Consecutive microwave irradiation induced substrate inhibition on the enzymatic esterification

The effect of consecutive microwave irradiation on the enzymatic esterification of pentanol with caprylic acid was investigated. Lipase from Mucor miehei was used as the catalyst under consecutive microwave irradiation and conventional heating, respectively. Microwave irradiation was found to increase the enzyme affinity to pentanol, which has not been reported in the literature. The conventional heating reaction conducted at an initial concentration of caprylic acid lower than 0.25 mol/L was found to fit the Ping-Pong Bi-Bi mechanism better than the microwave-assisted reaction. The microwave-assisted esterification departed from the Ping-Pong Bi-Bi mechanism independent of the initial caprylic acid concentration when the caprylic acid concentration was 0.25 mol/L and above. In addition, a pentanol inhibition under microwave irradiation was observed if the pentanol concentration was higher than 0.75 mol/L and caprylic acid concentration was over 0.25 mol/L. The pentanol inhibition caused by the microwave irradiation did not happen in the conventional heating reaction. This phenomenon has not previously been reported in the literature. The experimental results are correlated to the polarity of the substrates. These findings will benefit the understanding of nonthermal effect of microwave irradiation.