DNA electron transfer mechanism and dynamics.

The dynamics and efficiency of photoinduced charge separation in synthetic DNA hairpins and dumbbells possessing stilbenedicarboxamide (Sa) and stilbene-diether (Sd) linkers separated by short poly(dA)-poly(dT) base pair sequences have been investigated by means of time resolved fluorescence and transient absorption spectroscopy. Charge separation occurs via a single step superexchange mechanism at short distances and by a multistep hole transport mechanism when the linkers are separated by two or more base pairs. The dynamics and efficiency of hole transport are strongly distance dependent over the first few base pairs, but are relatively insensitive to distance at larger separations. The distance dependence is attributed to a Coulomb cage effect. Hole transport across alternating AT base sequences is less efficient than across poly(dA) sequences. Introduction of guanine into the poly(dA) sequence can either enhance or diminish the efficiency of charge separation, depending upon the location of G. Our experimental results are interpreted using several theoretical models.