Comformation and absolute configuration of -methyllanthionine

If the bicyclic peptide ring proposed by Gross $\text{et}$$\text{al}$. (1,2) does in fact exist in nisin and related antibiotics, then the unusual β-methyllanthionine component must be significantly distorted from its conformation in the free state, as determined by x-ray structure analysis. The torsion angles about the SC_β bonds are 50–100° from the torsion angles in models of the sulfur-bridged peptide ring proposed for nisin. The chirality of the methylated β-carbon atom is (S). The conformation of the amino acid differs from that of $\text{meso}$-lanthionine only by a 180° rotation of a carboxyl group about the $\text{C}{}_{\mathrm{\alpha }}{}^{\mathrm{D}}\text{C}{}_{\mathrm{\beta }}\text{(CH}{}_3\text{)}$ bond.