Isatin as Catalyst for the Anodic Oxidation of Amino-Acids

Abstract

Langenbeck found some time ago, that isatin and some of its derivatives were model catalysts for the amino-acid dehydrogenases, active in biological systems. In numerous solvents (pyridine, water, acetic acid etc.) isatin can oxidize amino-acids to aldehydes, CO2 and NH3. In this reaction, it is reduced to isatyd, which is re-oxidized to isatin by atmospheric oxygen. This principle also can be applied to the electrochemical oxidation of amino-acids. For this purpose the model catalyst and the amino-acid to be oxidized (alanin, Phenylalanin, glycocoll) are dissolved in buffered acetic or formic acid or in an NaCl solution. If a network of inert material is placed in the solution and is maintained potentiostatically at 500 mV against a hydrogen electrode in the same solution, then, on vigorous stirring, a weak anodic current flows. This current can increase up to 100 fold if finely-divided carbon is suspended in the solution. This corresponds to the anodic oxidation of amino-acids, each molecule of aminoacid losing not 2 electrons (as predicted by the Langenbeck theory), but 8 to 9 electrons. Possible mechanisms for this reaction are discussed.