Anomalously Small Reorganization Energy of the Half Redox Reaction of Azurin

Small values of the reorganization energy, 0.2-0.3 eV, were reported by electrochemical kinetic measurements for the half redox reaction of the redox-active protein azurin. This theoretical study explores possible mechanisms for the low activation barrier for electrochemical protein electron transfer: (1) electronic polarizability of the active site, (2) altering protonation states of far-away histidine residues not directly connected to the active site, and (3) a partial desolvation of the protein when attached to the electrode. The last mechanism provides the most robust explanation of the observations. Constraints imposed by the protein fold on its ability to sample the configuration space lead to the breakdown of the fluctuation-dissipation relation (FDR) and a strong separation of the Stokes-shift and variance reorganization energies. The resulting nonergodic kinetic reorganization energy observed experimentally is significantly lowered compared to predictions of standard models based on Gibbsian statistics and the FDR. The fast rate of protein electron transfer is directly related to the ability of the protein scaffold to maintain nonequilibrium statistics of electrostatic fluctuations projected on the electron-transfer reaction coordinate.