Femtosecond Charge Transfer Dynamics of a Modified DNA Base: 2-Aminopurine in Complexes with Nucleotides

As a fluorescent isomer of adenine, 2-aminopurine (Ap) is a powerful probe of DNA dynamics and DNA-mediated charge transfer processes. Here, we report studies with femtosecond resolution of the excited-state dynamics of Ap in various solvents and in bimolecular complexes with nucleotides. Using time-resolved transient absorption and fluorescence up-conversion methods we identify charge transfer as the origin for the quenching of the Ap fluorescence by all four DNA nucleotides. The direction of the redox process is, however, dependent on the base, and from the rates we deduce the nature of the transfer, hole versus electron transfer. The pH and the kinetic isotope effects of these charge transfer reactions revealed no evidence for proton transfer involvement in the rate-determining step. From the measured rates and using electron transfer theory we estimate the driving force for charge transfer between all four nucleobases and Ap. The results are important for the studies of dynamics using Ap in DNA assemblies