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

We have employed hybrid CASPT2//CASSCF/AMBER calculations to map the L-1(a)((1)pi pi*) 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 L-a relaxation route is flatter in DNA than in vacuo and (b) the L-a relaxation energy in DNA is much larger than the stabilization energy of the corresponding L-a 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.