Ab initio study of long-distance electron tunneling in a model peptide system

The method of tunneling: currents developed earlier by one of the authors i s applied to study electron tunneling dynamics in a model organometallic do nor-bridge-acceptor system, in which the donor is a blue copper center in t he reduced form, the bridge is a polypeptide (5 glycine residues), and the acceptor is the -HisRu(III)bpy(2)Im complex. This system mimics pretty clos ely the major amino acid tunneling path in Ru-modified azurin studied by Gr ay and co-workers recently. It is demonstrated that the tunneling matrix el ement as small as 10(-4) cm(-1) can be reliably evaluated using ab initio t unneling currents method. The method consists of an ab initio electronic st ructure calculation of the spatial distribution of quantum mechanical flux in the tunneling transition occurring in the system, when an electron/hole tunnels from the donor site in the molecule to the acceptor site. The analy sis is based on the calculation of two diabatic nonorthogonal electronic st ates corresponding to localization of a tunneling electron on donor and acc eptor sites, respectively, and subsequent evaluation of the matrix element of current density operator between these two states. All electrons in the system are taken into account at the Hartree-Fock level, and therefore the method allows one to examine the reaction of the background electrons to th e tunneling charge in a self-consistent way. Results for this system confir m earlier reported finding that in the tunneling flow there exist quantized vortices, similar to those of a superfluid liquid, such as liquid He.