Ab initio calculations of long-distance electron tunneling in organometallic systems of biological origin

The method of tunneling currents developed earlier by the author is applied to study electron tunneling dynamics in a model organometallic donor-bridg e-acceptor system in which the donor is the blue copper center in the reduc ed form, the bridge is a polypeptide (5 glycine residues), and the acceptor is -HisRu(III)(NH3)(5) complex. This system mimics pretty closely the majo r amino-acid tunneling path in Ru-modified azurin studied by Gray and co-wo rkers recently For the first time it is demonstrated that the tunneling mat rix element as small as 10(-4) cm(-1) can be reliably evaluated using the a b initio tunneling currents method. The method consists of an ab initio cal culation of the spatial distribution of quantum mechanical flux of probabil ity density occurring during the tunneling transition in the system, when a n electron/hole tunnels from the donor site in the molecule to the acceptor site. The analysis is based on the calculation of two diabatic nonorthogon al electronic states corresponding to localization of a tunneling electron on donor and acceptor sites, respectively, and subsequent evaluation of the matrix element of current density operator between these two states. All e lectrons in the system are taken into account at the Hartree-Fock level, an d therefore the method allows one to examine the reaction of the background electrons to the tunneling charge in a self-consistent way. Results for th is system confirm earlier reported finding that in the tunneling flow there exist quantized vortices, similar to those of a superfluid liquid, such as Liquid He. (C) 2000 John Wiley & Sons, Inc.