Direct Recovery of Free Astaxanthin from Haematococcus pluvialis: Bifunctional Deep Eutectic Solvent-Mediated Extraction and Conversion in a Single-Step.

In this work, a series of deep eutectic solvents (DESs) were designed, using diamines as hydrogen bond acceptors (HBAs) and phenol derivatives as hydrogen bond donors (HBDs). The DES composed of 1,3-propanediamine and m-cresol was screened as the most efficient system for astaxanthin recovery from Haematococcus pluvialis. This dual-functional DES enables the one-step extraction and conversion of astaxanthin esters to astaxanthin. The structure of the DES was characterized by ¹H NMR, while computational chemistry confirmed the OH⋯N hydrogen bond within the DES. Quantum chemical calculations further clarified the conversion mechanism, demonstrating DES-catalyzed ester bond cleavage in astaxanthin esters to form free astaxanthin. Furthermore, optimizations of cell disruption and extraction processes improved the recovery efficiency, achieving a yield of 35.8 ± 0.13mg/g under optimal conditions (molar ratio 3:1, extraction time 2.1h, and material-to-liquid ratio 1:110g/mL). Overall, this strategy offers an efficient, eco-friendly approach for the direct production of free astaxanthin from Haematococcus pluvialis.