Dynamics and equilibrium for single step hole transport processes in duplex DNA

The dynamics of single step hole transport processes from a guanine cation radical (G(.)(+)) to GG, GGG and deazaguanine (Z) secondary electron donors separated by one or two A:T base pairs have been investigated by means of nanosecond time-resolved absorption spectroscopy with kinetic modeling. Photoinduced electron transfer from G to a stilbenedicarboxamide (Sal hairpin linker is used to generate a G(+.)/Sa(-.) radical ion pair separated by two or three A:T base pairs. The occurrence of hole transport from G(+.) to the secondary donors results in an increase in the Sa(-.) decay time. Kinetic modeling provides both the forward and return rate constants for hole transport, from which the hole transport equilibrium constant and free energy can be obtained. No other experimental method has yielded equilibrium data. Hole transport dynamics depend on the identity of the secondary donor, the number and identity of the bases separating the primary and secondary donor, and the location of the donor within the same strand as the primary donor or in the complementary strand. Secondary GG and GGG donors form very shallow hole traps, whereas Z forms a much deeper hole trap. These results are correlated with other transient absorption studies, selected strand cleavage data, and current theories of charge transport in DNA.