RATES OF DNA-MEDIATED ELECTRON-TRANSFER BETWEEN METALLOINTERCALATORS

Ultrafast emission and absorption spectroscopies were used to measure the kinetics of DNA-mediated electron transfer reactions between metal complexes intercalated into DNA, In the presence of rhodium(III) acce ptor, a substantial fraction of photoexcited donor exhibits fast oxida tive quenching (>3 x 10(10) per second). Transient-absorption experime nts indicate that, for a series of donors, the majority of back electr on transfer is also very fast (similar to 10(10) per second). This rat e is independent of the loading of accepters on the helix, but is sens itive to sequence and pi stacking. The cooperative binding of donor an d acceptor is considered unlikely on the basis of structural models an d DNA photocleavage studies of binding. These data show that the DNA d ouble helix differs significantly from proteins as a bridge for electr on transfer.