Cystine, an essential determinant of protein tertiary structure, is also a target for electrochemical manipulation: Cyclotides

The ability for thiol residues to undergo oxidation to disulfides is biochemically important affecting antioxidant activity (e.g., glutathione) and determining protein tertiary structure. The formation of cystine disulfide bridges from two cysteine residues not only influences protein confirmation but also renders it electrochemically active. Although peptides can be quantified by electrochemical oxidation of the disulfide bond, the commonly used working electrode, glassy carbon, suffers from two major issues – a limited oxidation potential and the persistent fouling of the electrode's surface necessitating frequent cleaning procedures. The boron-doped diamond (BDD) working electrode, on the other hand, can be used at extreme oxidation potentials and, as its surface is inert, does not suffer from adsorption problems. The ability for HPLC with electrochemical detection to quantify a variety of disulfide containing peptides was evaluated and included measurement of glutathione disulfide, cyclic nonapeptides (vasopressin and oxytocin), an example of intra and inter chain disulfide cross-linking (insulin) and a cyclic, disulfide rich cyclotide (cycloviolacin O2).