Electron-transfer reactions in proteins: a calculation of electronic coupling

The distance and medium dependences of the electronic coupling for electron-transfer reactions in proteins (particularly the ruthenated myoglobins studied by Gray et al.) are calculated. An extended-Hückel method is used to treat individually the donor, the acceptor, and the intervening protein. A search method is used to select a subset (~15-20) of the approximately 150 amino acid residues as being the most relevant for the electron transfer. Approximate agreement is found between ratios of experimental matrix elements (approximated as ratios of square roots of rate constants) and of calculated electronic matrix elements. No adjustable parameters were introduced in the diagonalization. Only parameters available in the literature from other (non-electron-transfer) sources were employed