Specific intramolecular cyclization to form disulfide bridges in peptides represents a major challenge to the synthetic art. The N-dithiasuccinoyl (Dts) function was originally proposed as an amino protecting group, removable under mild conditions by thiolysis [Barany, G. and Merrifield, R.B., J. Am. Chem. Sec., 99 (1977) 7363; 102 (1980) 3084]. We demonstrate here that this chemistry can be 'inverted', i.e., Dts-amines can be used as mild oxidation reagents that promote formation of intramolecular disulfide bridges. With oxytocin and deamino-oxytocin as models, and with Dts-glycine as the oxidant, we have shown the efficacy of this method under a variety of conditions: both components in solution; dithiol-peptide on a polymeric support and Dts-glycine in solution; and soluble dithiol-peptide with Dts-glycyl-resin. Kinetics have been determined under a range of conditions in mixtures of acetonitrile-phosphate buffer. The logarithm of the reaction rate (based on a simplified first-order assumption) varies linearly with pH; the slope of these correlations is 0.68+/-0.02. Under suboptimal conditions, by-products have been observed that include parallel and antiparallel dimers as well as trisulfide analogues. Optimized conditions give good yields and purities of the desired disulfides. Particular advantages of this approach include the lack of reactivity of Dts-glycine with all non-sulfhydryl side-chain functionalities found in peptides, and the fact that Dts-mediated oxidation is irreversible.
