Isolation and Identification of Cis-2,5-Diketopiperazine from a Novel Bacillus Strain and Synthesis of Its Four Stereoisomers

The <i>Bacillus horneckiae</i>-like strain 2011SOCCUF3 was isolated from the marine sponge <i>Spongia officinalis</i> and its metabolome was studied for secondary metabolites with antimicrobial activity. Culturing in the presence of Diaion HP-20 resin and purifying the culture extract identified cyclo-phenylalanine-proline (cyclo-(Phe-Pro)), a 2,5-diketopiperazine (2,5-DKP), isolated as a major metabolite. Further, LCMS analysis of the extract showed the presence of two isomers of the molecule in the culture broth. To confirm the stereochemistry of the isomers observed in the natural extract, all four stereoisomers of cyclo-(Phe-Pro) were synthesised. NMR and LCMS studies identified the presence of both <i>cis-</i> and <i>trans</i>-cyclo-(Phe-Pro) isomers. Stability and epimerisation studies on synthetic isomers and the effect of culturing conditions suggested that the less stable <i>cis</i> isomer was naturally produced, which epimerised in culture broth.