation.32 Ground-state energies were calculated also with the CCSD(T) (coupled cluster singles and doubles with noniterative triples) method33 for more accurate estimation of relative energies of uric acid tautomers and monohydrate isomers. The CC2 and CCSD(T) calculations were performed using TURBOMOLE 6.334 and Gaussian 09,35 respectively. Equilibrium geometries in the S0 state were optimized at the CC2 level with the aug-cc-pVDZ basis set.36,37 For bare uric acid, the S0 single-point energies of the optimized structure were recalculated at the CCSD(T) level using the aug-cc-pVTZ basis set36,37 and corrected with CC2/aug-cc-pVDZ zero-point vibrational energies. For monohydrates, single-point energy calculations of the S0 equilibrium geometry were performed with the CCSD(T)/aug-cc-pVDZ method, followed by basis set superposition error (BSSE) correction with the counterpoise procedure38,39 at the same computational level and zeropoint energy calculation at the CC2/aug-cc-pVDZ level. We could not complete the CCSD(T)/aug-cc-pVTZ calculation of monohydrates because it requires much more computational cost than that of bare uric acid. Computing time for bare uric acid is reduced by symmetry (the S0 equilibrium geometry belongs to the Cs point group; see section 3.1), while that for monohydrate is not. Because the aug-cc-pVDZ and aug-ccpVTZ basis sets give similar relative energies for bare uric acid tautomers in the S0 state, it is expected that the same applies for the S0 relative energy of monohydrates. The vertical IP was calculated from the CC2/aug-cc-pVTZ energies of the ground and cationic states at the optimized S0 structure. For the adiabatic IP, the equilibrium geometry of the cationic state was optimized at the CC2/aug-cc-pVDZ level, and then, the energy difference between the neutral and cationic species for the respective optimized geometries was calculated with the CC2/aug-cc-pVTZ method. Minimum-energy structures in the excited state were also optimized with the CC2/aug-cc-pVDZ method. Adiabatic excitation energies were estimated by the CC2/aug-cc-pVTZ single-point calculations for the ground- and excited-stateoptimized structures, while vertical excitation energies were calculated at the same level for the S0 structure. For dissociation of the NH bond and migration of the water molecule, excited-state reaction paths were determined by using CC2/aug-cc-pVDZ geometry optimization. A selected driving coordinate was fixed, while the other internal coordinates were optimized in the target excited state. The Cs symmetry of the molecular structure was enforced for the NH dissociation, while 1133
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the C1 symmetry was used for the water migration. The potential energy profiles along the determined paths were recalculated at the CC2/aug-cc-pVTZ level. In the CC2 single-point calculations of IPs and excitation energies, the aug-cc-pVDZ and aug-cc-pVTZ basis sets exhibit similar values that differ on the order of 0.1 eV. Thus, the discussion in the following sections is little affected by the choice of basis set between them.
3. RESULTS AND DISCUSSION 3.1. Ground-State Energies. The lower part of Figure 1 shows the ground-state equilibrium geometries of the three lowest-energy tautomers of isolated uric acid. The respective CCSD(T)/aug-cc-pVTZ relative energies, corrected with CC2/ aug-cc-pVDZ zero-point vibrational energies, are indicated in parentheses. All three tautomers exhibit a planar structure belonging to the Cs point group. The keto tautomer is calculated to be energetically the most stable form, while the enol(C2-OH) and enol(C8-OH) tautomers are less stable by 7.5 and 8.6 kJ/mol, respectively. These relative energies are consistent with previous theoretical results18,40−42 and support that the keto tautomer is the most likely to be detected in the R2PI spectrum of bare uric acid due to the largest population in the S0 state. Note that the energetic order of the enol(C2-OH) and enol(C8-OH) forms is different from the order of CCSD(T) energies in ref 18 (7.9 and 7.4 kJ/mol, respectively, relative to the keto form). This is because a different