Protein Electron Transfer in Solution, Protein Powders, and Electrode Confinement

Transport of electrons in biological electron transport chains involves interfaces and confinement, while measurements of rates of protein electron transfer and computer simulations are mostly conducted in solutions. Here, we employ molecular dynamics simulations of the azurin protein to compare activation parameters of electron transfer in solution with the corresponding parameters in confinement. We study the redox chemistry of azurin in protein powders and in confinement between two gold slabs, modeling conditions of an electrochemical experiment. The polarizability of the active site of the protein is included in the calculations and is found to be the main factor in lowering the reaction activation barrier. It couples to strong and inhomogeneous electric fields in confinement and protein powders, resulting in an observable reorganization energy of ∼0.1-0.2 eV, comparable to electrochemical measurements. Corresponding low activation barriers, ∼2<i>k</i> _B <i>T</i>, yield a weak temperature effect on the reaction rate. No dynamical medium control of protein half-reactions is found in our calculations, producing rate constants exponentially decaying with the distance to the electrode.