A THEORETICAL-STUDY OF THE CYTOSINE EXCIMER STATE - THE ROLE OF GEOMETRY OPTIMIZATION

A study of the lowest excited dimer (excimer) singlet state of the DNA base cytosine was performed with fully optimized geometry (near the c is-syn form) using a slightly modified version of MOPAC6 (AM1 hamilton ian) and 112 excited configurations involving an active space consisti ng of the four highest occupied and the two lowest empty MO of the dim er. A binding energy of 3 kcal/mol for the excimer state is demonstrat ed, and the excimer fluorescence is predicted to be shifted 150 nm to longer wavelengths relative to that of the monomer fluorescence. The e xcimer geometry has the planes nonparallel (40-45 degrees interplanar angle) and distorted, with the C-5-C-6 showing quite close contact (2. 4 Angstrom). Excitation is found to be more localized in the region of the C-5-C-6 bond than expected from the monomer wavefunction. No stab le excimer was found if the planes were constrained to be planar, nor if the triplet state was selected. The results suggest that the excime r geometry found in this study may be a precursor to the cis-syn cyclo butane photodimer.