Electron Flow through Metalloproteins
Electron flow through proteins and protein assemblies in the photosynthetic and respiratory machinery commonly occurs between metal centers or other redox cofactors that are separated by relatively large molecular distances, often in the 10−20 Å range. To inorganic chemists, such long-distance electron transfer was a mystery for many years, as we were accustomed to close-contact models for the transition states of simple self-exchange reactions between metal ions in aqueous solution. One of our favorite reactions was the ferrous/ferric self-exchange, which had been investigated thoroughly by Richard (Dick) Dodson at the Brookhaven National Laboratory in the 1940s (and published in Journal of the American Chemical Society in 1950). Dodson showed that the half-time for this reaction was on the order of seconds, which he noted was “fast” but in today’s femtosecond world is very “slow”. Indeed, this very simple electron transfer, where no bonds are formed or broken, was found to be orders of magnitude slower than long-distance electron flow through metalloprotein molecules. How could this be?