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Spectral origins and ionization potentials of guanine tautomers: Theoretical elucidation of experimental findings
Chemical Physics Letters 429 (2006) 261–265 www.elsevier.com/locate/cplett
Spectral origins and ionization potentials of guanine tautomers: Theoretical elucidation of experimental findings M.K. Shukla, Jerzy Leszczynski
*
Computational Centre for Molecular Structure and Interactions, Department of Chemistry, Jackson State University, Jackson, MS 39217, United States Received 17 July 2006; in final form 8 August 2006 Available online 12 August 2006
Abstract The HF, B3LYP and MP2 methods along with the 6-311G(d,p) and 6-311++G(d,p) basis sets have been used to compute the relative stability, ionization potentials and spectral origins corresponding to the electronic lowest singlet pp* excited states of 28 possible guanine tautomers in the gas phase to supplement the experimental data. The CIS/6-311G(d,p) level of theory was used to optimize electronic singlet pp* excited state geometries. The TDDFT method and the 6-311G(d,p) basis set was used to compute spectral origins (0–0 transitions) using the ground and excited state optimized geometries. Significant variations in the ionization potentials and spectral origins among guanine tautomers were revealed. We believe that the computed results would be helpful to interpret the complex R2PI spectra of guanine obtained in the supersonic jet-beam experiments. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Guanine is one of the important building blocks of nucleic acids and shows distinct characteristics among all nucleic acid bases. It has the maximum number of tautomers in different environments, lowest ionization potential and maximum negative vertical electron affinity among all bases [1–4]. Therefore, due to the lowest ionization potential, guanine is the most susceptible site for oxidation in DNA. The investigations of tautomeric properties of nucleic acid bases have paramount importance due to the possibility of their involvement in mutation. The relative stability of guanine tautomers is found to be significantly influenced by the molecular environment [1]. Earlier experimental investigation based upon the infra-red (IR) spectroscopic analysis of guanine in the argon matrix has suggested the presence of both keto and enol tautomers in the equal proportions [5]. However, in the polar solvent the keto-N9H form dominates [1]. In general, theoretical methods agree that in the gas phase the keto-N7H tautomer is the most stable and in water solution the tautomeric *
Corresponding author. E-mail address: [email protected] (J. Leszczynski).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.08.037
stability is shifted towards the keto-N9H tautomer, consequently, the latter is the most stable in the aqueous environment. At the MP2 and CCSD(T) level along with several large basis sets, the four low energy tautomers of guanine (keto-N9H, keto-N7H and cis- and trans- forms of enol-N9H) have been shown to be within 1 kcal/mol of energy [6]. Recently, state-of-the-art experimental studies have been carried out to investigate the phenomena of tautomerism of guanine in the gas phase including that of the spectral origins of the first singlet transitions and vibrational frequencies. In a jet-cooled spectroscopic investigation Nir et al. [7] have shown the existence of three tautomers of guanine, namely, the enol-N9H (32870 cm 1), ketoN7H (33274 cm 1) and keto-N9H (33914 cm 1). In a similar study Mons et al. [8] have identified the four tautomers (keto-N9H, keto-N7H, enol-N9H and enol-N7H) of guanine. The main difference between these two studies was that the enol-N9H tautomer assigned by earlier authors was reassigned as the enol-N7H tautomer and the enol-N9H tautomer was assigned with the spectral origin at 34755 cm 1 by the latter authors. A recent study by Choi and Miller [9] based on the IR-spectroscopy of guanine trapped in helium droplets and the MP2 level of theoretical
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calculation using the 6-311++G(d,p) and aug-cc-pVDZ basis sets shows the presence of only keto-N9H, ketoN7H and cis- and trans-forms of enol-N9H tautomer. Based on the results of Choi and Miller [9], Mons et al. [10] have proposed the reassignment of their experimental findings and accordingly the enol-N9H-trans, enol-N7H and two rotamers of the keto-N7H-imino tautomers of guanine are present in the supersonic jet-beam. However, it is surprising, since imino tautomers are much less stable than the canonical form of guanine and probably they are formed during the laser desorption of guanine in the experiments [7,8]. On the other hand, LeBreton and coworkers [11] studied photoelectron spectra of guanine and some methyl derivatives in the gas phase by heating the samples. Based on the similarity of photoelectron spectra of guanine and 7-methylguanine, it was concluded that the keto-N7H tautomer of guanine is the most stable form in the gas phase. Thus, the current knowledge regarding the tautomers of guanine in the gas phase is still somewhat uncertain and therefore, detailed information about the relative stability, ionization potentials and the spectral origin of the first pp* excited state of all possible tautomers including rotamers of guanine is needed. In this letter, we present results of above mentioned properties of all possible guanine tautomers with the objective that our theoretical results would be very valuable for experimentalists in analyzing the complex resonance enhanced two photon ionization (R2PI) experimental data. 2. Computational details Ground state geometries were optimized at the B3LYP level and single point energies were computed at the MP2 level using the 6-311++G(d,p) basis set at both levels. Geometries of all tautomers in the electronic lowest singlet excited states were optimized at the CI-Singles (CIS) level using the 6-311G(d,p) basis set. Since the CIS level is the HF analog for the excited state [12], therefore, ground state geometries were also optimized at the HF/6-311G(d,p) level. In order to obtain transition energies at the same level of the theoretical accuracy, the vertical singlet electronic transition energies were computed at the time-dependent density functional theory (TDDFT) level employing the B3LYP functional and the 6-311G(d,p) basis set using the HF ground state optimized geometries. The spectral origins (0–0 transitions) corresponding to the lowest singlet pp* excited states of guanine tautomers were computed as the energy difference between the ground state energy obtained at the B3LYP/6-311G(d,p)//HF/6-311G(d,p) level and the corresponding singlet pp* excited state energy obtained at the TD-B3LYP/6-311G(d,p)//CIS/6-311G(d,p) level. The vibrational frequency analysis was performed to ascertain the ground and excited state potential energies surfaces, all geometries were found to be minima at the corresponding potential energy surfaces. Relative total energies at the B3LYP/6-311++G(d,p) level were corrected for the scaled (scaling factor 0.9877) zero point energy
(ZPE) [13]. The GAUSSIAN-03 program was used in all calculations [14].
