A melanin–chitin complex from the shell of Mytilus edulis: structural characterization and evidence for possible intermolecular interactions

In mollusk shells, the chitin-based framework provides critical support for the attachment of shell-associated proteins and biomineralization. Our research has proposed that this chitin framework may also be closely associated with melanin deposition in the shell of Mytilus edulis. Using thermal acid hydrolysis, a melanin–chitin complex (M–C complex) was extracted from mussel shells. Subsequently, scanning electron microscopy and Micro-CT were employed to reveal the morphological and structural characteristics of this complex, showing that melanin tightly adheres to the chitin framework. Further characterization by FTIR and Raman mapping confirmed their co-localization. ssNMR and XPS analyses also provided some evidence that seems to suggest that hydrogen bonding may be a key factor mediating their interaction. Based on these, a novel possible melanin–chitin structural model was proposed. This discovery enhances our understanding of the shell architecture and provides crucial insights into the composition and formation mechanisms of shells.