ELECTROCATALYTIC OXIDATION OF REDUCED NICOTINAMIDE COENZYMES AT GOLD AND PLATINUM-ELECTRODE SURFACES MODIFIED WITH A MONOLAYER OF PYRROLOQUINOLINE QUINONE - EFFECT OF CA2+ CATIONS

Chemisorption of cystamine and cysteamine was used for functionalization of Au and Pt electrodes. respectively with amino groups. The functionalized electrodes were used for covalent immobilization of pyrroloquinoline quinone (PQQ) as a monolayer by carbodiimide coupling of the PQQ carboxylic groups with the surface amino groups. The electrochemical properties of the PQQ-modified electrodes (formal potential E-degrees, peak currents I(p) and peak-to-peak separation DELTAE) were changed after addition of Ca2+ cations, probably owing to the formation of a complex between PQQ and Ca2+. Electrocatalytic oxidation of NADH and NADPH was shown at the PQQ-modified electrodes. This process was strongly enhanced in the presence of Ca2+ cations. Cyclic voltammetry, steady-state current, rotating disk electrode and flow-injection measurements were applied to study this electrocatalytic process. The kinetic parameters of this electrocatalytic process were evaluated in the presence or absence of Ca2+ cations assuming the formation of an intermediate charge-transfer complex between the immobilized PQQ and NADH. The only influence of Ca2+ cations on the kinetics of the catalytic process is an equilibrium shift towards the formation of the charge-transfer complex [NADH ... Ca2+ ... PQQ]. The decay of this complex leading to the formation of the final products (NAD+ and PQQH2) was found to be independent of the presence of Ca2+ cations. The process studied is considered as a model for the mechanism of catalysis showed by PQQ- and Ca2+-containing dehydrogenases. These PQQ-modified electrodes were stable enough even in a flow-injection system and can be considered very promising for practical applications.