3. Results and discussion The ground state relative energies of guanine tautomers in the gas phase obtained at the MP2/6-311++G(d,p)// B3LYP/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels are presented in the Table 1. Structures of all tautomers along with their nomenclatures are shown in the Fig. 1. It should be noted that all tautomers in the Table 1 are arranged in the order of the increasing relative energy obtained at the MP2/6-311++G(d,p)//B3LYP/6311++G(d,p) level. It is evident from the Table 1 that both methods generally predict the similar order of relative stability of guanine tautomers, except few exceptions which are for higher energy tautomers. The keto-N7H tautomer is predicted to be the most stable in the gas phase and this results is in agreement with earlier theoretical results obtained at the CCSD(T)//MP2 level using several large basis sets [6]. The gas phase ultraviolet photoelectron study of guanine and methylguanine also suggested the ketoN7H tautomer to be more stable than the keto-N9H form and this conclusion was based on the similarity of the photoelectron spectra of guanine with 7-methylguanine [11]. The four tautomers of guanine (keto-N7H, keto-N9H, enol-N9H and enol-N9H-trans) are within the range of 0–1.1 kcal/mol (Table 1). Further, all enol-imino tautomers are found to be about 20–50 kcal/mol less stable than the most stable keto-N7H tautomer in the gas phase. The computed dipole moments of all tautomers are also shown in the Table 1. It is evident that the keto-N7H tautomer has the lowest and the keto-N9H tautomer has the highest dipole moment among tautomers within the 0–1.1 kcal/ mol energy range. Further, due to the large dipole moment, the keto-N9H tautomer will be more stable than the ketoN7H tautomer in the water solution [1]. The tautomeric stability order will also be modified in the water solution, but due to the large energy difference it is highly unlikely that unstable tautomers will become stable in the water solution. Computed first vertical ionization potentials of all tautomers in the gas phase obtained at the B3LYP/6311++G(d,p) level are shown in the Table 1. There are several theoretical investigations performed to study the vertical and adiabatic ionization potentials of guanine, but all these studies are limited to only relatively stable tautomers whose energies are within few kcal/mol of the most stable tautomer [15–17]. These investigations have suggested that the B3LYP is a reliable method to predict the ionization potential of nucleic acid bases. Due to the recent gas phase spectroscopic investigations of guanine tautomers and difficulty in the R2PI spectral assignments, a reliable knowledge for the ionization potentials of all guanine tautomers is necessary [7–10]. Quantitative information about ionization potentials of different tautomers will be
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Table 1 Computed relative energies (DE, kcal/mol), dipole moments (l, Debye), vertical ionization potentials (IP, eV), electronic lowest singlet pp* vertical (TEv, eV) and adiabatic transition energies (0–0, eV) and corresponding oscillator strengths of guanine tautomersa Tautomers
K-N7H K-N9H E-N9H E-N9H-trans E-N7H K-N7H-N3H K-N7H-imino-cis K-N7H-imino E-N7H-trans K-N9H-imino K-N9H-imino-cis K-N9H-N3H E-N7H-IMN3 E-N9H-IMN3 E-N7H-IMN3-cis E-N9T-IMN1 E-N9T-IMN3 E-N7T-IMN3 E-N9H-IMN3-cis E-N9H-IMN1 E-N9T-IMN1-cis E-N9T-IMN3-cis E-N7T-IMN3-cis E-N9H-IMN1-cis E-N7T-IMN1 E-N7H-IMN1 E-N7T-IMN1-cis E-N7H-IMN1-cis
IP
Transition Energy
MP2
DFT
MP2
DFT
DFT
TEv
f
0–0
0.0 0.3 0.4 1.0 3.5 6.4 8.1 8.2 11.8 16.0 17.9 19.0 19.7 24.3 24.8 25.2 25.9 29.7 31.2 31.9 32.4 33.8 35.7 40.2 40.7 40.8 48.5 49.5
0.0 0.4 1.6 2.3 4.4 6.4 6.2 5.9 11.9 13.8 15.5 19.2 18.3 23.0 23.5 22.6 24.5 27.3 29.5 28.5 29.1 31.8 33.4 36.1 36.5 35.7 43.8 43.8
2.0 6.3 3.1 3.8 4.1 4.4 3.8 2.7 4.9 5.8 8.7 10.6 5.6 4.4 5.3 4.4 6.9 7.7 7.2 1.7 4.6 9.3 7.9 3.3 8.7 7.9 10.1 9.8
1.8 6.7 3.2 3.7 3.9 4.8 4.1 2.7 4.8 6.1 9.0 11.0 5.9 4.5 5.6 4.6 7.0 8.0 7.2 1.9 4.7 9.4 8.3 3.4 8.9 8.0 10.3 9.9
