Cysteine alkylation in unprotected peptides: synthesis of a carbavasopressin analog by intramolecular cysteine alkylation

An expedient method that allows modification of a cysteine residue side chain through alkylation of unprotected peptides is described. In order to test the generality of the method, an unprotected cysteine-containing tetrapeptide possessing several potentially competing nucleophilic functionalities such as amino, hydroxy, and carboxyl groups was chosen as a model peptide. The model tetrapeptide, H-Ser-Lys-Cys-Phe-OH was synthesized by standard solid-phase methods, cleaved with HF, and purified by reverse-phase HPLC. Reaction of the unprotected peptide with 1.3 equiv of various alkylating agents in saturated ammonia in methanol at 0 o C proceeded cleanly to yield single alkylation products within one hour as shown by HPLC analysis. In most cases acidic, basic, and neutral side chains were introduced by this method in over 80% yield. The methodology is also applicable to the synthesis of cyclic peptides by intramolecular cysteine alkylation and provides a useful alternative to cyclizations that occur through disulfide or amine bond-forming reactions as illustrated by the synthesis of a carbavasopressin