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 functionalizati on of Au and Pt electrodes. respectively with amino groups. The functi onalized electrodes were used for covalent immobilization of pyrroloqu inoline quinone (PQQ) as a monolayer by carbodiimide coupling of the P QQ carboxylic groups with the surface amino groups. The electrochemica l properties of the PQQ-modified electrodes (formal potential E-degree s, 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 N ADPH was shown at the PQQ-modified electrodes. This process was strong ly enhanced in the presence of Ca2+ cations. Cyclic voltammetry, stead y-state current, rotating disk electrode and flow-injection measuremen ts were applied to study this electrocatalytic process. The kinetic pa rameters of this electrocatalytic process were evaluated in the presen ce or absence of Ca2+ cations assuming the formation of an intermediat e charge-transfer complex between the immobilized PQQ and NADH. The on ly influence of Ca2+ cations on the kinetics of the catalytic process is an equilibrium shift towards the formation of the charge-transfer c omplex [NADH ... Ca2+ ... PQQ]. The decay of this complex leading to t he formation of the final products (NAD+ and PQQH2) was found to be in dependent of the presence of Ca2+ cations. The process studied is cons idered as a model for the mechanism of catalysis showed by PQQ- and Ca 2+-containing dehydrogenases. These PQQ-modified electrodes were stabl e enough even in a flow-injection system and can be considered very pr omising for practical applications.