Distance dependence of hole transfer rates from G radical cations to GGG traps in DNA

Abstract

Relative reaction rates for hole transfer between G radical cations and GGG triplets in DNA, through different bridges of varying lengths, are numerically calculated and the obtained results are compared with corresponding experimental observations [Giese et al., 2001, Nature, 412, 318; Angew. Chem., Int. Ed., 1999, 38, 996]. Hole donors and acceptors are separated either by (T-A)n bridges or by N repeated barriers consisting of (T-A,T-A) double base-pairs which are connected through single G-C base-pairs. In the former case, hole transfer rates show a strong exponential decrease with the length of the bridge for short bridges, while a switching to weak distance dependence has been observed for longer bridges. In the latter case, a power law seems to better describe the distance dependence of charge transfer rates. All these experimental observations are qualitatively reproduced by our simulations without any adjustable parameter, considering only tunneling as the charge transfer mechanism. Physical insights into the mechanism providing the switching behavior in the case of (T-A)n bridges are presented through an analysis of the eigenfunctions of the system.

A systematic study of electron or hole transfer along DNA dimers, trimers and polymers

“…It is hoped that the present work will contribute in this direction. β values for different systems include ≈ 1.0 -1.4 Å−1 for protein-bridged systems [22][23][24], ≈ 1.55 -1.65 Å−1 for aqueous glass bridges [22], ≈ 0.2 -1.4 Å−1 for DNA segments [25][26][27][28][29][30], ≈ 0.8 -1.0 Å−1 for saturated hydrocarbon bridges [31,32], ≈ 0.2 -0.6 Å−1 for unsaturated phenylene [33,34], polyene [35][36][37] and polyyne [38][39][40] bridges, and much smaller values (< 0.05 Å−1 ), suggesting a molecular-wire-like behavior, for a p-phenylenevinylene bridge [41]. Hence, it seems that charge transfer in ATATAT..., poly(dG)-poly(dC) and poly(dA)-poly(dT) is almost molecular-wire-like.…”

Length-independent transport rates in biomolecules by quantum mechanical unfurling

“… 14 , 21 The sharp drop was therefore attributed to exponentially decreasing CT rates via an off-resonant (super-exchange) or resonant (flickering 21 ) quantum tunneling mechanism, whereas the observed mild dependence on the bridge length for longer bridges was attributed to thermally activated charge propagation through the poly-A stack by sequential hopping. 18 , 20 , 22 – 28 The hopping mechanism implies that the hole propagates in multiple steps along the bridge and therefore the longer the bridge, the slower the transfer, with a typical 1/ N fall of the transfer rate with the number of bridge sites. Therefore, the observation that doubling the donor–acceptor separation had strictly no effect on the CT efficiency (and presumably on the CT rate) through long bridges is not accounted for by any of these mechanisms, and calls for revisiting some basic assumptions underlying models of CT through DNA and biomolecules in general.…”

“…This implies that preparation of the hole at the donor G site amounts to populating primarily a single quasiparticle eigenstate. The vanishingly small projection of the donor site wave function on other eigenstates would lead to coherent oscillations between the donor and the acceptor, 26 , 28 even if the system retains its equilibrium geometry, but the majority of the hole population would remain at the donor site at all times in this case. Fluctuations in the DNA molecule and/or in its environment are therefore necessary in order to facilitate the hole transfer kinetics from the donor ([G] + ) to the acceptor ([GGG] + ).…”