Four pairs of proline-containing cyclic dipeptides from Nocardiopsis sp. HT88, an endophytic bacterium of Mallotus nudiflorus L.

Strain HT88 was isolated from the fresh stems of Mallotus nudiflorus L, and it was identified as Nocardiopsis sp. by analyzing its morphology and the 16S rRNA sequence. The extracts of fermented HT88 showed potent antimicrobial activities. Bioassay guided separation of extracts led to eight proline (or hydroxyproline, Hyp)-containing cyclic dipeptides. Their structures were determined by 1D and 2D NMR spectroscopy and ESI mass spectrometry and further comparison with existing ¹H and ¹³C NMR, melting points and specific rotation data. The eight 2,5-diketopiperazines (DKPs) were identified as cyclo(L-Pro-L-Leu) (1), cyclo(Pro-Leu) (2), cyclo(L-trans-Hyp-L-Leu) (3), cyclo(D-trans-Hyp-D-Leu) (4), and cyclo(D-Pro-L-Phe) (5), cyclo(L-Pro-L-Phe) (6), and cyclo(D-cis-Hyp-L-Phe) (7), cyclo(L-trans-Hyp-L-Phe) (8), respectively. Up to date, this is the first isolation of four pairs of proline based DKPs from Nocardiopsis sp.