A facile method for the direct synthesis of lanthionine containing cyclic peptides

A series of disulfide bridged peptides were designed as potential inhibitors of protein-protein interactions. Following solid phase synthesis, completely deprotected linear peptides were first oxidized to their disulfide analogs and then transformed into their lanthionine equivalents via a base-assisted reaction in water. Peptides consisting of cystine bridges of length i, i+3, with and without discrimination of the chiral centers, were studied for this transformation. Lanthionine peptides were also obtained directly from the reduced linear peptides under mild alkaline treatment, and the reaction proceeded via disulfide bond formation. The extent of conversion of a disulfide bridge into its lanthionine counterpart varied according to the primary sequence. Product characterization revealed diastereomeric lanthionine formation. The presence of D-amino acids, peptide conformation, and/or position of the cystine bridge are among the factors determining the facility of this reaction. Elimination of the backbone proton beta to the sulfur atom followed by intramolecular thiol Michael addition is the most likely mechanism for this transformation.