Investigation On Microwave Irradiation-Enzyme Coupling Catalysis

Microwave irradiation and enzymatic catalysis are two powerful catalysis strategies in synthetic chemistry. A new trend in organic catalyzing synthesis is to utilize these two strategies simultaneously, which is called microwave irradiation-enzyme coupling catalysis (MIECC). In this paper, the MIECC reactor was improved and the effects of MIECC on initial reaction rates, kinetic characteristics and region-selectivity were investigated on different enzymatic reactions. Additionally, the mechanism of microwave cooking inducing acrylamide from microwave-cooked rice was discussed. The main contents and results are as follows:1. A special MIECC reactor was designed and assembled to study MIECC effects, in which the kernel microwave generator was taken from a domestic microwave oven. The cooling effect of different media on reaction system was studied. Accurate temperature display and control were achieved. In the range of 40~70℃, the enzymatic esterification between n-caprylic acid and n-pentanol was investitated by using the reactor. The results revealed that initial esterification rates under microwave irradiation were higher than those achieved via conventional heating, which showed apparent MIECC effect. Thus, the relevant reactions were suitable to be executed in the reactor.2. The effect of MIECC on reaction rates was investigated through enzymatic esterification between n-caprylic acid and n-alcohol. The optimum reaction temperature slightly shifted from that in enzymatic esterification by conventional heating. n-Alcohol homologs used in this experiment showed substrate specificity in terms of the odd and even carbon numbers. THF expressed abnormal solvent effect, whereas in the contrastive enzymatic esterification by conventional heating, the above mentioned substrate specificity and solvent effect were not observed. All the above phenomena could be satisfactorily explained by both thermal and non-thermal effect of microwave on enzyme and substrates. Further investigation revealed that microwave irradiation reduced the apparent activation energy of the enzymatic reaction according to Arrhenius equation.3. The reaction system with the substrates of n-caprylic acid and n-pentanol and the solvent n-octane was selected for investigating the effect of MIECC on the kinetic characteristics of the reaction. There was slight difference between Michaelis constants resulted from the two heating modes. Microwave irradiation reinforced the affinity of enzyme and alcohol substrate and had no apparent impact on the complex of enzyme and acid substrate, whichbehaved substrate specificity. Microwave irradiation did not entirely change the Ping-Pong Bi-Bi mechanism of reaction but made the reaction slightly deflected from the Ping-Pong Bi-Bi mechanism with higher concentration of alcohol (above 0.5 mol/L).4. The effect of heating modes on the region-selectivity of esterification of M-caprylic acid and glycerol was investigated. The reactions were catalyzed by the 1,3 specific lipase -Novo435. Microwave irradiation did not represent obvious effects on the reaction rates. With different water contents and substrate concentrations, microwave irradiation weakened the region-selectivity which appeared in the case of conventional heating. Whereas pre-irradiating the lipase via microwave enhanced the selectivity.5. A method of determining acrylamide (Am) in cereal food by extracting, centrifugal separating , adsorbing and GC inspecting was established. The threshold limit of GC testing for Am was investigated through four modes as follows: (1) conventional heating, (2) conventional heating plus microwave reheating, (3) microwave heating, (4) microwave heating plus microwave reheating. The content of Am through mode (2), (3), (4) was 2-9 fold over (1), in which the Am content was 30 jig/kg rice. The difference of Am contents from different modes was resulted by the special heating and non-thermal effect from microwave irradiation.