Spectroelectrochemical Studies of Metalloenzymes and Polymer-Coated Graphite Transparent Electrodes

Thermodynamic parameters of metalloprotein electron transfer reactions, namely reduction potentials, enthalpies and entropies, are determined by means of spectroelectrochemical techniques. The studies concentrate on blue copper protein (Rhus vernicifera stellacyanin, Phaseolus vulgaris plastocyanin and Pseudomonas aeruginosa azurin), and c-type cytochromes (horse heart cytochrome c, Pseudomonas aeruginosa cytochrome c-551 and cytochrome oxidase). The results show that within the same class of metalloprotein studied, i.e., blue copper protein and cytochrome c, the observed enthalpy and entropy changes can be used as criteria for determining solvent access of the metal centers. The more access the metal center has to the solvent, the less negative enthalpy and entropy changes are associated with its reduction process. The increase in solvent access of the metal center is in the order cytochrome cd < cytochrome c-551 ~ cytochrome c and azurin < plastocyanin < stellacyanin. The conclusions agree well with those from kinetic studies. The reduction potentials of heme b5 of liver sulfite oxidase have been determined. The potentials of the intact enzyme and those in which the molybdenum moeity has been removed are very similar, indicating that there is no interaction such as to alter the reduction potential of heme b5 between the heme and molybdenum prosthetic groups. The reduction of the heme b5 of chicken liver sulfite oxidase by Fe(EDTA)2- has also been investigated. The rate of electron transfer has been analyzed within the framework of the relative Marcus theory of outer sphere electron transfer. The electrostatic corrected self-exchange rate constant is two orders of magnitude greater than the corresponding value of cytochrome c indicating more accessibility of the heme of sulfite oxidase than in those of cytochrome c. The activation parameters have also been determined. It is proposed that electron transfer mechanism employed by heme b5 of sulfite oxidase is very similar to that employed by cytochrome c, i.e., via exposed heme edge. Spectroelectrochemical studies of the polymer-coated graphite transparent electrodes have been performed. The amount of polymer adsorbed on electrode surface was determined spectrally. The rates of attachment and detachment of Ru(III)(EDTA) to and from the pyridine group on the polymer backbone were studied. It was found that reactivity of the formation and detachment of Ru(EDTA) complex was less than the corresponding homogeneous reactions. The electrochemical properties of such electrodes were also reported.