CASSCF investigation of electronic excited states of 2-aminopurine

2-Aminopurine is a highly fluorescent analogue of adenine that can be incor porated synthetically into DNA with little perturbation of the native doubl e helical structure. The sensitive dependence of the quantum yield of this fluorophore on nucleic acid conformation has made it an invaluable probe of DNA structure, dynamics, and interactions. To assist in the development of models for the molecular interpretation of fluorescence measurements, the electronic structure of 2-amino-9-methylpurine has been calculated in the g round state and the lowest singlet pi pi* and n pi* excited states. These c omputations employed the complete active space multiconfigurational self-co nsistent field method (CASSCF), supplemented by multiconfigurational quasid egenerate perturbation theory (MCQDPT). The predicted energies for pi pi* e xcitation and emission and n pi" excitation are in good agreement with prev ious experimental values. The permanent molecular dipoles of the ground and pi pi* excited states are similar in magnitude and direction, consistent w ith experimental observations of weak solvatochromic shifts in pi pi* absor ption and emission spectra. However, the permanent dipole of the n pi* stat e is rotated approximately 60 degrees relative to that of the ground state, implying that the n pi* excitation energy will increase in more polar solv ents due to the relative destabilization of this slate by unfavorably orien ted solvent dipoles. This result demonstrates that the "blue-shift" of the n pi* State in polar solvents, which is commonly attributed to the effect o f hydrogen bonding, can arise entirely from a general solvent effect. The e nergy of a radiationless vibronic transition from the pi pi* state to the n pi* state will increase in more polar solvents, provided that the solvent does not rearrange during the transition. Consequently, the efficiency of f luorescence quenching by vibronic coupling between the pi pi* and n pi* sta tes is predicted to decrease significantly in such solvents. The geometry o f the fluorescent emitting state, obtained by CASSCF optimization of the pi pi* state, is moderately buckled due to the occupation of an antibonding o rbital localized to C6. This buckling implies an out-of-plane vibration dur ing the relaxation of the pi pi* state, which is required for vibronic coup ling between this state and the n pi* state. Such a solvent-sensitive intra molecular quenching mechanism may account for the observed dependence of th e fluorescence lifetime of 2-aminopurine on the local environment both in p ure solvents and in DNA.