Water mediated electron transfer between proteins

Does natural selection optimize molecular biomachinery at the quantum level? We present statistical characterizations of molecular dynamics at an interprotein electron transfer interface in the respiratory chain of the bacterium Paracoccus denitrificans. This work is motivated by a recent experiment in which anomalous electron transfer rates were measured in vitro for this redox reaction [Ma et al., Biochemistry 46, 11137 (2007)]. We argue that specific amino acids are likely to harness nearby water molecules to bridge the interprotein gap, enhancing the quantum mechanical tunneling rate.