Aerobic respiration and photosynthesis work in concert: the oxygen that is evolved by photosynthetic organisms is the oxidant that sustains life in aerobic microbes and animals; and, in turn, the end products of aerobic respiratory metabolism, carbon dioxide and water, nourish photosynthetic organisms. Electron flow through proteins and protein assemblies in the respiratory and photosynthetic machinery commonly occurs between metal-containing cofactors that are separated by large molecular distances, often in the 10-25 angstrom range. Although these cofactors are weakly coupled electronically, the reactions are remarkably rapid and specific. Understanding the underlying physics and chemistry of these distant electron transfer processes is the goal of the experimental work in my laboratory.

We have investigated free energy, temperature, and distance dependences of electron transfer rates in Ru(diimine)-modified iron and copper proteins. Employing laser flash/quench triggering methods, we have shown that 20-angstrom, coupling-limited Fe(II) to Ru(III) and Cu(I) to Ru(III) electron tunneling can occur on the microsecond timescale both in solutions and crystals; and, further, that analysis of these rates suggests that distant donor-acceptor electronic couplings are mediated by a combination of sigma and hydrogen bonds in folded polypeptide structures. In recent work, we have found that 20-angstrom hole hopping through intervening tryptophan residues is several hundred-fold faster than single-step electron tunneling in Re-modified copper proteins. 1. Gray, H. B. & Winkler, J. R. (2003). Electron tunneling through proteins. Quart. Rev. Biophys. 36, 341-372. 2. Gray, H. B. & Winkler, J. R. (2005). Long-range electron transfer. Proc. Natl. Acad. Sci. USA 102, 3534-3539.