A THEORETICAL-STUDY OF THE CYTOSINE EXCIMER STATE

A study of the lowest excited singlet state of the stacked dimer (cis- syn form) for the DNA base cytosine was performed with fully optimized geometry using a slightly modified version of MOPAC6 (AM1 Hamiltonian ) and four singly excited configurations. A binding energy of 7-8 kcal /mol for the excimer state is demonstrated, and the excimer fluorescen ce is predicted to be shifted 160-180 nm to longer wavelengths relativ e to that of the monomer fluorescence. The excimer geometry has the pl anes non-parallel and distorted, with the C5-C6 bonds showing quite cl ose contact (2.4 angstrom), suggesting that the excimer is a precursor to photodimerization.