Modeling Fungal Melanin Buildup: Biomimetic Polymerization of 1,8‐Dihydroxynaphthalene Mapped by Mass Spectrometry

Due to the emerging biomedical relevance and technological potential of fungal melanins, and prompted by the virtual lack of information about their structural arrangement, an optimized synthetic protocol has been devised for a potential structural model of Ascomyces allomelanin through enzyme‐catalyzed oxidative polymerization of 1,8‐dihydroxynaphthalene (1,8‐DHN). Electrospray ionization mass spectrometry (ESI‐MS) measurements of freshly synthesized DHN‐polymer recorded in the negative ion mode allowed detection of oligomers up to m/z 4000, separated by 158 Da, corresponding to the in‐chain DHN‐unit. The dominant peaks were assigned to singly‐charged distribution, up to 23 repeating units, whereas a doubly charged polymer distribution was also detectable. Chemical derivatization, ultra‐performance liquid chromatography (UPLC)‐ESI MS, and MS/MS data confirmed that oxidative polymerization of 1,8‐DHN proceeds through C−C coupling of the naphthalene rings. The new insights reported here into synthetic 1,8‐DHN oligomers/polymers as a mimic of fungal melanins may guide novel interesting advances and applications in the field of biomimetic functional materials. Modeling of fungal melanin using ESI‐MS: The successful mass spectrometric investigation of 1,8‐DHN‐based allomelanin structure obtained in vitro by enzyme‐mediated oxidative polymerization is reported here. Electrospray ionization mass spectrometry (ESI‐MS), coupled with liquid chromatography, has proven a valuable tool to gain novel insights on the structural model of fungal melanin as well as on the coupling mechanism of 1,8‐DHN monomeric units.