Evidence of structural non-planarity in excited state: New findings provided by vibrational analysis of the guanine–cytosine base pair M.K. Shukla a , G.M. Kuramshina b , Jerzy Leszczynski a, * a Computational Center for Molecular Structure and Interactions, Department of Chemistry, Jackson State University, Jackson, MS 39217, USA b Department of Physical Chemistry, Faculty of Chemistry, Moscow State University, Moscow 119992, Russia Received 22 March 2007; in final form 11 September 2007 Available online 15 September 2007 Abstract Vibrational frequency analysis for the Watson–Crick guanine–cytosine (GC) base pair in the ground and lowest single pp * excited state was performed based on the reference geometry optimized at the HF/6-311G(d,p) and CIS/6-311G(d,p) levels, respectively. It was found that different NH and CH stretching vibrations can be used for the qualitative prediction of excited state geometrical non- planarity and the localization of electronic excitations in the base pair complexes. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Genetic characteristics are inherited through the well defined hydrogen bonding patterns formed between purine (adenine and guanine) and pyrimidine (thymine and cyto- sine) bases in the deoxynucleic acid (DNA). Nucleic acid bases (NABs) absorb ultra-violet (UV) radiation efficiently, but the quantum efficiency of radiative emission is very poor and absorbed energy is released in the form of effi- cient ultrafast non-radiative decay [1]. However, the under- lying mechanism is still not very well understood. Recent extensive theoretical calculations [2,3] and some experi- mental evidences [4,5] suggest that electronic excited state geometries of NABs are generally non-planar. The out- of-plane structural deformation in the excited state plays an important role in the ultrafast non-radiative deactiva- tion of NABs and base pairs where excited state potential energy surface conically intersect with ground state through reaction path [2,6–8]. Further, we have also shown that the mode of excited state structural deformation significantly depends upon the hydrogen bonding environ- ments [9,10]. Recently, state-of-the-art spectroscopic techniques have been used to analyze structures and properties of DNA fragments in the gas phase [1]. The resonance enhanced multiphoton ionization (REMPI) UV-spectra provide information about the vibrations of electronically excited state while the IR–UV double-resonance or spectral hole burning techniques measure the ground state vibrations. The R2PI, IR–UV and UV–UV double-resonance spectro- scopic investigations of GC base pair in the supersonic jet-cooled beam have been performed including that of the measurement of intermolecular excited state vibrational frequencies [11]. However, these frequencies were com- pared with ground state frequencies obtained at the HF/ 6-31G(d,p) level [11]. It was found that the GC pair is in the Watson–Crick (WC) configuration, but cytosine is in the enol form [11]. Interestingly, in another experiment per- formed in the mid-IR region, the GC pair was assigned to complex in which guanine is in the keto-N7H form and cytosine is in the enol form [12]. The WC-GC base pair in the supersonic beam was formed in the complex of 9- ethylguanine and 1-methylcytosine and the absorption 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.020 * Corresponding author. Fax: +1 601 979 7823. E-mail address: jerzy@ccmsi.us (J. Leszczynski). www.elsevier.com/locate/cplett Available online at www.sciencedirect.com Chemical Physics Letters 447 (2007) 330–334