Femtosecond direct observation of charge transfer between bases in DNA

Charge transfer in supramolecular assemblies of DNA is unique because of th e notion that the pi -stacked bases within the duplex may mediate the trans port, possibly leading to damage and/or repair. The phenomenon of transport through pi -stacked arrays over a long distance has an analogy to conducti on in molecular electronics, but the mechanism still needs to be determined . To decipher the elementary steps and the mechanism, one has to directly m easure the dynamics in real time and in suitably designed, structurally wel l characterized DNA assemblies. Here, we report our first observation of th e femtosecond dynamics of charge transport processes occurring between base s within duplex DNA. By monitoring the population of an initially excited 2 -aminopurine, an isomer of adenine, we can follow the charge transfer proce ss and measure its rate. We then study the effect of different bases next t o the donor (acceptor), the base sequence, and the distance dependence betw een the donor and acceptor. We find that the charge injection to a nearest neighbor base is crucial and the time scale is vastly different: 10 ps for guanine and up to 512 ps for inosine. Depending on the base sequence the tr ansfer can be slowed down or inhibited, and the distance dependence is dram atic over the range of 14 Angstrom. These observations provide the time sca le, and the range and efficiency of the transfer. The results suggest the i nvalidity of an efficient wire-type behavior and indicate that long-range t ransport is a slow process of a different mechanism.