Proton-coupled electron transfer from electrodes to myoglobin in ordered biomembrane-like films

Voltammetry and visible and infrared spectroscopy were used to explore protonation equilibria coupled to electron transfer between electrodes and the heme protein myoglobin (Mb) in thin liquid crystal films of didodecyldimethylammonium bromide (DDAB) and phosphatidylcholines (PC). Mb conformation and heme iron ligation in the films were controlled by the pH of the external solution. Acid-base equilibrium models successfully explained pH dependencies of Soret band absorbances, formal potentials, electron transfer rate constants, and electroactive surface concentrations of Mb in the films. A pK(a1) of 4.6 in the Mb-lipid films is associated with protonation of histidine residues in hydrophobic regions of the Mb structure, possibly involving the proximal histidine bound to iron and/or the distal histidine in the heme pocket. At pH < 4.6, a partly unfolded molten globule form of Mb predominates in the films and is reduced directly. Native metmyoglobin [MbFe(III)-H2O] appears to be the major species in films between pH 5.5 and 8. In this pH range, protonation of MbFe(III)-H2O occurs prior to electron transfer, and a protonated form which may be a kinetic conformer accepts the electron. MbFe(III)-OH is formed in the films at pH > 9, and its one-electron reduction is also coupled to protonation.