Effective coupling in biological electron transfer: exponential or complex distance dependence?

Calculations for a simple model electron transfer system and tuna cytochrome c demonstrate a dichotomy in the distance dependence of the effective coupling. In one regime, the effective coupling varies exponentially with distance and depends primarily on the average properties of the bridging material; in the other regime, the effective coupling has a complex distance dependence and is more sensitive to the details of the bridging material. Experiments and theory indicate that both regimes may occur in biological systems, providing a perspective on a recent controversy over the nature of the distance dependence.