Direct electron transfer for hemoglobin in surfactant films cast on carbon electrodes

Direct electron transfer is reported for hemoglobin in liquid-crystal films of didodecyldimethylammonium bromide (DDAB) cast on glassy-carbon electrodes. Cyclic and square-wave voltammograms were obtained for two types of electrodes with DDAB films in the presence and in the absence of oxygen. At pH 7.1, in the absence of oxygen, a redox couple with a large cathodic and a small anodic peak was obtained, while for pH 5.5, in the presence of oxygen, only the cathodic wave could be obtained up to scan rates of 1 V/s. Based on the voltammogram pattern and the scan rate dependence of the peak potential and current, the mechanism of the electroreduction process was discussed in terms of the equilibrium between the T (tense) and R (relaxed) conformations. Two different reaction pathways were found, depending on the pH value and oxygen concentration. In neutral solutions and in the absence of oxygen, the electroreduction is followed by a slow release of the water molecule bound in the sixth coordination site of the heme, which determines the appearance of the reduction in a potential range characteristic for the R state. In acidic media (pH 5.5) and in the presence of oxygen, the electrode process involves successive dissociation of the water molecule, electroreduction, and oxygenation of deoxyhemoglobin; in this case, the cathodic peak potential is shifted to positive values, as expected for an increased contribution of the T conformation.