Theoretical calculation of the UV spectrum of cytosine

1. 13th ICQC, International Congress of Quantum Chemistry Helsinki, Finland, 22–27 June, 2009 Theoretical calculation of the UV spectrum of cytosine Attila Tajti, Géza Fogarasi and Péter G. Szalay Eötvös University, Institute of Chemistry, Laboratory of Theoretical Chemistry,Pf. 32, Budapest, Hungary, H-1518 Introduction Any structural change in the nucleotide bases is of literally vital importance for DNA as such a change may completely destroy the hydrogen bond system of the double helix. Two types of changes can be distinguished. Beyond vertical excitations, the complete vibronic spectrum has been determined for the first time, using the Linear Vibronic Coupling, LVC method [9]. The four lowest excited electronic states (Table 1) and fourteen vibrational modes with several quan-ta on each of them were included. Applying a new development (see separate poster by Szalay and Tajti), non-adiabatic couplings were calculated at the CCSD level. Comparison with an experimental spectrum [10] is shown in Fig. 1. (see also the higher resolution spectrum in the poster-head). i) In the electronic ground state, tautomerism is a possible cause of genetic point mutations. ii) Changes involving excited states [1]: when UV light is absorbed  leading potentially to photochemical damage the chromophores responsible for absorption are the bases, including cytosine, subject of the present study. Results We report here results on the excited states of cytosine (canonical oxo-form), obtained from coupled cluster theory combined with the equation of motion method (EOM-CC). Using the program package CFOUR [2], vertical excitation energies were calculated at the singles and doubles level (EOM-CCSD) [3], with the effect of triple Figure 1. The UV absorption spectrum of cytosine. a) present theoretical results: free molecule, electronic transitions by EOM-CCSD/cc-pCVDZ, vibronic treatment by LVC; solid line: with non-adiabatic coupling, dashed line: without coupling; band half-widths 0.15. b) experiment: aqueous solution, redrawn from ref.[10] substitutions tested by the EOM-CC3 method [4]. Based on the theoretical results, all available experimental data have been critically reviewed. We are in contradiction with a REMPI study [11] which assigned an electronic transition in the region around 32 000 cm-1 to the present oxo-tautomer (and the region 36 – 38 000 cm-1 to the enol form.) However, the majority of experimental results, especially a recent resonant Raman spectrum [12] – with appropriate reinterpretation - do support our conclusion below. Linear dichroism data on single crystals and oriented films [10, 13] can also be interpreted by the calculated transition moments (Scheme 1). References [1] Recent reviews: a) M. K. Shukla, L. Leszczynski, J. Biomolecular Structure & Dynamics2007, 25, 93-117; b) C. E. Crespo-Hernandez, B. Cohen, P. M. Hare, B. Kohler, Chem. Rev.2004, 104, 1977-2019. [2] CFOUR, a quantum chemical program package written by J. F. Stanton, J. Gauss, M. E. Harding, P. G. Szalay, with contributions from: A. A. Auer,R. J. Bartlett, U. Benedikt, C. Berger, D. E. Bernholdt, O. Christiansen, M. Heckert, O. Heun, C. Huber, D. Jonsson, J. Juslius, K. Klein, W. J. Lauderdale, D. Matthews, T. Metzroth, D. P. O’Neill, D. R. Price, E. Prochnow, K. Ruud, F. Schiffmann, S. Stopkowicz, A. Tajti, M. E. Varner, J. Vzquez, F. Wang, J. D. Watts; and the integral packages MOLECULE (J. Almlçf, P. R. Taylor), PROPS (P. R. Taylor), ABACUS (T. Helgaker, H. J. Aa. Jensen, P. Jørgensen,J. Olsen), and ECP routines by A. V. Mitin, C. van Wllen. For the current version, see http://www.cfour.de. [3] J. F. Stanton, R. J. Bartlett, J. Chem. Phys.1993, 98, 7029-7039. [4] O. Christiansen, H. Koch, P. Jørgensen, J. Chem. Phys.1995, 103, 7429. [5] M. P. Fülscher, B. O. Roos, J. Am. Chem. Soc.1995, 117, 2089-2095. [6] N. Ismail, L. Blancafort, M. Olivucci, B. Kohler, M. A. Robb, J. Am. Chem. Soc.2002, 124, 6818-6819. [7] M. Merchan, L. Serrano-Andres, J. Am. Chem. Soc.2003, 125, 8108-8109. [8] K. Tomic, J. Tatchen, C. M. Marian, J. Phys. Chem. A2005, 109, 8410-8418. [9] H. Köppel, W. Domcke, L. S. Cederbaum, Adv. Chem. Phys.1984, 57, 59-246. [10] F. Zaloudek, J. S. Novros, L. B. Clark, J. Am. Chem. Soc.1985, 107, 7344-7351. [11] E. Nir, M. Mller, L. I. Grace, M. S. de Vries, Chem. Phys. Lett. 2002, 355, 59–64. [12] B. E. Billinghurst, G. R. Loppnow, J. Phys. Chem. A 2006, 110, 2353–2359. [13] B. E. Billinghurst, G. R. Loppnow, J. Phys. Chem. A 2006, 110, 2353–2359. Scheme 1. Calculated transition moment directions of electronic excitations I and IV in cytosine. CONCLUSION While experimental studies have consistently interpreted the UV spectrum on the basis of four, -* transitions, the present study has led to the surprising conclusion that cytosine has only two, rather than four, observable electronic transitions in the UV range up to 60000 cm-1. The result may be of crucial importance for future photochemical studies. Acknowledgement.Financial support has been provided by Hungarian science grants NKTH-OTKA-A07, no. K 68427 and OTKA K72423. The authors thank Prof. J.F. Stanton for providing access to and assistance with the SIM code.