Antibacterials

Work by the group in this area has been focused on the synthesis of the peptide antibiotic lysobactin and, more recently, towards the synthesis of subtilin.

Lysobactin

Lysobactin is a depsipeptide antibiotic first isolated from Lysobacter ATCC 53042. It contains a number of unusual amino acids including L-hydroxyasparagine, L-hydroxyleucine, L-threo-b-hydroxyphenylalanine and D- and L-allo-threonine. Lysobactin has been shown to parallel the biological activity of vancomycin but it displays some toxicity. Therefore, a solid phase approach to the total synthesis of lysobactin would allow rapid production of many analogues, possibly leading to a more active and less toxic derivative.

The Structure of Lysobactin.

The 11mer peptide was synthesised by solid phase FMOC chemistry with resin attachment via the carboxylic acid of aspartic acid to the rink amide linker, thereby creating an asparagine residue on resin cleavage. The resin bound reactions were followed by MALDI-TOF MS (see combinatorial methods). Macrolactonisation was found to proceed on activation with HOBt / DIC / DMAP to give the lysobactin analogue in an overall yield of 15%.

Subtilin

Subtilin belongs to a family of polycyclic peptide antibiotics called the lantibiotics. They are characterised by the unusually high proportion of unnatural amino acids which include dehydroalanine, dehydroaminobutyrate, lanthionine and methyl-lanthionine.

These unusual amino acids are created by post-translational modification of a ribosomally derived precursor peptide. The first step is thought to be an enzyme catalysed dehydration of serine / threonine to give dehydroalanine / dehydroaminobutyrate. Subsequent Michael addition of a cysteine thiol onto the ab-unsaturated amino acids gives lanthionine and methyl-lanthionine respectively. This reaction is highly stereospecific giving D, L-(meso)-lanthionine. Research is directed towards the investigation of the regio- and stereospecificity observed on conversion of a dehydrated precursor into the thioether bridged rings.

Novel methods for the synthesis of multiply dehydrated peptides have been developed. S-Methyl-cysteine is incorporated into peptides and is converted to dehydroalanine by oxidation and elimination. Elimination can be affected by either pyrolysis (in refluxing dioxane) or by treatment with base (DBU or NaOH) giving the desired ab-unsaturated amino acid in very good yield.

Work is directed towards the stereospecific synthesis of orthogonally protected L, L-lanthionine and D, L-lanthionine, and the synthesis of a templated library with the ultimate aim to produce smaller subtilin fragments with increased stability and retention of activity.