8.16 8.02 8.00 8.02 8.05 8.44 8.25 8.18 8.08 8.18 8.26 8.41 7.71 7.75 7.78 7.47 7.77 7.73 7.81 7.46 7.49 7.84 7.80 7.47 7.48 7.38 7.53 7.46
4.96 5.19 5.00 4.96 4.60 5.40 4.62 4.53 4.54 5.10 5.19 5.38 3.92 4.41 4.05 3.74 4.42 3.86 4.54 3.76 3.78 4.54 3.99 3.79 3.44 3.39 3.53 3.43
0.1028 0.1510 0.1151 0.1241 0.0677 0.1843 0.0630 0.0641 0.0627 0.0559 0.0445 0.0439 0.0770 0.0906 0.0843 0.0539 0.0919 0.0713 0.0922 0.0541 0.0560 0.0939 0.0778 0.0564 0.0487 0.0514 0.0517 0.0602
4.42 4.30 4.55 4.56 4.19 4.81 4.29 4.19 3.83 4.34 4.34 4.59 3.33 3.77 3.44 3.18 3.69 2.89 3.85 2.89 3.23 3.80 2.97 2.88 2.69 2.72 2.82 2.79
l
DE
The MP2 represents the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) level and DFT represents the B3LYP/6-311++G(d,p) level. The relative energies at the B3LYP/6-311++G(d,p) level was corrected for the scaled (scaling factor 0.9877) ZPE. The vertical and adiabatic transition energies were obtained at the TD-B3LYP/6-311G(d,p) level using the HF/6-311G(d,p) and CIS/6-311G(d,p) geometries (see Section 2 for an explanation). a K-represents keto and E-represents enol.
very informative for an experimentalist to selectively ionize guanine tautomeric forms in the multiphoton ionization experiment in the supersonic jet. LeBreton and coworkers [11,18] have performed an extensive gas phase He(I) photoelectron spectroscopic investigations on guanine and different methylguanine. The first vertical ionization energies of guanine, 1-methylguanine, 7-methylguanine, 9-methylguanine, 1,9-dimethylguanine and O6,9-dimethylguanine were found to be 8.28, 7.98, 8.16, 8.02, 8.09 and 7.96 eV, respectively [11,18]. Since, the photoelectron spectra of guanine and 7-methylguanine were found similar [11] therefore, it would be important to compare the computed ionization potentials of guanine tautomers with the corresponding methyl substituted guanine. The keto-N7H tautomer is predicted to have the largest ionization potential (8.16 Debye) among tautomers whose relative energies are within 1.1 kcal/mol of the most stable one (Table 1). Thus our computed ionization potential for the ketoN7H tautomer at the B3LYP/6-311++G(d,p) level is in the excellent agreement with the corresponding experimental value of the 7-methylguanine in the photoelectron spectra [11]. Further, the calculated value is also close to the experimental ionization potential of guanine at the 8.28 eV. Thus, our calculation also support the findings
of LeBreton and coworkers [11] that in the gas phase the keto-N7H form of the guanine will be prevalent. The computed ionization potential of the keto-N9H tautomer at 8.02 eV is also in the excellent agreement with the corresponding experimental value at 8.02 and 7.98 eV for the 9-methylguanine and 1-methylguanine respectively. Similarly, the computed ionization potential of enol-N9H tautomer at the 8.0 eV can also be compared with the corresponding experimental value of O6,9-methylguanine at 7.96 eV [18]. Thus, B3LYP/6-311++G(d,p) level calculation is able to predict the first vertical ionization potentials of guanine tautomers accurately. We believe that the computed first vertical ionization potentials of other tautomer, for which the corresponding experimental data is not available, are also reliable. Further, it is clear from the Table 1 that the ionization potentials of all enol-imino tautomers are generally less than 8.0 eV, while for the keto-N3H tautomer it is more than 8.4 eV. The vertical and adiabatic transition energies corresponding to the lowest singlet pp* excited state of all guanine tautomers are also presented in the Table 1. It has been shown earlier by us as well as other authors that the time-dependent density functional theory method provides efficient and reliable option for the transition energies
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Fig. 1. Structures of the N9H tautomers of guanine. The corresponding N7H forms can be obtained by replacing hydrogen attached to the N9 site to the N7 site of guanine. K represents the keto and E represents the enol.
