Lipase catalyzed synthesis of l-alanyl esters of carbohydrates

Synthesis of l-alanyl esters of carbohydrates like aldohexoses (d-glucose, d-galactose and d-mannose), ketohexose (d-fructose), pentoses (d-arabinose and d-ribose) and disaccharides (lactose, maltose and sucrose) were carried out using Rhizomucor miehei lipase (RML) in organic solvents with conversion yields in the range 8–56%. Enzymatic esterification between l-alanine and d-glucose using RML and porcine pancreas lipase (PPL) was investigated in terms of incubation period, enzyme concentration, substrate concentration, buffer salts (pH and concentration) and enzyme reusability. RML has shown a maximum conversion yield of 28% (1.12–0.88 mmol monoesters and 0.24 mmol diesters) at 1 mmol d-glucose and 4 mmol l-alanine in presence of 30% RML (w/w, d-glucose) and 0.1 mM (0.1 ml) pH 4.0 acetate buffer in 72 h. PPL showed a maximum yield of 18% (0.36–0.28 mmol monoesters and 0.08 mmol diesters) at 1 mmol d-glucose and 2 mmol l-alanine in presence of 40% lipase (w/w, d-glucose) and 0.1 mM (0.1 ml) pH 5.0 acetate buffer in 72 h. In presence of buffer salts, RML showed 22% higher ester yield than in its absence under the same experimental conditions. However, buffer salts did not enhance esterification with PPL. Two-dimensional HSQCT NMR confirmed formation of 1-O-, 2-O-, 3-O-, 4-O-, 5-O-, 6-O- and 6′-O-monoesters and 1,6-di-O-, 2,5-di-O-, 2,6-di-O-, 3,5-di-O-, 3,6-di-O-, 4,6-di-O and 6,6′-di-O-diesters to varying extents depending on the carbohydrate employed.