Extraction and characterization of a novel glycosylated naphthazarin pigment from mangrove Aspergillus unguis AUMC15225

Abstract

Natural naphthoquinone (NQ) dyes are gaining attention as safer and more sustainable alternative to synthetic colorants, and their glycosylation can enhance solubility and stability. In the present study, a novel, extracellular, highly stable, glycosylated derivative of NQ pigments (2-O-β-L-arabinofuranosyl-5,8-dihydroxy-1,4-naphthoquinone, C₁₅H₁₆O₈) was produced by the mangrove epiphytic fungus, Aspergillus unguis. Pigment production kinetics and solubility testing indicated the secondary metabolic behavior and high polarity of the pigment. HPLC separated the pigment into two isomeric fractions with retention times of 4.840 and 6.020 min. Structural characterization was supported by several analyses, with a λ_max at 300 nm and R_f values of 0.76-0.8. Furthermore, FTIR spectroscopy has revealed the key peaks of naphthazarin core at 1642, 1255-1262, 3331-3335, 1650, 918, 700, and 553 cm⁻1. In addition, glycosylation characteristic peaks were found at 3396, 1200-1260, 1144, 1101, and 1028 cm⁻1. Quinone carbonyls and α hydroxyls' hydrogen bonding in the naphthazarin core was indicated by the peaks at 2920-2933 cm^-1. The molecular weight of the neutral compound was found to be 324 Da via LCMS. NMR spectroscopy has displayed characteristic signals confirming the proposed structure. Proton NMR showed signals at δ 6.586 and 6.214 ppm (aromatic protons: H-6 and H-3 of the naphthazarin core); 4.892 ppm (anomeric proton H-1΄ of the sugar moiety); 4.85-4.22 ppm (anomeric protons of the pentose sugar). Moreover, ¹³C NMR displayed signals at δ 97.38 ppm (anomeric carbon C-1΄), 77-70 ppm (oxygenated sugar carbons of pentose), 61.61 ppm (hydroxymethyl carbon C-5΄ that confirms the furanose structure).