Amino acid modification and structural characterization of Auricularia heimuer melanin

In order to improve the solubility of Auricularia heimuer melanin, amino acid modification was performed on the melanin extracted from the fermentation broth of A. heimuer. The structure of amino acid melanin with better modification effects was selected for characterization, and its solubility, stability and antioxidant activity were analyzed. The results showed that arginine, lysine, and histidine had better modification effects on A. heimuer melanin, with absorbance values of 1.970, 1.560, and 1.193, respectively. The ultraviolet spectral analysis of the modified amino acid melanin showed that the maximum absorption peaks all shifted towards shorter wavelengths. Infrared spectroscopy analysis revealed that there was an absorption peak of amino acid melanin at wave number 2200-2000 cm^-1. The solubility of arginine, histidine and lysine melanin in water were significantly increased, being 5.62 g/L, 5.43 g/L and 5.39 g/L, respectively. Under different temperature, light and metal ion conditions, with the extension of time, the absorbance values of A. heimuer melanin and amino acid melanin did not change significantly, and the melanin retention rates were all more than 94.97%. When the mass concentration of A. heimuer melanin and amino acid melanin was 4 mg/ml, the clearance rates of DPPH free radicals, superoxide anion free radicals and hydroxyl free radicals were all more than 96%, and the iron ion reducing power (absorbance value) was 1.863 to 2.252.