Biological activity and quantitative determination of free amino acids from muskmelon (Cucumis melo L.) extracts

Muskmelon (Cucumis melo L., family Cucurbitaceae) processing generates substantial quantities of underutilized by-products, including peel/rind, stems, and leaves, which may serve as valuable sources of bioactive compounds. This study aimed to evaluate the phytochemical composition, quantify free amino acids, assess antimicrobial activity, and investigate molecular interactions of these by-products to support their potential for food and pharmaceutical valorization.

Phytochemical screening and thin-layer chromatography (TLC) were employed to identify major secondary metabolites. Free amino acids were quantified using gas chromatography-mass spectrometry (GC-MS). Antimicrobial activity of methanolic extracts from different plant parts was evaluated against Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Candida albicans using the agar well diffusion method. Molecular docking analysis was performed to assess interactions between identified amino acids and target proteins, including S. aureus tyrosyl-tRNA synthetase (PDB: 1JIJ) and C. albicans Sap1 protease (PDB: 2QZW).

Phytochemical analysis confirmed the presence of sterols, triterpenes, flavonoids, phenolics, saponins, and carotenoids, with peel/rind and leaves showing the richest profiles. GC-MS analysis revealed glutamine (28.0%), alanine (11.2%), and aspartic acid (9.3%) as the predominant free amino acids. Methanolic extracts exhibited significant, concentration-dependent antimicrobial activity, with stronger inhibition observed against Gram-positive bacteria and C. albicans. Molecular docking demonstrated favorable binding affinities of selected amino acids toward target enzymes, supporting the observed biological activity.

The findings highlight muskmelon by-products as promising sources of bioactive compounds with notable antimicrobial potential. While amino acids demonstrated favorable in silico interactions, the antimicrobial activity is primarily attributed to phytochemical constituents. These results support the potential application of muskmelon waste in functional food development and natural antimicrobial formulations. Further studies, including MIC/MBC determination and phytochemical-targeted docking, are recommended to validate and expand these findings.