Lanthionine bridges

From a stereochemical viewpoint, the sulfur extrusion methods12122 using P(NEt2)3 seem to have certain advantages, because chiral centers at alpha carbon atoms are not involved in the lanthionine formation. However, due to the possible formation of ionic intermediates prior to formation of the sulfide bridge, undesired symmetrically substituted or also ste-reochemically undesired lanthionines may inadvertently be formed. Possible solutions to circumvent these problems have also been reported. 23,26 ... 

Scheme 5 Synthesis of Unsymmetrically Substituted Lanthionines Using the Desulfurization Reaction of Ethylene Glycol Bridged Cystines 23 ...

The lantibiotics differ extensively from the class II bacteriocins in that they contain post-translationally modified amino acids, as for example dehydrated amino acids and lanthionine residues, forming intramolecular thioether bridges [39, 184]. The chemical modification reactions leading to the typical lanthionines were first proposed by Ingram [185] and are assumed to be catalyzed by specific enzymes encoded in the lantibiotic gene cluster. In the lantibiotic lactocin S, p-alanine residues were discovered, probably by conversion of dehydrated serine residues via a dehydrogenation reaction [82]. In some... 

Microorganisms and plants can synthesize many uncommon amino acids. For example, some microbes make 2-aminoisobutyric acid and lanthionine, which is a sulfide-bridged derivative of alanine. Both of these amino acids are found in peptidic lantibiotics such as alamethicin. While in plants, 1-aminocyclopropane-l-carboxylic acid is a small disubstituted cyclic amino acid that is a key intermediate in the production of the plant hormone ethylene. 

Gross, E., and J. L. Morell Nisin. The Assignment of Sulfide Bridges of p-Methyl-lanthionine to a Novel Bicyclic Structure of Identical Ring Size. J. Amer. Chem. Soc. 92, 2919 (1970). 

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