calculation of nucleic acid bases [19–21]. Further, computed first vertical singlet pp* transition of the keto-N7H tautomer is red-shifted compared to the corresponding transition of the keto-N9H tautomer. This prediction is in agreement with the experimental result which shows that the first absorption transition of the 7-methylguanine is red-shifted compared to the corresponding transition of guanosine monophosphate (GMP) [22]. Further, it is evident from the data shown in the Table 1 that the first vertical singlet pp* transition of other tautomers of guanine are generally red-shifted compared to the corresponding transition of the keto-N9H tautomer, except for ketoN9H-N3H and keto-N7H-N3H tautomers which show significant blue-shift. Generally all unstable tautomers show significant red-shift in the first absorption transition compared to the corresponding transition of the keto-N9H tautomer. An elaborated description of several absorption transitions of guanine can be found in our recent publication, where a detailed study about vertical singlet transitions of nucleic acid bases obtained at the TDDFT level with different large basis sets including several sets of diffuse functions are reported [19]. Therefore, we will discuss mainly the adiabatic transitions corresponding to the lowest singlet pp* excited state of guanine tautomers in an attempt to provide a guide for experimentalists working
in the area of multiphoton ionization of nucleic acid bases in the supersonic jet expansion. Mons et al. [8] have assigned the spectral origin of the enol-N9H, keto-N9H, keto-N7H and enol-N7H tautomers in the R2PI experiments at 4.31, 4.20, 4.12 and 4.07 eV, respectively. However, these authors have recently reassigned their R2PI spectra of guanine and accordingly transitions at 4.31, 4.20, 4.12 and 4.07 eV correspond to the spectral origin of enol-N9H-trans, keto-N7H-imino-cis, keto-N7H-imino and enol-N7H forms, respectively [10]. This reinterpretation was based on the comparison of the experimental IR frequency of guanine tautomers in the He droplet [9], experimental IR frequency in the R2PI experiments [8] and the theoretical vibrational frequencies at the B3LYP/6-31+G(d) level. Our theoretical results qualitatively show the similar trend in the spectral origin of guanine tautomers as suggested in the reassigned R2PI spectra [10]. Thus it appears that the computed disagreement in the spectral origin of the keto-N9H and ketoN7H tautomers in our earlier theoretical investigation [23] was due to the older assignment of the R2PI data [8]. It should be noted that in our investigation the spectral origin of the keto-N9H tautomer was predicted to be redshifted compared to that of the keto-N7H tautomer while the contradictory result was found in the experiment
M.K. Shukla, J. Leszczynski / Chemical Physics Letters 429 (2006) 261–265
[8,23]. Thus, in view of the qualitative agreement in between our computed spectral origin and the reinterpreted R2PI spectra [10], it appears that the spectral origin of the keto-N9H tautomer will be red-shifted than that of the enol-N9H tautomer and the spectral origin of the ketoN7H tautomer will be in between that of the keto-N9H and the enol-N9H tautomer. 4. Conclusions The keto-N9H, keto-N7H and two rotamers of the enolN9H tautomer have similar stability in the gas phase with electronic energy within 0–1.0 kcal/mol range; the ketoN7H tautomer being the most stable. The first vertical ionization potentials of these tautomers are about 8.0 eV, except the keto-N7H tautomer which has the 8.16 eV. Among all possible guanine tautomers, we have found that generally higher energy tautomers (the relative energy more than 20 kcal/mol) have lower ionization potentials. The computed spectral origin values corresponding to the lowest singlet pp* excited state of guanine tautomers are in the qualitative agreement with the reinterpreted R2PI data. We hope that computed ionization potentials and spectral origins of guanine tautomers would be very useful for experimentalists in identifying the stable tautomers of guanine in the R2PI experiments. Acknowledgements We are thankful to Prof. Michel Mons, Laboratoire Francis Perrin – URA CNRS 2453 – Service des Photons, Atomes et Molecules, CEA Saclay, Bat 522, 91191 Gif-surYvette, Cedex, France for reading the final version of our manuscript and for his critical suggestions. Authors are also thankful to financial supports from NSF-CREST Grant No. HRD-0318519, ONR Grant No. N00034-031-0116 and NSF-EPSCoR Grant No. 02-01-0067-08/ MSU. Authors are also thankful to the Mississippi Center for Supercomputing Research (MCSR) for the generous computational facility.
