trans-Dichlorotetracyanoplatinate(IV) as a Reagent for the Rapid and Quantitative Formation of Intramolecular Disulfide Bonds in Peptides.

Oxidation of cysteine thiol groups by trans-dichlorotetracyanoplatinate(IV) to form intramolecular peptide disulfide bonds has been studied for a series of dithiol peptides ranging from 4 to 15 amino acid residues in length. The dithiol peptides are rapidly and quantitatively transformed to their intramolecular disulfide forms by a slight excess of [Pt(CN)(4)Cl(2)](2)(-), as shown by HPLC. Quantitative analyses by HPLC and by spectrophotometric titration confirm a [Pt(IV)]:[dithiol peptide] stoichiometry of 1:1. Under the low pH conditions used, oxidation to form a 38-membered ring in the case of reduced somatostatin is as rapid as that to form much smaller rings, suggesting that ring closure is not the rate-determining step. The oxidation rates increase as the pH is increased. Time-resolved spectra show two isosbestic points, indicating that no peptide-platinum intermediates accumulate to a significant amount. A reaction mechanism similar to that for reduction of [Pt(CN)(4)Cl(2)](2)(-) by monothiols is proposed. [Pt(CN)(4)Cl(2)](2)(-) is a mild oxidant and essentially substitution inert; its reduction product, [Pt(CN)(4)](2)(-), is stable, has no redox chemistry with peptides, and does not form complexes with peptides. Moreover, [Pt(CN)(4)Cl(2)](2)(-) and [Pt(CN)(4)](2)(-) are nontoxic and readily separable from peptides by HPLC, and the cost of the Pt(IV) complex is negligible compared with that of peptides. The only unwanted side reaction observed with [Pt(CN)(4)Cl(2)](2)(-) is oxidation of the sulfur of methionine to the sulfoxide form. These characteristics and the results of this study suggest that [Pt(CN)(4)Cl(2)](2)(-) is an excellent reagent for the formation of intramolecular peptide disulfide bonds.