basis set is used for the single-point energy calculation and a different computational method is applied for the zero-point vibrational energy correction. In the ground-state calculation of uric acid monohydrates, we consider the doubly hydrogen-bonded isomers, which have two intermolecular N−H···O or O−H···O bonds. Figure 2 shows the molecular structure of the seven isomers that are calculated to be the most stable at the present computational level. The five isomers in Figure 2a and the two isomers in Figure 2b are monohydrates of the keto and enol(C8-OH) tautomers, respectively, represented by the letters “k” and “e” in the label of each isomer. For monohydrates of the enol(C2-OH) form, all considered isomers exhibit higher S0 energies than the isomers in Figure 2. The following letter “W” in the label is short for water, and the last two digits indicate the position of the uric acid atomic sites that are hydrogen-bonded by the water molecule. All isomers in Figure 2 other than kW39 exhibit a nonplanar structure at the S0 minimum, where the hydrogen atom of a free (non-hydrogen-bonded) OH bond is out of the molecular plane. For the kW39 isomer, the equilibrium geometry is in the Cs symmetry, where the two free OH bonds are placed symmetrically with respect to the molecular plane. Figure 2 also shows CCSD(T)/aug-cc-pVDZ relative energies of monohydrate isomers, where the BSSE correction and zero-point energy correction are employed. The groundstate calculations in the present work indicate that the first and second lowest-energy isomers are kW89 and kW23 forms, respectively, in agreement with previous theoretical results.18,43 The third most stable form is the kW67 isomer, which lies 2.9 kJ/mol higher in energy than the kW89 isomer at the present computational level. The S0 energies of the kW39, kW78, eW67, and eW89 isomers are calculated to be 4.8, 7.6, 3.8, and 9.3 kJ/mol, respectively, relative to the kW89 isomer. 3.2. Photophysics of Isolated Uric Acid. Table 1 summarizes ionization energies of isolated uric acid tautomers
Figure 2. Molecular structure of lowest-energy isomers of uric acid monohydrate: (a) keto and (b) enol forms. The values in parentheses show the CCSD(T)/aug-cc-pVDZ energy (in kJ/mol) of the S0 equilibrium structure relative to the kW89 isomer, including CC2/ aug-cc-pVDZ zero-point vibrational energy correction and CCSD(T)/ aug-cc-pVDZ counterpoise correction.
Table 1. IPs (in eV) of Uric Acid Tautomers Calculated at the CC2/aug-cc-pVTZ Level keto Vertical Ionization 8.62 π−1 n−1 10.55 Adiabatic Ionization π−1 8.24 n−1 10.22
enol(C2-OH)
enol(C8-OH)
8.47 10.51
8.81 10.31
8.13 10.09
8.42 10.04
calculated at the CC2/aug-cc-pVTZ level. In the keto tautomer, vertical and adiabatic IPs for ionization from the highest π orbital (denoted as π−1 ionization) are calculated to be 8.62 and 8.24 eV, respectively, which are in good agreement with the experimental values determined by photoelectron spectroscopy (8.55 and 8.15 eV).18,44 The enol(C2-OH) tautomer exhibits similar IPs for the π−1 ionization. On the other hand, the π−1 IPs of the enol(C8-OH) tautomer are considerably higher than those of other tautomers, which suggests that this tautomer is less likely to be ionized via the R2PI method. The vertical and adiabatic IPs for the ionization from the lone pair orbital (referred to as n−1 ionization) are more than 10 eV for all three tautomers. This result indicates that the n−1 ionization is unlikely to contribute to the R2PI spectrum reported in ref 18. Vertical excitation energies of the lowest 1ππ*, 1πσ*, and 1 nπ* states calculated at the CC2/aug-cc-pVTZ level are listed in Table 2. The lowest 1ππ* states of the keto, enol(C2-OH), and enol(C8-OH) tautomers exhibit excitation energies of 4.59, 4.45, and 4.92 eV, respectively. The calculated energies of the keto and enol(C2-OH) tautomers, which correspond to the 1134
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Table 2. Excitation Energies (in eV, Oscillator Strengths in Parentheses) of Uric Acid Tautomers Calculated at the CC2/aug-cc-pVTZ Level keto Vertical Excitation ππ* 4.59 (0.19) 1 πσ* 4.77 (2.5 × 10−4) 1 nπ* 5.11 (
