Tryptophan electrode reactions

Individual amino acids in proteins undergo oxidation reactions at potentials on the positive side of ca. 0.7 V. Tryptophan and tyrosine residues are oxidizable at carbon, " platinum, and gold electrodes while histidine is oxidizable at carbon. The sulfur-containing amino acids cysteine, cystine, and methionine have been shown to be oxidized at carbon. Adsorption processes are important in these reactions. 

Others, such as penicillinase, will promote the reaction of many substrates. Thus, ampicillin, naficillin, penicillin G, penicillin V, cyclibillin, and dicloxacillin can be determined with a penicillinase electrode. D- and L-amino acid oxidases are even less selective. The former, when coupled to an electrode, responds to D-phenylalanine, D-alanine, o-valine, D-methionine, D-leucine, o-norleucine, and o-isoleucine, while the latter responds to L-leucine, L-tyrosine L-phenylala-nine, L-tryptophan, and methionine. 

For instance, Guilbault and Lubrano [220] prepared an electrode for assaying L-amino acids by the chemical bonding of L-amino oxidase to a Pt electrode that sensed the hydrogen peroxide produced in the above reaction. The L-amino acids cysteine, leucine, tyrosine, phenylalanine, tryptophan, and methionine were assayed. 

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