Quantum dynamics in strong fluctuating fields

A large number of multifaceted quantum transport processes in molecular systems and physical nanosystems can be treated in terms of quantum relaxation processes which couple to one or several fluctuating environments. A thermal equilibrium environment can conveniently be modelled by a thermal bath of harmonic oscillators. An archetype situation provides a two-state dissipative quantum dynamics, commonly known under the label of a spin-boson dynamics. An interesting and nontrivial physical situation emerges, however, when the quantum dynamics evolves far away from thermal equilibrium. This occurs, for example, when a charge transferring medium possesses nonequilibrium degrees of freedom, or when a strong time-dependent control field is applied externally. Accordingly, certain parameters of underlying quantum subsystem acquire stochastic character. Herein, we review the general theoretical framework which is based on the method of projector operators, yielding the quantum master equations for systems that are exposed to strong external fields. This allows one to investigate on a common basis the influence of nonequilibrium fluctuations and periodic electrical fields on quantum transport processes. Most importantly, such strong fluctuating fields induce a whole variety of nonlinear and nonequilibrium phenomena. A characteristic feature of such dynamics is the absence of thermal (quantum) detailed balance.Comment: review article, Advances in Physics (2005), in pres

Tunneling energy effects on GC oxidation in DNA

Hole-mediated electronic couplings, reorganization energies, and electron transfer (ET) rates are examined theoretically for hole-transfer reactions in DNA. Electron transfer rates are found to depend critically on the energy gap between the donor/acceptor states and the intervening bases-the tunneling energy gap. The calculated distance decay exponent for the square of the electronic coupling, β, for hole transfer between GC base pairs (and pi-electron D/A pairs) ranges from 0.95 to 1.5 Å -1 in the model structures as the tunneling energy gap varies from 0.3 to 0.8 eV (which we argue is the range of energy gaps for GC oxidation probed in recent experiments). We show that the tunneling energy gap depends on the ET reorganization energy, which itself grows rapidly with distance for ET over 1-5 base pairs. Inclusion of the distance dependence of reorganization energies for these hole transfer reactions gives the tunneling rates an apparent decay exponent of ∼1.5-2.5 Å -1. We show that ET rates observed in DNA across one and two base pairs are reasonably well described with single-step ET theories, using our calculated couplings and reorganization energies. However, the computed single-step tunneling (superexchange) ET rates for donor and acceptor species separated by three or more base pairs are much smaller than observed. We conclude that longer-distance ET probably proceeds through thermal population of intermediate hole states of the bridging bases. Switching between mechanisms as distance grows beyond a few base pairs is likely to be a general characteristic of ET in small tunneling energy gap systems.link_to_subscribed_fulltex

Hole size and energetics in double helical DNA: Competition between quantum delocalization and solvation localization

The transition between single step long-range tunneling and multistep hopping transport in DNA electron transfer depends on a myriad of factors including sequence, distance, conformation, solvation and, consequently, hole state energetics. We show that the solvation energetics of hole (radical cation) states in DNA is comparable to the quantum delocalization energetics of the hole. That is, the solvation forces that tend to localize the hole compete with the quantum effects that give rise to hole delocalization. The net result is that the hole states are predicted to be relatively compact (one to three base pairs in length) and that the "trap depth" of these holes is expected to be much shallower than anticipated by gas-phase quantum chemical analysis of base stacks. This analysis predicts guanine oxidation potential dependence on the length of GC runs to be modest (differences <0.1 V for holes from one to three base pairs). The lowering of the trapped hole binding energy has significant implications for the structure and mobility of hole states in DNA.link_to_subscribed_fulltex

Bond-mediated electron tunneling in ruthenium-modified high-potential iron-sulfur protein

Bond-mediated electron tunneling in ruthenium-modified high-potential iron-sulfur protein was investigated through electrochemical method