Direct voltammetric investigation of the electrochemical properties of human hemoglobin: Relevance to physiological redox chemistry

Voltammetric measurements on solutions of human hemoglobin using gold elect rodes modified with omega -hydroxyalkanethiols have yielded the first direc t measure of the reorganization energy of the protein. The value obtained b ased on extrapolation of the experimentally measured currents, 0.76 eV, is independent of pH (i.e., over the physiologically relevant rage, pH 6.8-7.4 ) and is remarkably similar to values obtained for myoglobin. This result i s perhaps surprising given the marked dependence of the measured reduction potential of hemoglobin on pH (i.e., the redox Bohr effect). Electron trans fer rates from the electrode to hemoglobin were also measured. Using simila rly measured heterogeneous electron-transfer rates for cytochrome b(5), it is possible to predict the magnitude of the homogeneous electron-transfer r ate from cytochrome bs to methemoglobin using a formalism developed by Marc us. These predicted rates are in reasonable agreement with reported rates o f this physiological reaction based on stopped-flow kinetics experiments. T hese results suggest that the intrinsic electroreactivity of these heme pro teins is sufficient to account for physiologically observed rates. Residual differences between homogeneous phase kinetics and those predicted by hete rogeneous phase reactions are suggested to be due to small reductions in th e outer-sphere reorganization energy of both component proteins which arise due to solvent exclusion at the interface between the two proteins in comp lex.