Quantification of cyclic dipeptides from cultures of Lactobacillus brevis R2Δ by HRGC/MS using stable isotope dilution assay

Lactic acid bacteria (LAB) play an important role as natural preservatives in fermented food and beverage systems, reducing the application of chemical additives. Thus, investigating their antifungal compounds, such as cyclic dipeptides, has gained prominence. Previous research has primarily focussed on isolation of these compounds. However, their precise quantification will provide further information regarding their antifungal performance in a complex system. To address this, deuterated labelled standards of the cyclic dipeptides cyclo(Leu-Pro), cyclo(Pro-Pro), cyclo(Met-Pro) and cyclo(Phe-Pro) were synthesized, and stable isotope dilution assays were developed, enabling an accurate quantification of cyclo(Leu-Pro), cyclo(Pro-Pro), cyclo(Met-Pro) and cyclo(Phe-Pro) in MRS-broth and wort. Quantitative results showed that, in the Lactobacillus brevis R2Δ fermented MRS-broth, the concentrations of cyclo(Leu-Pro), cyclo(Pro-Pro) and cyclo(Phe-Pro) were significantly higher (P < 0.05), than in wort for cyclo (Leu-Pro) when compared with their controls. This indicates that the formation of these three cyclic dipeptides is related to L. brevis R2Δ metabolism. Furthermore, this represents the first report of cyclic dipeptides quantification using stable isotope dilution assays in LAB cultures both in vitro and in a food system.