Kinetics and equilibrium for thermolysin-catalyzed syntheses of dipeptide precursors in aqueous/organic biphasic systems

Efficient Synthesis of Theaflavin 3-Gallate by a Tyrosinase-Catalyzed Reaction with (-)-Epicatechin and (-)-Epigallocatechin Gallate in a 1-Octanol/Buffer Biphasic System

Theaflavins, the orange-red pigments contained in black tea, have attracted attention as a result of their health-promoting effects. However, their synthetic preparation, in which the enzymatic oxidation of catechol-type catechin is followed by the quinone-induced oxidative dimerization of selectively combined catechol- and pyrogallol-type catechins, provides only a low yield. In the present study, we found that a 1-octanol/buffer biphasic system improved the yield of theaflavin 3-gallate in a tyrosinase-catalyzed synthetic reaction with (-)-epicatechin and (-)-epigallocatechin gallate. When the enzymatic reaction proceeded in a buffer solution, oxidized (-)-epigallocatechin gallate was preferentially used for self-dimerization. However, self-dimerization was suppressed in the octanol phase, allowing oxidized (-)-epigallocatechin gallate to participate in coupling with (-)-epicatechin quinone, leading to effective production of theaflavin 3-gallate. Furthermore, the preferential localization of theaflavin 3-gallate in the octanol phase prevented (-)-epicatechin-quinone-induced degradation.

Kinetic analysis for synthesis of a dipeptide precursor using an immobilized enzyme in water-immiscible organic solvents

N-(Benzyloxycarbonyl)-L-aspartyl-L-phenylalanine methyl ester (Z-AspPheOMe), a precursor of the synthetic sweetener aspartame, was synthesized, using thermolysin immobilized onto Amberlite XAD-7, both in ethyl acetate and in tert-amyl alcohol. The initial rates for synthesis of Z-AspPheOMe in the organic solvents were predicted on the basis of a model proposed for an aqueous/organic biphasic reaction and compared with the experimentally observed substrate concentration dependencies. The experimental synthetic rates using the enzyme immobilized at a high enzyme concentration were lower than the calculated ones over a wide range of the substrate concentration. It was suggested as a reason for this discrepancy that the enzyme molecules form compact aggregates and those existing inside the aggregates cannot be utilized for reaction. The experimental results with the enzyme immobilized at a low concentration in ethyl acetate coincided well with the calculated ones. On the other hand, when tert-amyl alcohol was used, the experimental results were different in tendency irrespective of the amount of enzyme loaded, probably due to the fact that a distinct water phase does not exist around the enzyme aggregates inside the support.