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 electrode s 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 succe ssfully explained pH dependencies of Soret band absorbances, formal po tentials, electron transfer rate constants, and electroactive surface concentrations of Mb in the films. A pK(a1) of 4.6 in the Mb-lipid fil ms is associated with protonation of histidine residues in hydrophobic regions of the Mb structure, possibly involving the proximal histidin e 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] ap pears to be the major species in films between pH 5.5 and 8. In this p H range, protonation of MbFe(III)-H2O occurs prior to electron transfe r, 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-e lectron reduction is also coupled to protonation.