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References [1] J. Leszczynski, in: M. Hargittai, I. Hargittai (Eds.), Advances in Molecular Structure and Research, vol. 6, JAI Press Inc., Stamford, Connecticut, 2000. [2] X. Li, Z. Cai, M.D. Sevilla, J. Phys. Chem. B 105 (2001) 10115, and references cited therein. [3] K. Aflatooni, G.A. Gallup, P.D. Burrow, J. Phys. Chem. A 102 (1998) 6205. [4] X. Li, Z. Cai, M.D. Sevilla, J. Phys. Chem. A 106 (2002) 1596, and references cited therein. [5] G.G. Sheina, S.G. Stepanian, E.D. Radchenko, Yu.P. Blagoi, J. Mol. Struct. 158 (1987) 275. [6] L. Gorb, A. Kaczmarek, A. Gorb, A.J. Sadlej, J. Leszczynski, J. Phys. Chem. B 109 (2005) 13770. [7] E. Nir, Ch. Janzen, P. Imhof, K. Kleinermanns, M.S. de Vries, J. Chem. Phys. 4604 (2001) 115. [8] M. Mons, I. Dimicoli, F. Piuzzi, B. Tardivel, M. Elhanine, J. Phys. Chem. A 5088 (2002) 106. [9] M.Y. Choi, R.E. Miller, J. Am. Chem. Soc. 128 (2006) 7320. [10] M. Mons, F. Piuzzi, I. Dimicoli, L. Gorb, J. Leszczynki, J. Phys. Chem. A (2006), accepted for publication. [11] J. Lin, C. Yu, S. Peng, I. Akiyama, K. Li, L.K. Lee, P.R. LeBreton, J. Phys. Chem. 84 (1980) 1006. [12] J.B. Foresman, M. Head-Gordon, J.A. Pople, M.J. Frisch, J. Phys. Chem. 96 (1992) 135. [13] M.P. Anderson, P. Uvdal, J. Phys. Chem. A 109 (2005) 2937. [14] M.J. Frisch et al., GAUSSIAN 03, Revision D.01, Gaussian, Inc., Wallingford, CT, 2004. [15] C.E. Crespo-Hernandez, R. Arce, Y. Ishikawa, L. Gorb, J. Leszczynski, D.M. Close, J. Phys. Chem. A 108 (2004) 6373. [16] O. Dolgounitcheva, V.G. Zakrzewski, J.V. Ortiz, J. Am. Chem. Soc. 122 (2000) 12304. [17] S.D. Wetmore, R.J. Boyd, L.A. Eriksson, Chem. Phys. Lett. 322 (2000) 129. [18] P.R. LeBreton, X. Yang, S. Urano, S. Fetzer, M. Yu, J. Am. Chem. Soc. 112 (1990) 2138. [19] M.K. Shukla, J. Leszczynski, J. Comput. Chem. 25 (2004) 768. [20] A. Tsolakidis, E. Kaxiras, J. Phys. Chem. A 109 (2005) 2373. [21] D. Varsano, R.D. Felice, M.A.L. Marques, A. Rubio, J. Phys. Chem. B 110 (2006) 7129. [22] R.W. Wilson, J.P. Morgan, P.R. Callis, Chem. Phys. Lett. 36 (1975) 618. [23] M.K. Shukla, Jerzy Leszczynski, J. Phys. Chem. A 109 (2005) 7775.
Spectral origins and ionization potentials of guanine tautomers: Theoretical elucidation of experimental findings M.K. Shukla, Jerzy Leszczynski
*
Computational Centre for Molecular Structure and Interactions, Department of Chemistry, Jackson State University, Jackson, MS 39217, United States Received 17 July 2006; in final form 8 August 2006 Available online 12 August 2006
Abstract The HF, B3LYP and MP2 methods along with the 6-311G(d,p) and 6-311++G(d,p) basis sets have been used to compute the relative stability, ionization potentials and spectral origins corresponding to the electronic lowest singlet pp* excited states of 28 possible guanine tautomers in the gas phase to supplement the experimental data. The CIS/6-311G(d,p) level of theory was used to optimize electronic singlet pp* excited state geometries. The TDDFT method and the 6-311G(d,p) basis set was used to compute spectral origins (0–0 transitions) using the ground and excited state optimized geometries. Significant variations in the ionization potentials and spectral origins among guanine tautomers were revealed. We believe that the computed results would be helpful to interpret the complex R2PI spectra of guanine obtained in the supersonic jet-beam experiments. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Guanine is one of the important building blocks of nucleic acids and shows distinct characteristics among all nucleic acid bases. It has the maximum number of tautomers in different environments, lowest ionization potential and maximum negative vertical electron affinity among all bases [1–4]. Therefore, due to the lowest ionization potential, guanine is the most susceptible site for oxidation in DNA. The investigations of tautomeric properties of nucleic acid bases have paramount importance due to the possibility of their involvement in mutation. The relative stability of guanine tautomers is found to be significantly influenced by the molecular environment [1]. Earlier experimental investigation based upon the infra-red (IR) spectroscopic analysis of guanine in the argon matrix has suggested the presence of both keto and enol tautomers in the equal proportions [5]. However, in the polar solvent the keto-N9H form dominates [1]. In general, theoretical methods agree that in the gas phase the keto-N7H tautomer is the most stable and in water solution the tautomeric *
Corresponding author. E-mail address: [email protected] (J. Leszczynski).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.08.037
stability is shifted towards the keto-N9H tautomer, consequently, the latter is the most stable in the aqueous environment. At the MP2 and CCSD(T) level along with several large basis sets, the four low energy tautomers of guanine (keto-N9H, keto-N7H and cis- and trans- forms of enol-N9H) have been shown to be within 1 kcal/mol of energy [6]. Recently, state-of-the-art experimental studies have been carried out to investigate the phenomena of tautomerism of guanine in the gas phase including that of the spectral origins of the first singlet transitions and vibrational frequencies. In a jet-cooled spectroscopic investigation Nir et al. [7] have shown the existence of three tautomers of guanine, namely, the enol-N9H (32870 cm 1), ketoN7H (33274 cm 1) and keto-N9H (33914 cm 1). In a similar study Mons et al. [8] have identified the four tautomers (keto-N9H, keto-N7H, enol-N9H and enol-N7H) of guanine. The main difference between these two studies was that the enol-N9H tautomer assigned by earlier authors was reassigned as the enol-N7H tautomer and the enol-N9H tautomer was assigned with the spectral origin at 34755 cm 1 by the latter authors. A recent study by Choi and Miller [9] based on the IR-spectroscopy of guanine trapped in helium droplets and the MP2 level of theoretical
