Adenine deactivation in DNA resolved at the CASPT2//CASSCF/AMBER level

We have employed hybrid CASPT2//CASSCF/AMBER calculations to map the 1La(1ππ*) deactivation path of a single quantum mechanical adenine in a d(A)10·d(T)10 double strand in water that is treated at the molecular mechanics level. We find that (a) the La relaxation route is flatter in DNA than in vacuo and (b) the La relaxation energy in DNA is much larger than the stabilization energy of the corresponding La excimer. An intra-monomer relaxation process is found to be compatible with the multiexponential decay recorded in DNA, possibly including the longer (>100 ps) lifetime component.