Efficient two-step chemo-enzymatic synthesis of all-trans-retinyl palmitate with high substrate concentration and product yield.

A new two-step chemo-enzymatic approach for highly efficient synthesis of all-trans-retinyl palmitate is constructed in this study. In the first step, retinyl acetate as starting material was fully hydrolyzed to retinol by potassium hydroxide. In the hydrolysis system, anhydrous ethanol was the best co-solvent to increase the solubility of retinyl acetate. The addition amounts of 5 M potassium hydroxide and anhydrous ethanol were 8 and 10 mL against 10 g retinyl acetate, respectively, and 100 % hydrolysis rate was obtained. In the second step, esterification was catalyzed by immobilized lipase on macroporous acrylic resin AB-8 using the extracted retinol and palmitic acid as substrates in non-aqueous system. After optimization, the parameters of esterification reaction were confirmed as follows: non-aqueous solvent was selected as n-hexane, washing times of extraction solution was four times, retinol concentration was 300 g/L, substrate molar ratio of retinol to palmitic acid was 1:1.1, the amount of immobilized enzyme was 10 g/L, and the esterification temperature was 30 °C. Under the optimal conditions, this protocol resulted in a 97.5 % yield of all-trans-retinyl palmitate in 700-L reactor. After purification, all-trans-retinyl palmitate was obtained with above 99 % of purity and 88 % of total recovery rate. This methodology provides a promising strategy for the large-scale production of all-trans-retinyl palmitate.