在α-蒎烯生物环氧化反应过程中酯对反应的影响及机理探讨

Chemo-enzymatic epoxidation of α-pinene:ester as influencing factors and mechanism of the reaction

在以过氧化脲(UHP)为氧源、Nov435为催化剂的体系中进行α-蒎烯生物环氧化反应的研究,重点考察不同酯作为酰基供体对反应的影响。从反应速率和酶批次稳定性综合考虑,以丙酸乙酯为最佳溶剂,30℃下α-蒎烯3h的环氧转化率可达到87%左右,经6批次反应后仍可达到47.6%,发现过酸和H2O2的协同作用对酶稳定性的影响要高于单一因素。酶催化乙酸乙酯、丙酸乙酯、乙酸戊酯、己酸乙酯等不同酰基供体过水解反应均可在60min达到平衡,但过酸平衡浓度和α-蒎烯化学环氧化反应速率受酯影响较大,进而对总反应化学-酶法联合催化环氧化产生影响。最后探讨了丙酸乙酯为唯一酰基供体时的反应机制,认为酶利用丙酸产过酸的反应速率和过酸平衡浓度远低于酯;丙酸乙酯的水解和过水解反应为一对竞争性反应,因此有机体系更有利于环氧化反应。

Chemo-enzymatic epoxidation of α-pinene was achieved in a non-aqueous system which employed esters as solvent and perhydrolysis substrate,urea-hydrogen peroxide（UHP） as an oxygen source and lipase as catalyst. Different esters were found to affect the reaction significantly. The optimal reaction condition was 30℃ and ethyl propionate as the solvent considering both the reaction rate and the stability of enzyme. Under these conditions,the conversion could achieve 87% after 3h and remain 47.6% after 6 cycles. The synergy toxic effect of H_2O_2 and peracid was more significant than that of single one. It was found that using different acyl donors such as ethyl acetate,ethyl propionate,amyl acetate,ethyl caproate and the like,the reaction of lipase-catalyzed production of peracid can all reach equilibrium within 60 min. However,the equilibrium concentration of peracid and the rate of chemical epoxidation greatly depended on the ester employed,which in turn affected the overall epoxidation of α-pinene. The mechanism of the reaction with ethyl propionate as the exclusive acyl donor was discussed. The reaction rate and the equilibrium concentration for producing peracid by enzyme with propionic acid was significantly lower than those with esters and it was found that there are competitions between the enzymatic hydrolysis with H_2 O and the perhydrolysis with H_2O_2 when H_2O existed,and therefore the nonaqueous environment was more suitable for the epoxidation.