Computer simulations of the effect of protein dynamics on the long distance tunneling mediated by the protein matrix have been carried out for a Ru-modified (His 126) azurin molecule. We find that the tunneling matrix element is a sensitive function of the atomic configuration of the part of the protein matrix in which tunneling currents (pathways) are localized. Molecular dynamics simulations show that f luctuations of the matrix element can occur on a time scale as short as 10 fs. These short time f luctuations are an indication of a strong dynamic coupling of a tunneling electron to vibrational motions of the protein nuclear coordinates. The latter results in a modification of the conventional Marcus picture of electron transfer in proteins. The new element in the modified theory is that the tunneling electron is capable of emitting or absorbing vibrational energy (phonons) from the medium. As a result, some biological reactions may occur in an activationless fashion. An analytical theoretical model is proposed to account for thermal f luctuations of the medium in long distance electron transfer reactions. The model shows that, at long distances, the phonon-modified inelastic tunneling always dominates over the conventional elastic tunneling.Electron transfer is an integral part of many biological processes, such as photosynthesis and respiration. Much effort, therefore, has been directed toward understanding transport properties of various biological materials. In particular, recent experimental studies have provided information on the distance and structural dependence of electron transfer rates in various natural and modified proteins (1-4). In these systems, electron transfer typically occurs over distances of 10-30 Å and is due to tunneling mediated by the intervening medium between donor and acceptor.It is commonly believed that fundamental principles of long distance electron transfer are essentially the same as those of any other electron transfer reaction (5). The only difference seems to be in the nature of electronic coupling; in short distance reactions, electronic orbitals of donor and acceptor directly overlap whereas in long distance reactions this coupling is indirect because of sequential overlaps of atomic orbitals of the donor, the intervening medium (bridge), and the orbitals of the acceptor. These sequential overlaps give rise to the concept of superexchange. It is assumed that all states in the bridging medium are virtual, i.e., there are no other resonant states in the system but those of donor and acceptor. The resonance between donor and acceptor occurs in the course of thermal fluctuations of the polar environment. The absence of real intermediate states and direct coupling physically means that electron transfer occurs via tunneling. In this picture, the overall rate of electron transfer is proportional to the frequency at which donor and acceptor states come to resonance and the probability to transfer an electron between donor and acceptor states at the transition state (i...
