Synthesis, characterization, and direct aqueous superoxide anion scavenging of a highly water-dispersible astaxanthin-amino acid conjugate

Abstract

The aqueous solubility and/or dispersibility of synthetic carotenoid analogs can be improved by varying the chemical structure(s) of the esterified moieties. In the current study, a highly water-dispersible astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione) derivative was synthesized by esterification to the amino acid L-lysine, and subsequently converted to the tetrahydrochloride salt. Deep violet, evenly colored aqueous suspensions were obtained with addition of the novel derivative to USP purified water up to a maximum of 181.6 mg/mL. These aqueous suspensions were obtained without the addition of heat, detergents, co-solvents, or other additives. At higher concentrations (above 181.6 mg/mL), the dispersion became turbid and viscous. There was no saturation point up to 181.6 mg/mL. The direct superoxide scavenging ability of the tetrahydrochloride dilysine astaxanthin salt was also evaluated by electron paramagnetic resonance (EPR) spectroscopy in a well-characterized in vitro isolated human neutrophil assay. The novel derivative was an extremely potent (micromolar concentration) aqueous-phase scavenger, with near-complete suppression of the superoxide anion signal (as detected by spin-trap adducts of DEPMPO) achieved at 100 microM. To the authors' knowledge, this novel carotenoid derivative exhibits the greatest aqueous dispersibility yet described for a natural and/or synthetic C40 carotenoid, and as such, will find utility in those applications for which aqueous-phase singlet oxygen quenching and direct radical scavenging are required.