Spectroscopy of 2-aminopurine: An MCSCF study

2-aminopurine is a highly fluorescent isomer of adenine that can be incorpo rated into DNA as a probe of structure, dynamics, and protein-DNA interacti ons. Interpretation of the fluorescence of 3-aminopurine in DNA requires a model of the electronic structure of this fluorophore in its ground and exc ited states. To this end, electronic structures and energies of the ground and lowest singlet excited states of 2-amino-9-methylpurine were calculated by the multiconfiguration self-consistent field method supplemented by mul ticonfiguration perturbation theory. The molecular geometry was optimized i n both of these electronic states to permit investigation of both electroni c excitation and fluorescence emission. The predicted energies and transiti on dipoles were in good agreement with experiment. The permanent molecular dipole of 2-amino-9-methylpurine increased upon excitation, suggesting that both the absorption and emission spectra should shift to slightly lower en ergies in polar solvents. The anomalous spectral shifts observed in water s uggest that 2-aminopurine undergoes hydrogen bonding that better stabilizes the ground state than the excited state. From the calculated electrostatic potentials of these two states, the position at which this hydrogen bond f orms was predicted. These results form a basis for understanding the excite d states and possible intermolecular interactions of 2-aminopurine in DNA.