Extraction and Structural Characterization of Melanin from Testae of Xanthoceras sorbifolia

Melanin from testae of Xanthoceras sorbifolia was extracted and purified by alkaline extraction, acid hydrolysis and organic solvent wash. The effects of solvent concentration, temperature, time, and liquid/solid ratio on the extraction were studied. The melanin was characterized by diagnostic tests and spectroscopies of ultraviolet visible light (UV-Vis), infrared spectrum (IR) and electron paramagnetic resonance (EPR). Results showed that the melanin was suitable to be extracted with 1 mol/L NaOH and liquid/solid ratio of 15 mL/g at 50℃ for 12 h, twice. Under these conditions, the yield of melanin crude extract was 14.42%, and the yield of the purified melanin was 5.23% through further acid hydrolysis and organic solvent washing. After the purification, the color value at 400 nm was enhanced by 21.0%. The physicochemical properties of the melanin showed a strong similarity to these of the synthetic DOPA melanin. They were insoluble in both water and common organic solvents and dissolved in 1 mol/L KOH, precipitated below pH3, bleached in NaClO and H₂O₂, and the qualitative reaction of polyphenols were positive. The melanin absorption spectrum represented the decreasing line in the ultraviolet and visible range of wavelengths. The infrared spectrum had a broad absorption peak around 3400 cm^-1 and a strong one at 1650~1620 cm^-1. The EPR spectrum was a single line devoid of hyperfine splitting.