Quantitative Monitoring of Cyclic Glycine–Proline in Marine Mangrove-Derived Fungal Metabolites

This study developed and validated a robust UPLC-MS/MS method for quantifying cyclic glycine-proline (cGP) in mangrove-derived <i>Penicillium</i> and <i>Aspergillus</i> strains. The method demonstrated excellent linearity, precision, and recovery, with detection limits as low as 4.8 ng/mL. <i>Penicillium pedernalense</i> extract achieved a cGP content of 67.45 ± 1.11 ng/mL, with a corresponding fermentation yield of 29.31 ± 0.61 mg/L. This surpassed <i>Penicillium steckii</i>, which reached a content of 31.71 ± 0.31 ng/mL, with a yield of 8.51 ± 0.15 mg/L. This quantitative approach for metabolite analysis provides a viable method for screening these fungal strains, highlighting their potential for sustainable production of cyclic glycine-proline (cGP).