Canonical Watson-Crick base pair interactions in π → π* type triplet states

Ground state and triplet pi ->pi states of canonical Watson-Crick base pairs have been studied at the B3LYP level of theory. Excited states were found to be localized at either of the monomers forming the base pair ( guanine, cytosine, adenine and thymine), geometry relaxation of the excited base pair being similar to that occurring in the isolated nucleobase. For thymine and cytosine, triplet pi ->pi excitation produces a significant elongation of the C-5-C-6 bond whereas for guanine and adenine there is a significant increase of the N-3-C-2 bond and pyramidalization of the NH2 group. Adenine-thymine energy pairing remains almost unaffected by triplet excitation. However, for guanine-cytosine, with excitation localized at the guanine moiety, base pairing energy decreases about 5 kcal/mol due to pyramidalization of the amino group of guanine.