Mechanism of intramolecular charge transfer in DNA helix as probed by the use of the fluorescent 2-aminopurine

As a structural analogue of adenine, 2-aminopurine (2Ap) is often used as a fluorescent probe to study the intramolecular charge transfer reaction in DNA. We have designed and synthesized a series of model DNA helix with the variation in the distance between the 2Ap probe and the GGG sequence, and have investigated, by means of picosecond time-resolved fluorescence spectroscopy, the effect of the length of the bridge (consisting of a number of inosines, I) separating the electron donor (...GGG...) and the acceptor (2Ap) on the charge transfer dynamics. The fluorescence dynamics of 2Ap exhibited three exponential decay components, the one with a time constant of a few hundred picoseconds is assigned to the intramolecular charge transfer from GGG to 2Ap. Within 2.4 nm of the donor-acceptor separation, the rate of charge transfer decreased exponentially upon increasing the separation, from which the decay factorβ is determined to be 1.3 nm−1. Beyond 2.4 nm, however, the rate started to increase, this abnormal behavior of charge transfer is interpreted in terms of the match of electronic energies between the l-bridge and the donor/acceptor couple.