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 ariodic 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.
InfroduclionThe heterogenous electron transfer between electrode and heme-containing molecules has been extensively studied as a means to provide a better understanding of the mechanism of electron transfer in biological systems. The heme group in these proteins is placed in a hydrophobic environment, a "ligand pocket" provided by the polypeptide chain. As a result, the direct electron transfer (ET) at conventional electrodes is extremely slow and often accompanied by an adsorption induced passivation. In view of the enormous practical and fundamental interest of the subject, impressive work was devoted to investigating the methods to make the ET between the heme and the electrode more accessible and thus, more rapid. Three different approaches have been proposed: (i) Redox reactions on mediator-modified electrodes.1-4 The mediators are generally redox dyes, with rapid electrode kinetics, and able to undergo a rapid homogeneous chemical reaction with the heme-containing protein; (ii) Direct electron transfer between electrodes and heme proteins. Most studies in this field are concerned with the direct ET reaction for electron-transport proteins such as cytochrome c at bare glassy carbon,5 Hg,6 Ag,7 and Au8 electrodes. For myoglobin (Mb), direct ET was reported on tin doped 1n203 electrodes.9 Recently, Li et al.'° have also reported direct ET for Hb at bare Ag electrodes. However, their data seem rather questionable since the reported redox couple is in the 0.09 to 0.27 V range (vs SCE), i.e., within a range where the voltammetric techniques are limited by the dissolution of silver fr...