Cyclo(Gly-L-Leu)

Liquid chromatography mass spectrometry (LC-MS) was used to quantify the levels of the 2,5-diketopiperazines cis-cyclo(L-Leu-L-Pro) and cis-cyclo(L-Phe-L-Pro) in acidified dough and bread. Dough acidification led to a significant increase in the level of cis-cyclo(L-Leu-L-Pro) and cis-cyclo(L-Phe-L-Pro) over 48 h compared to a nonacidified dough. However, no differences were found between chemically (mix of lactic and acetic acid in the presence of antibiotics) and biologically acidified doughs. On examination of the levels of cis-cyclo(L-Leu-L-Pro) and cis-cyclo(L-Phe-L-Pro) in bread crumb and crust, it was found that temperature is the main causative agent of 2,5-diketopiperazine formation during the baking process. Bread crumb and crust contained almost 100 and 2000 times respectively the levels found in dough prior to baking. cis-Cyclo(L-Leu-L-Pro) and cis-cyclo(L-Phe-L-Pro) were also found to be at sensorally active levels in bread crust, however both 2,5-diketopiperazines were found to be below the minimum inhibitory concentration for antifungal activity in bread.

The taste compounds inducing the blood-like, metallic bitter taste sensation reported recently for a dichloromethane extract prepared from roasted cocoa nibs were identified as a series of 25 diketopiperazines by means of HPLC degustation, LC-MS/MS, and independent synthesis. Among these 25 compounds, 13 cis-configured diketopiperazines, namely, cyclo(L-IIe-L-Phe), cyclo(L-Val-L-Leu), cyclo(L-Pro-L-Pro), cyclo(L-IIe-L-Pro), cyclo(L-Val-L-Tyr), cyclo(L-Ala-L-Tyr), cyclo(L-Phe-L-Ser), cyclo(L-Ala-L-IIe), cyclo(L-Leu-L-Phe), cyclo(L-Pro-L-Val), cyclo(L-Pro-L-Thr), cyclo(L-Pro-L-Tyr), and cyclo(L-Val-L-Val) were identified for the first time in cocoa. In addition, the taste recognition thresholds for the metallic as well as the bitter taste of the diketopiperazines were determined, and after quantitative analysis by using two diastereomeric diketopiperazines as the internal standards, the sensory impact of the diketopiperazines was evaluated on the basis of their dose-over-threshold (DoT) factors calculated as the ratio of the concentration and the threshold concentration of a compound. These data revealed DoT factors above 1.0 exclusively for cis-cyclo(L-Pro-L-Val), cis-cyclo(L-Val-L-Leu), cis-cyclo(L-Ala-L-Ile), cis-cyclo(L-Ala-L-Leu), and cis-cyclo(L-Ile-L-Pro), whereas all of the other diketopiperazines were present below their individual bitter taste threshold concentrations and should therefore not contribute to the cocoa taste. Because the DoT factors do not consider the nonlinear relationship between the concentration and gustatory response of an individual compound, we, for the first time, report on the recording of dose/response functions describing the human bitter taste perception of diketopiperazines more precisely.

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.