Differences between water-soluble and water-insoluble melanin derived from Inonotus hispidus mushroom

Melanin is a natural pigment with a high content and a complex polymer structure. In this study, both water-soluble and water-insoluble melanin were obtained from the fruiting bodies of the mushroom Inonotus hispidus, and scanning electron microscopy (SEM), ultraviolet (UV) absorption, Fourier transform infrared (FTIR) spectroscopy, elemental analysis, nuclear magnetic resonance (NMR) spectroscopy, pyrolysis gas chromatography mass spectroscopy (Py-GCMS), and matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS/MS) were used to analyze the water-soluble and water-insoluble I. hispidus fungal melanin (IHFM). Differences in the chemical composition and structure of I. hispidus fungal melanin (IHFM) were found. Studies showed that both have a maximum absorbance at 226 nm, and the maximum mass-to-charge ratios are [M + H] ⁺ m/z = 272.2434 and 272.0308, respectively. Elemental analysis revealed that they both contain C, H, N, O, and do not contain sulfur, which is only found in eumelanin. The differences between water-soluble and water-insoluble IHFM may relate to the following factors: (i) the microscopic particles of water-soluble IHFM are relatively small, the arrangement is relatively neat, and they contain a large quantity of benzene, phenol, and indole compounds; and (ii) the microscopic particles of water-insoluble IHFM are larger and the arrangement is irregular. The types and quantities of benzene, phenol, and indole compounds are relatively small, rich in a variety of sugar monomers, and may be insoluble in water due to the presence of a greater number of aliphatic groups. Studies have found that water-soluble IHFM has good cellular antioxidant activity, can reduce oxidative damage by hydrogen peroxide (H₂O₂) on LO2 cells effectively and can significantly reduce the level of intracellular reactive oxygen species (ROS) to protect the liver. These results are beneficial in order to develop applications of melanin in the food industry and in other fields.