Endogenous opioid peptides are of particular interest as they modulate pain and reward processing, and the real-time <i>in vivo</i> quantification of these molecules is invaluable to developing therapeutic strategies to treat these medical problems. A novel method has been developed for the electrochemical detection of methionine-enkephalin in the brain by exploiting the oxidation of the electroactive residues, tyrosine and methionine, on a carbon electrode. However, uncovering the fundamental mechanism of electron transfer is necessary to further progress this technological advance. Density functional theory was used to optimize the structures for the amino acids methionine and tyrosine in zwitterionic form to determine redox potentials and ground state energies. Electrochemical square schemes were used to study the simple proton coupled electron transfer for the amino acids. The initial calculated square scheme for methionine showed that the proposed intermediate redox structures were unlikely to occur, and this was supported by literature. Several new schemes were proposed where the oxidation of methionine occurs through reactions with hydroxyl radicals. Mass spectral evidence from previous studies support a scheme whereby hydroxyl radicals interact with the methionine molecule to form the sulfoxide and sulfone intermediates at the electrode surface. This study will reveal the fundamental nature of the electrochemical detection of methionine-enkephalin at carbon electrodes to permit advances in the sensing technology and in turn deepen our understanding of peptidergic mechanisms implicated in drug addiction.

Theoretical analysis of the standard reduction potential for the irreversible redox process of methionine

253rd National Meeting (2017)

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