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calculation using the 6-311++G(d,p) and aug-cc-pVDZ basis sets shows the presence of only keto-N9H, ketoN7H and cis- and trans-forms of enol-N9H tautomer. Based on the results of Choi and Miller [9], Mons et al. [10] have proposed the reassignment of their experimental findings and accordingly the enol-N9H-trans, enol-N7H and two rotamers of the keto-N7H-imino tautomers of guanine are present in the supersonic jet-beam. However, it is surprising, since imino tautomers are much less stable than the canonical form of guanine and probably they are formed during the laser desorption of guanine in the experiments [7,8]. On the other hand, LeBreton and coworkers [11] studied photoelectron spectra of guanine and some methyl derivatives in the gas phase by heating the samples. Based on the similarity of photoelectron spectra of guanine and 7-methylguanine, it was concluded that the keto-N7H tautomer of guanine is the most stable form in the gas phase. Thus, the current knowledge regarding the tautomers of guanine in the gas phase is still somewhat uncertain and therefore, detailed information about the relative stability, ionization potentials and the spectral origin of the first pp* excited state of all possible tautomers including rotamers of guanine is needed. In this letter, we present results of above mentioned properties of all possible guanine tautomers with the objective that our theoretical results would be very valuable for experimentalists in analyzing the complex resonance enhanced two photon ionization (R2PI) experimental data. 2. Computational details Ground state geometries were optimized at the B3LYP level and single point energies were computed at the MP2 level using the 6-311++G(d,p) basis set at both levels. Geometries of all tautomers in the electronic lowest singlet excited states were optimized at the CI-Singles (CIS) level using the 6-311G(d,p) basis set. Since the CIS level is the HF analog for the excited state [12], therefore, ground state geometries were also optimized at the HF/6-311G(d,p) level. In order to obtain transition energies at the same level of the theoretical accuracy, the vertical singlet electronic transition energies were computed at the time-dependent density functional theory (TDDFT) level employing the B3LYP functional and the 6-311G(d,p) basis set using the HF ground state optimized geometries. The spectral origins (0–0 transitions) corresponding to the lowest singlet pp* excited states of guanine tautomers were computed as the energy difference between the ground state energy obtained at the B3LYP/6-311G(d,p)//HF/6-311G(d,p) level and the corresponding singlet pp* excited state energy obtained at the TD-B3LYP/6-311G(d,p)//CIS/6-311G(d,p) level. The vibrational frequency analysis was performed to ascertain the ground and excited state potential energies surfaces, all geometries were found to be minima at the corresponding potential energy surfaces. Relative total energies at the B3LYP/6-311++G(d,p) level were corrected for the scaled (scaling factor 0.9877) zero point energy
(ZPE) [13]. The GAUSSIAN-03 program was used in all calculations [14].
3. Results and discussion The ground state relative energies of guanine tautomers in the gas phase obtained at the MP2/6-311++G(d,p)// B3LYP/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels are presented in the Table 1. Structures of all tautomers along with their nomenclatures are shown in the Fig. 1. It should be noted that all tautomers in the Table 1 are arranged in the order of the increasing relative energy obtained at the MP2/6-311++G(d,p)//B3LYP/6311++G(d,p) level. It is evident from the Table 1 that both methods generally predict the similar order of relative stability of guanine tautomers, except few exceptions which are for higher energy tautomers. The keto-N7H tautomer is predicted to be the most stable in the gas phase and this results is in agreement with earlier theoretical results obtained at the CCSD(T)//MP2 level using several large basis sets [6]. The gas phase ultraviolet photoelectron study of guanine and methylguanine also suggested the ketoN7H tautomer to be more stable than the keto-N9H form and this conclusion was based on the similarity of the photoelectron spectra of guanine with 7-methylguanine [11]. The four tautomers of guanine (keto-N7H, keto-N9H, enol-N9H and enol-N9H-trans) are within the range of 0–1.1 kcal/mol (Table 1). Further, all enol-imino tautomers are found to be about 20–50 kcal/mol less stable than the most stable keto-N7H tautomer in the gas phase. The computed dipole moments of all tautomers are also shown in the Table 1. It is evident that the keto-N7H tautomer has the lowest and the keto-N9H tautomer has the highest dipole moment among tautomers within the 0–1.1 kcal/ mol energy range. Further, due to the large dipole moment, the keto-N9H tautomer will be more stable than the ketoN7H tautomer in the water solution [1]. The tautomeric stability order will also be modified in the water solution, but due to the large energy difference it is highly unlikely that unstable tautomers will become stable in the water solution. Computed first vertical ionization potentials of all tautomers in the gas phase obtained at the B3LYP/6311++G(d,p) level are shown in the Table 1. There are several theoretical investigations performed to study the vertical and adiabatic ionization potentials of guanine, but all these studies are limited to only relatively stable tautomers whose energies are within few kcal/mol of the most stable tautomer [15–17]. These investigations have suggested that the B3LYP is a reliable method to predict the ionization potential of nucleic acid bases. Due to the recent gas phase spectroscopic investigations of guanine tautomers and difficulty in the R2PI spectral assignments, a reliable knowledge for the ionization potentials of all guanine tautomers is necessary [7–10]. Quantitative information about ionization potentials of different tautomers will be
