Isolation and structure identification of novel brominated diketopiperazines from Nocardia ignorata - A lichen-associated actinobacterium

Abstract

Actinobacteria are well known for their potential in biotechnology and their production of metabolites of interest. Lichens are a promising source of new bacterial strains, especially Actinobacteria, which afford a broad chemical diversity. In this context, the culture medium of the actinobacterium Nocardia ignorata, isolated from the terrestrial lichen Collema auriforme, was studied. The strain was cultivated in a BioFlo 115 bioreactor, and the culture medium was extracted using an XAD7HP resin. Five known diketopiperazines: cyclo (L-Pro-L-OMet) (1), cyclo (L-Pro-L-Tyr) (2), cyclo (D-Pro-L-Tyr) (3), cyclo (L-Pro-L-Val) (4), cyclo (L-Pro-L-Leu) (5), and one auxin derivative: indole-carboxaldehyde (8) were isolated, along with two new brominated diketopiperazines: cyclo (D-Pro-L-Br-Tyr) (6) and cyclo (L-Pro-L-Br-Tyr) (7). Structure elucidation was performed using HRMS and 1D and 2D NMR analysis, and the synthesis of compounds 6 and 7 was carried out in order to confirm their structure.