Voltammetric behavior of trolox in the presence of amino acid in unbuffered dimethylsulfoxide

The voltammetric behavior of trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) in the presence of amino acids was studied at a plastic formed carbon working electrode in an unbuffered dimethylsulfoxide solution containing 50 mmol L−1 LiClO4. A well-defined oxidation peak was observed on the linear potential sweep voltammogram of trolox at 0.6 V vs. Ag/AgCl. Following the addition of amino acid (such as alanine, arginine, asparagine, glutamic acid, glutamine, glycine, histidine, homocysteine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, or valine) to the solution, a new peak (termed a prepeak) appeared on the voltammogram at more negative potentials than the original oxidation peak of trolox. The half-peak potential (Ep/2) of the prepeak shifted to a more negative value accompanied by a decrease in the pKb1 of the added amino acid. A plot of the Ep/2 of the prepeak against the pKb1 shows an approximately linear line with a slope of 31 mV p K b 1 − 1 and correlation coefficient (r) of 0.709. Since a plot of the Ep/2 of the prepeak against the pH at the isoelectric point (pI) shows a linear line with a slope of 19 mV pI−1 and r of 0.958, amino acid would contribute to the proton transfer process in this electrode reaction not as a conjugated base but as an amphoteric zwitterion under the unbuffered condition. The prepeak height was proportional to the amino acid concentration ranging from 0.01 to 0.2 or 0.3 mmol L−1 with an r of more than 0.997. The present method has a possibility for the electrochemical determination of amino acids.