Synthesis and kinetics of oxidation of some tripeptides of elastin sequences with anodically generated manganese(III) sulfate: A mechanistic study

The fragments of elastin sequences, glycyl-alanyl-proline (GAP), glycyl-valyl-proline (GVP), glycyl-isoleucyl-proline (GIP) and glycyl-leucyl-proline (GLP) were synthesized by a classical solution phase method and characterized. The kinetics of oxidation of these tripeptides (TP) by Mn(III) has been studied in the presence of sulfate ions in acidic solution at 25 degreesC. The reaction was followed spectrophotometrically at lambda (max) = 500 nm. A first-order dependence of rate on both [Mn(Ill)] and [TP] was observed. The rate is independent of the concentration of the reduction product, Mn(II), and hydrogen ions. The effects of varying the dielectric constant of the medium and addition of anions such as sulfate. chloride or perchlorate were studied. Activation parameters have been evaluated using Arrhenius and Eyring plots. The oxidation products were isolated and characterized. A mechanism involving the reaction of TP with Mn(III) in the rate-limiting step is suggested. The effect of hydrophobicity of the amino acids on the rate of oxidation is discussed.