Salt and pH effects on electrochemistry of myoglobin in thick films of a bilayer-forming surfactant

Salt concentration and pH of external solutions were shown to control the electrochemistry of the heme protein myoglobin (MbFe(III)-H2O) in stable, ordered films of didodecyldimethylammonium bromide (DDAB). Protonation of (a)quometmyoglobin (MbFe(III)-H2O) in these films precedes electron transfer from electrodes, causing formal potentials to shift negative as pH increases from 5 to 8. At pH > 8, MbFe(III)-H2O dissociates to MbFe(III)-OH, which is reduced directly at the electrode at higher rates than MbFe(III)-H2O. Correlations of voltammetric data with FT-IR spectra suggested that at pH < 4.6, an unfolded form of Mb resides in the films and is reduced directly. The concentration of salt in solution influences electrochemical properties of Mb-DDAB films by its influence on Mb conformation and by effects on interfacial Donnan potentials. NMR indicated strong binding of anions to Mb within DDAB films. Bound anions may neutralize positive charge on Mb's surface so that it can reside in a partly hydrophobic environment, as postulated on the basis of previous ESR and linear dichroism studies. (C) 1997 Elsevier Science B.V.