Optimization of reaction conditions and the donor substrate in the synthesis of hexyl-β-d-galactoside

Abstract Reaction conditions were optimized and the donor substrate selected for maximizing the reaction yield and productivity of the enzymatic synthesis of hexyl-β-galactoside with β-galactosidase from Aspergillus oryzae . We independently studied the effect of water content, type of cosolvent, temperature, donor substrate concentration, and leaving group of the donor substrate on the yield and productivity of hexyl-β-galactoside synthesis. Reaction yield was maximum in the medium with 70% water content and acetone as cosolvent, corresponding to a water activity of 0.94. Temperature and donor substrate concentration had very little effect on the yield. The leaving group of the donor substrate was the most relevant variable. Lactose, lactulose, o-nitrophenyl-β- d -galactopyranoside, and propyl- and butyl-β- d -galactoside (the last two are enzymatically synthesized from lactose) were evaluated as donor substrates. Use of propyl- and butyl-β-galactoside as donor substrates allowed us to increase the product yield by 683% and 716% [vs. lactose (0.06 mol/mol)]. Because propyl- and butyl-β-galactosides can be synthesized at low cost from lactose, using them in a two-step process could be much better alternative than a one-step process with lactose; leading to a theoretical global molar yield of 0.41 and 0.37 mol/mol from propyl- and butyl-β-galactosides, respectively.