Chemical synthesis of ComX pheromone and related peptides containing isoprenoidal tryptophan residues

The ComX pheromone is a post-translationally modified oligopeptide that stimulates natural genetic competence controlled by quorum sensing in Bacillus subtilis. Recently, the structure of the ComX(RO-E-2) pheromone produced by strain RO-E-2 was determined. Based on the NMR analysis, a geranyl group is bound to the tryptophan residue, which results in the formation of a tricyclic ring structure. It was proposed that one of the four possible stereochemical isomers was based on a conformational search for model compounds and the assumption that amino acid residues in the natural pheromone have the L-configuration. All possible modified tryptophan residues and the corresponding COmX(RO-E-2) peptides were synthesized to confirm the precise stereochemistry. Here, the synthesis of the modified tryptophan derivatives was reported in detail. It was succeeded in synthesizing four optically active modified tryptophan methyl esters from which the four diastereomeric COmX(RO-E-2) peptides were prepared. Since only one of the four diastereomers was spectroscopically identical to the natural pheromone and exhibited biological activity, the absolute structure of the COmX(RO-E-2) pheromone was able to be established unambiguously. Furthermore, it was noticed that two other bioactive pheromones were present in the culture broth that were co-purified with COmX(RO-E-2) pheromone. These pheromones were presumed to be the N-terminal truncated peptides Of COmX(RO-E-2) pheromone, i.e., [2-6]COmX(RO-E-2) and [3-6]COmX(RO-E-2), by LC-MS and NMR analyses. Using Fmoc solid-phase peptide synthesis, COmX(RO-E-2) pheromone and the [2-6]COmX(RO-E-2) and [3-6]COmX(RO-E-2) peptides were prepared. The synthetic peptides were identical to the natural pheromones and also showed significant biological activity. (c) 2006 Elsevier Ltd. All rights reserved.