Abstract
Autoxidation of cysteine catalyzed by Cu (II) in glycylglycine buffer was examined spectrophotometrically. Upon mixing the Cu (II) and cysteine solutions was produced a transient purple color, due to a Cu (II)-cysteine complex, giving a broad electronic spectrum with λmax=530 nm. The purple Cu (II) complex, which is decomposed fleetingly to Cu (I) species, is stabilized relatively as pH increases. The autoxidation of cysteine does not exhibit Michaelis-Menten kinetic behaviors, and appears to be half order in concentration of cysteine over pH 7.3. Increasing concentrations of oxygen in the medium promote the autoxidation. As cysteine is autoxidized, hydrogen peroxide is produced and partially accumulated. The plot of the reciprocal of the rate as a function of the reciprocal of the oxygen concentration gives a straight line. One molar oxygen participates probably in a step of the reoxidation of the Cu (I) species. A pH-rate profile of the autoxidation displays a bell-shaped curve with a maximum at pH 7.4.
