Ultrasound-promoted enzymatic synthesis of troxerutin esters in nonaqueous solvents

Comparative studies of enzymatic acylation of troxerutin by the alkaline protease from Bacillus subtilis under ultrasound and shaking were carried out in nonaqueous media. Using divinyl dicarboxylates (CH2CH–OOC–(CH2)n–COO–CHCH2, n = 2, 3, 4, 7, 8, 11) featuring different chain length as acyl donors, troxerutin was regioselectively acylated at B ethoxyl group, whether under ultrasound or shaking. Ultrasonic treatment increased the reaction rate and led to high conversion. Several factors, such as pre-irradiation on the enzyme, the power and frequency of the ultrasound, operation manner, as well as the length of the acyl donors were investigated. Using enzyme pre-irradiated for 8 h, the conversion of troxerutin was increased to 87.3% compared with 56.3% obtained from the untreated enzyme. Experimental results also showed that continual ultrasound caused greater rate acceleration than interval ultrasound. Powers of 100, 150 and 200 W, frequencies of 40, 80 and 100 kHz all showed significant improvement on the transesterification, with the greatest effect observed at 150 W, 80 kHz. The acceleration effect increased as the chain length of the acyl donors decreased from C13 to C4.