Discovery of a Highly Selective and Efficient Reagent for Formation of Intramolecular Disulfide Bonds in Peptides

We have discovered that trans- [Pt(en) 2 Cl 2 ] 2+ (en = ethylenediamine) is a highly selective and efficient reagent for the quantitative formation of intramolecular disulfide bonds in peptides. A series of 14 dithiol peptides which form disulfide-containing rings ranging in size from 14 to 53 atoms were used to characterize the reagent. The dithiol peptides are cleanly and rapidly converted to their disulfide forms by a slight excess of the platinum complex under mild reaction conditions (slightly acidic and neutral media). For all the dithiol peptides studied, including a penicillamine-derived peptide, the oxidation yields range from 97% to 100%. No side reactions were observed, including no oxidation of the methionine side chain. The reaction kinetics for oxidation of reduced pressinoic acid were found to be second order overall: rate = k ‘[Pt(IV)][dithiol peptide], where k ‘ is a pH-dependent second-order rate constant. Values of 0.60 ± 0.01, 3.5 ± 0.2, and 22 ± 1 M - 1 s - 1 were determined for k ‘ at pH 3.0, 4.0, and 5.0, respectively (25 °C and 0.45 M ionic strength). A reaction mechanism for oxidation of dithiol peptides by [Pt(en) 2 Cl 2 ] 2+ is proposed. [Pt(en) 2 Cl 2 ] 2+ and its reduction product [Pt(en) 2 ] 2+ are essentially substitution inert under the conditions used for disulfide formation, they are nontoxic, and they are readily separated from peptides by HPLC. The characteristics of [Pt(en) 2 Cl 2 ] 2+ and its reaction properties with dithiol peptides suggest that [Pt(en) 2 Cl 2 ] 2+ is a universal reagent for the rapid and quantitative formation of intramolecular disulfide bonds in peptides.