Synthesis of amino acid surfactants bioinspired from melanin using ethyl protocatechuate

Amino acid surfactants are widely used in areas of life and industry. Biomimetic synthesis of a new type of amino acid surfactant was based on the melanogenesis using the natural substances ethyl protocatechuate. Ethyl protocatechuate was oxidized to o-benzoquinone using silver oxide. The amino group of amino acids, including arginine, cysteine, glutamic acid, histidine, leucine, and threonine, was conjugated with o-benzoquinone to synthesize melanin-like amino acid surfactants. o-Benzoquinone moiety behaved as hydrophobic part while amino acid residue served as hydrophilic head group. The critical micelle concentration of cysteine product was 1.18 mM. The melanin-like amino acid surfactant showed excellent foamability, wettability, emulsification, solubilization, and dispersibility properties. Arginine product showed the longest foam half lifetime of 18 min; the wetting time of threonine product was 23 s; the emulsion delamination time of cysteine product was 1034 and 1940 s for liquid paraffin and soybean oil, respectively; threonine product had the most solubilization capacity of 9430 mL mol⁻¹ of benzene; glutamic acid product showed the highest lime soap dispersing power value of 45.58%. It was the first invention for biomimetic synthesis of amino acid surfactant from aspect of both structure and route. Melanin-like amino acid surfactant had promising potential in both fundamental research and industrial application.