Electrooxidative Decarboxylation of Vanillylmandelic Acid: Voltammetric Differentiation between the Structurally Related Compounds Homovanillic Acid and Vanillylmandelic Acid

Abstract

Vanillylmandelic acid (VMA) and homovanillic acid (HVA) are the major end products of catecholamine metabolism. Abnoramally high levels in both plasma and urine may be indicative of a number of diseases including neuroblastoma and phaeochromocytoma. Commonly the VMA:HVA ratio is used as a disease marker, so that any measurement techniques need to be able to differentiate between these two structurally similar compounds. Electrochemistry is often limited in selectivity due to many organic molecules being oxidized or reduced at similar potentials. This work investigates the electrochemical oxidation mechanism of VMA at an edge-plane pyrolytic graphite electrode and highlights how, although structurally similar to HVA, their voltammetric responses may be differentiated through appropriate selection of the electrode material. The oxidation of VMA exhibits two clear peaks and the mechanism is shown to proceed through the decarboxylation of VMA to form vanillin, which is further oxidized resulting in the second peak. Modification of the electrode with a porous layer of multiwalled carbon nanotubes so as to change the mass transport to that of a thin layer system causes the voltammetric resolution between the two species to be enhanced. Differential pulse voltammetry is used to measure the limits of detection for VMA on an edge-plane pyrolytic graphite electrode and on commercially available multiwalled carbon nanotube screen printed electrode, with limits of detection of 1.7 and 1.0 microM, respectively. These limits of detection are well within the range of sensitivity required for clinical sample measurement.