Direct Measurement of the Dynamics of Hole Hopping in Extended DNA G-Tracts. An Unbiased Random Walk

We report the measurement of distance- and temperature-dependent rate constants for charge separation in capped hairpins in which a stilbene hole acceptor and hole donor are separated by A(3)G(n) diblock polypurine sequences consisting of 3 adenines and 1-19 guanines. The longer diblock systems obey the simplest model for an unbiased random walk, providing a direct measurement of k(hop) = 4.3 x 10(9) s(-1) for a single reversible G-to-G hole hopping step, somewhat faster than the value of 1.2 x 10(9) s(-1) calculated for A-tract hole hopping. The temperature dependence for hopping in A(3)G(13) provides values of E-act = 2.8 kcal/mol and A = 7 x 10(9) s(-1), consistent with a weakly activated, conformationally gated process.