CASSCF Investigation of Electronic Excited States of 2-Aminopurine

Abstract

2-Aminopurine is a highly fluorescent analogue of adenine that can be incorporated synthetically into DNA with little perturbation of the native double-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 ground state and the lowest singlet ππ* and nπ* excited states. These computations employed the complete active space multiconfigurational self-consistent field method (CASSCF), supplemented by multiconfigurational quasi-degenerate perturbation theory (MCQDPT). The predicted energies for ππ* excitation and emission and nπ* excitation are in good agreement with previous experimental values. The permanent molecular dipoles of the ground and ππ* excited states are similar in magnitude and direction, consistent with experimental observations of weak solvatochromic shifts in ππ* absorption and emission spectra. However, the permanent dipole of the nπ* state is rotated approximately 60° relative to that of the ground state, implying that the nπ* excitation energy will increase in more polar solvents due to the relative destabilization of this state by unfavorably oriented solvent dipoles. This result demonstrates that the “blue-shift” of the nπ* state in polar solvents, which is commonly attributed to the effect of hydrogen bonding, can arise entirely from a general solvent effect. The energy of a radiationless vibronic transition from the ππ* state to the nπ* state will increase in more polar solvents, provided that the solvent does not rearrange during the transition. Consequently, the efficiency of fluorescence quenching by vibronic coupling between the ππ* and nπ* states is predicted to decrease significantly in such solvents. The geometry of the fluorescent emitting state, obtained by CASSCF optimization of the ππ* state, is moderately buckled due to the occupation of an antibonding orbital localized to C6. This buckling implies an out-of-plane vibration during the relaxation of the ππ* state, which is required for vibronic coupling between this state and the nπ* state. Such a solvent-sensitive intramolecular quenching mechanism may account for the observed dependence of the fluorescence lifetime of 2-aminopurine on the local environment both in pure solvents and in DNA.