Sustainable production of multifunctional melanin-like amino acid surfactant by conjugating o-quinone building block from natural polyphenols

Surfactants occupy a prominent place in the domestic and industrial fields. Amino acid surfactants have attracted attention as alternatives to petrochemical-based surfactants owing to their good biocompatibility and biodegradability. Conventional amino acid surfactants with structures having only acyl linkages lack multifunctionality. Therefore, novel amino acid surfactants must be developed to achieve this feature. Inspired by melanogenesis, the biomimetic synthesis of melanin-like amino acid surfactants with structures similar to that of melanin was conceived. Natural polyphenols with catechol moieties serve as building blocks for hydrophobic moieties. As a proof-of-concept, caffeic acid phenethyl ester was oxidized to o-quinone using periodate resin. Melanin-like amino acid surfactants were synthesized by conjugating amino acids to o-quinones via Michael addition and Schiff base reactions. The structure was confirmed by mass spectrometry. Melanin-like amino acid surfactants have outstanding surface activity with a low critical micelle concentration and excellent capacity for foaming, emulsifying, dispersing, wetting, and solubilizing. Melanin-like amino acid surfactants exhibit low skin irritation, as well as high tolerance to ions and pH. The utilization of polyphenols introduces multifunctionality, affording antioxidant, antibacterial and whitening activities. This study proposes a modular strategy for fabricating new and multifunctional amino acid surfactants that mimic melanin by simply replacing the o-quinone building block. This biomimetic synthesis route is a sustainable process for the green production of amino acid surfactants. This new generation of amino acid surfactants will contribute to meeting the increasing demand for multifunctional surfactants in both academia and industry.