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Table 1 Computed relative energies (DE, kcal/mol), dipole moments (l, Debye), vertical ionization potentials (IP, eV), electronic lowest singlet pp* vertical (TEv, eV) and adiabatic transition energies (0–0, eV) and corresponding oscillator strengths of guanine tautomersa Tautomers
K-N7H K-N9H E-N9H E-N9H-trans E-N7H K-N7H-N3H K-N7H-imino-cis K-N7H-imino E-N7H-trans K-N9H-imino K-N9H-imino-cis K-N9H-N3H E-N7H-IMN3 E-N9H-IMN3 E-N7H-IMN3-cis E-N9T-IMN1 E-N9T-IMN3 E-N7T-IMN3 E-N9H-IMN3-cis E-N9H-IMN1 E-N9T-IMN1-cis E-N9T-IMN3-cis E-N7T-IMN3-cis E-N9H-IMN1-cis E-N7T-IMN1 E-N7H-IMN1 E-N7T-IMN1-cis E-N7H-IMN1-cis
IP
Transition Energy
MP2
DFT
MP2
DFT
DFT
TEv
f
0–0
0.0 0.3 0.4 1.0 3.5 6.4 8.1 8.2 11.8 16.0 17.9 19.0 19.7 24.3 24.8 25.2 25.9 29.7 31.2 31.9 32.4 33.8 35.7 40.2 40.7 40.8 48.5 49.5
0.0 0.4 1.6 2.3 4.4 6.4 6.2 5.9 11.9 13.8 15.5 19.2 18.3 23.0 23.5 22.6 24.5 27.3 29.5 28.5 29.1 31.8 33.4 36.1 36.5 35.7 43.8 43.8
2.0 6.3 3.1 3.8 4.1 4.4 3.8 2.7 4.9 5.8 8.7 10.6 5.6 4.4 5.3 4.4 6.9 7.7 7.2 1.7 4.6 9.3 7.9 3.3 8.7 7.9 10.1 9.8
1.8 6.7 3.2 3.7 3.9 4.8 4.1 2.7 4.8 6.1 9.0 11.0 5.9 4.5 5.6 4.6 7.0 8.0 7.2 1.9 4.7 9.4 8.3 3.4 8.9 8.0 10.3 9.9
8.16 8.02 8.00 8.02 8.05 8.44 8.25 8.18 8.08 8.18 8.26 8.41 7.71 7.75 7.78 7.47 7.77 7.73 7.81 7.46 7.49 7.84 7.80 7.47 7.48 7.38 7.53 7.46
4.96 5.19 5.00 4.96 4.60 5.40 4.62 4.53 4.54 5.10 5.19 5.38 3.92 4.41 4.05 3.74 4.42 3.86 4.54 3.76 3.78 4.54 3.99 3.79 3.44 3.39 3.53 3.43
0.1028 0.1510 0.1151 0.1241 0.0677 0.1843 0.0630 0.0641 0.0627 0.0559 0.0445 0.0439 0.0770 0.0906 0.0843 0.0539 0.0919 0.0713 0.0922 0.0541 0.0560 0.0939 0.0778 0.0564 0.0487 0.0514 0.0517 0.0602
4.42 4.30 4.55 4.56 4.19 4.81 4.29 4.19 3.83 4.34 4.34 4.59 3.33 3.77 3.44 3.18 3.69 2.89 3.85 2.89 3.23 3.80 2.97 2.88 2.69 2.72 2.82 2.79
l
DE
The MP2 represents the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) level and DFT represents the B3LYP/6-311++G(d,p) level. The relative energies at the B3LYP/6-311++G(d,p) level was corrected for the scaled (scaling factor 0.9877) ZPE. The vertical and adiabatic transition energies were obtained at the TD-B3LYP/6-311G(d,p) level using the HF/6-311G(d,p) and CIS/6-311G(d,p) geometries (see Section 2 for an explanation). a K-represents keto and E-represents enol.
very informative for an experimentalist to selectively ionize guanine tautomeric forms in the multiphoton ionization experiment in the supersonic jet. LeBreton and coworkers [11,18] have performed an extensive gas phase He(I) photoelectron spectroscopic investigations on guanine and different methylguanine. The first vertical ionization energies of guanine, 1-methylguanine, 7-methylguanine, 9-methylguanine, 1,9-dimethylguanine and O6,9-dimethylguanine were found to be 8.28, 7.98, 8.16, 8.02, 8.09 and 7.96 eV, respectively [11,18]. Since, the photoelectron spectra of guanine and 7-methylguanine were found similar [11] therefore, it would be important to compare the computed ionization potentials of guanine tautomers with the corresponding methyl substituted guanine. The keto-N7H tautomer is predicted to have the largest ionization potential (8.16 Debye) among tautomers whose relative energies are within 1.1 kcal/mol of the most stable one (Table 1). Thus our computed ionization potential for the ketoN7H tautomer at the B3LYP/6-311++G(d,p) level is in the excellent agreement with the corresponding experimental value of the 7-methylguanine in the photoelectron spectra [11]. Further, the calculated value is also close to the experimental ionization potential of guanine at the 8.28 eV. Thus, our calculation also support the findings
of LeBreton and coworkers [11] that in the gas phase the keto-N7H form of the guanine will be prevalent. The computed ionization potential of the keto-N9H tautomer at 8.02 eV is also in the excellent agreement with the corresponding experimental value at 8.02 and 7.98 eV for the 9-methylguanine and 1-methylguanine respectively. Similarly, the computed ionization potential of enol-N9H tautomer at the 8.0 eV can also be compared with the corresponding experimental value of O6,9-methylguanine at 7.96 eV [18]. Thus, B3LYP/6-311++G(d,p) level calculation is able to predict the first vertical ionization potentials of guanine tautomers accurately. We believe that the computed first vertical ionization potentials of other tautomer, for which the corresponding experimental data is not available, are also reliable. Further, it is clear from the Table 1 that the ionization potentials of all enol-imino tautomers are generally less than 8.0 eV, while for the keto-N3H tautomer it is more than 8.4 eV. The vertical and adiabatic transition energies corresponding to the lowest singlet pp* excited state of all guanine tautomers are also presented in the Table 1. It has been shown earlier by us as well as other authors that the time-dependent density functional theory method provides efficient and reliable option for the transition energies
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Fig. 1. Structures of the N9H tautomers of guanine. The corresponding N7H forms can be obtained by replacing hydrogen attached to the N9 site to the N7 site of guanine. K represents the keto and E represents the enol.
