Kinetic and Mechanistic Parameters of Laccase Catalyzed Direct Electrochemical Oxygen Reduction Reaction

This article presents the kinetic studies of oxygen reduction by one of the most important multicopper oxidases (fungal laccase) using the classic tool of electrochemistry: rotating ring-disk electrode (RADE). Laccase was immobilized on a multiwalled carbon nanotube (MWNT) modified inert disk electrode using 1-pyrenebutanoic acid succinimidyl ester (PBSE), as a tethering agent. The conditions for laccase immobilization on MWNT were optimized to prepare a highly active composite electro-catalyst for O-2 reduction. The mechanistic as well as kinetic parameters such as Tafel slopes, number of electrons transferred, electrochemical rate constants (for heterogeneous charge transfer) and electron transfer rate constant were calculated from the RRDE experiment results. The Tafel slope obtained was close to the value of that of ideal four-electron reduction of O-2 to water indicating a highly active laccase in the tethered composite. The RRDE results also suggested the presence of intermediate steps in the oxygen reduction reaction. A model pathway for O-2 reduction reaction at the laccase composite modified electrode was postulated, and rate constants for individual reactions in the pathway were calculated. The rate constant for four-electron O-2 reduction was determined to be 1.46 x 10(-3) mol s(-1), indicating excellent electro-catalytic activity of the laccase-MWNT composite catalyst.