Electron tunneling in azurin: the coupling across a beta-sheet.

Abstract

We would like to understand how electron flow is controlled in biological molecules. Standard theories calculate the rate for long distance electron transfer (ET) as the product of electronic coupling (the square of the electron tunneling matrix element) and nuclear (Franck-Condon) factors. Much attention has been directed to the role of protein secondary and tertiary structure in the tunneling coupling, focusing on the interplay between different types of chemical bonds. Here we have evaluated the relative contributions of covalent bonds, hydrogen bonds and through-space jumps in coupling through a beta-strand or across a beta-sheet section of a blue copper protein, azurin. We have analyzed four distant electronic couplings in azurin. Each coupling is between the copper atom and a Ru(bpy)2(im) complex attached to a histidine on the protein surface. In three experiments the intervening medium was a simple beta-strand, while in the fourth experiment it was a section of beta-sheet. We have shown that electron tunneling in a protein can be broken down into ET 'tubes' of pathways through specific covalent and hydrogen bonds. These ET tubes encapsulate trivial interference effects and can expose crucial inter-tube interference effects. In coupling through a beta-sheet, hydrogen bonds are as important as covalent links, and are the primary source for tube interference.