calculation of nucleic acid bases [19–21]. Further, computed first vertical singlet pp* transition of the keto-N7H tautomer is red-shifted compared to the corresponding transition of the keto-N9H tautomer. This prediction is in agreement with the experimental result which shows that the first absorption transition of the 7-methylguanine is red-shifted compared to the corresponding transition of guanosine monophosphate (GMP) [22]. Further, it is evident from the data shown in the Table 1 that the first vertical singlet pp* transition of other tautomers of guanine are generally red-shifted compared to the corresponding transition of the keto-N9H tautomer, except for ketoN9H-N3H and keto-N7H-N3H tautomers which show significant blue-shift. Generally all unstable tautomers show significant red-shift in the first absorption transition compared to the corresponding transition of the keto-N9H tautomer. An elaborated description of several absorption transitions of guanine can be found in our recent publication, where a detailed study about vertical singlet transitions of nucleic acid bases obtained at the TDDFT level with different large basis sets including several sets of diffuse functions are reported [19]. Therefore, we will discuss mainly the adiabatic transitions corresponding to the lowest singlet pp* excited state of guanine tautomers in an attempt to provide a guide for experimentalists working
in the area of multiphoton ionization of nucleic acid bases in the supersonic jet expansion. Mons et al. [8] have assigned the spectral origin of the enol-N9H, keto-N9H, keto-N7H and enol-N7H tautomers in the R2PI experiments at 4.31, 4.20, 4.12 and 4.07 eV, respectively. However, these authors have recently reassigned their R2PI spectra of guanine and accordingly transitions at 4.31, 4.20, 4.12 and 4.07 eV correspond to the spectral origin of enol-N9H-trans, keto-N7H-imino-cis, keto-N7H-imino and enol-N7H forms, respectively [10]. This reinterpretation was based on the comparison of the experimental IR frequency of guanine tautomers in the He droplet [9], experimental IR frequency in the R2PI experiments [8] and the theoretical vibrational frequencies at the B3LYP/6-31+G(d) level. Our theoretical results qualitatively show the similar trend in the spectral origin of guanine tautomers as suggested in the reassigned R2PI spectra [10]. Thus it appears that the computed disagreement in the spectral origin of the keto-N9H and ketoN7H tautomers in our earlier theoretical investigation [23] was due to the older assignment of the R2PI data [8]. It should be noted that in our investigation the spectral origin of the keto-N9H tautomer was predicted to be redshifted compared to that of the keto-N7H tautomer while the contradictory result was found in the experiment
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[8,23]. Thus, in view of the qualitative agreement in between our computed spectral origin and the reinterpreted R2PI spectra [10], it appears that the spectral origin of the keto-N9H tautomer will be red-shifted than that of the enol-N9H tautomer and the spectral origin of the ketoN7H tautomer will be in between that of the keto-N9H and the enol-N9H tautomer. 4. Conclusions The keto-N9H, keto-N7H and two rotamers of the enolN9H tautomer have similar stability in the gas phase with electronic energy within 0–1.0 kcal/mol range; the ketoN7H tautomer being the most stable. The first vertical ionization potentials of these tautomers are about 8.0 eV, except the keto-N7H tautomer which has the 8.16 eV. Among all possible guanine tautomers, we have found that generally higher energy tautomers (the relative energy more than 20 kcal/mol) have lower ionization potentials. The computed spectral origin values corresponding to the lowest singlet pp* excited state of guanine tautomers are in the qualitative agreement with the reinterpreted R2PI data. We hope that computed ionization potentials and spectral origins of guanine tautomers would be very useful for experimentalists in identifying the stable tautomers of guanine in the R2PI experiments. Acknowledgements We are thankful to Prof. Michel Mons, Laboratoire Francis Perrin – URA CNRS 2453 – Service des Photons, Atomes et Molecules, CEA Saclay, Bat 522, 91191 Gif-surYvette, Cedex, France for reading the final version of our manuscript and for his critical suggestions. Authors are also thankful to financial supports from NSF-CREST Grant No. HRD-0318519, ONR Grant No. N00034-031-0116 and NSF-EPSCoR Grant No. 02-01-0067-08/ MSU. Authors are also thankful to the Mississippi Center for Supercomputing Research (MCSR) for the generous computational facility.
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