Electron transfer between tryptophan and tyrosine: Theoretical calculation of electron transfer matrix element for intramolecular hole transfer

In this article, the electron transfer (ET) matrix element has been estimated for the gas-phase hole transfer between indole cation radical (InH+) and phenol (PhOH). This work consists of three parts. First, the ET reaction between the isolated indole and phenol was investigated using the AM1 method and ab initio calculation at the level of UHF/6-31++G*. Second, the intramolecular ET systems mediated by -(CH2)(n)-(n = 2-5) were discussed. The linear reaction coordinate approximation has been adopted to describe the variation of the nuclear configuration. Different methods have been used to examine the crossing point of the potential energy curves. All the values of R-c from different methods show very good consistency. ET matrix elements have been evaluated for InH-(CH2)(n)-PhOH systems. Also, the AM1 method and Koopmans' theorem were used to investigate the ET process between tyrosine and tryptophan, in which the oligoproline serves as the bridge. Conformations have been optimized using the AM1 method. Based on the linear reaction coordinate approximation, ET matrix elements for the intramoleeulatr hole transfer in those Trp--(Pro)(n)-TyrOH (n = 0-3) peptides have been estimated by searching for the minimum of the energy splitting factor. All the results show that if a hole transfer process exists between the indole (or indolyl side chain in tryptophan) and phenol (or phenolyl side chain in tyrosine), without deprotonation, it will be difficult for the the ET reaction to occur. All the values of R-c are found in the range of R > 1, no matter if a bridge intervenes or not. Hole transfer reactions are predicted to be greatly endothermic, and to have a large activation barrier in the gas-phase case. (C) 2001 John Wiley & Sons, Inc.