Influence of diffusion on the kinetics of an enzyme-catalyzed reaction in gelatin-based gels

The influence of mass-transport limitations on the initial reaction rates o f a lipase-catalyzed stereoselective esterification reaction has been inves tigated for two structurally different gelatin-based gels. The time to reac h equilibrium is much longer in pelleted hydrogels (pseudo-solid aqueous ge ls; PAGs) than in pelleted microemulsion-based gels (MBGs). R/S-(+/-)-2-Oct anol and hexanoic acid were used as substrates. The reaction takes place by imbibition of the substrate-containing organic solvent into pores of the p elleted gel. To minimize the diffusion distances, the macroscopic surface a reas of the gels were increased by granulating the gel pellets. The experim entally obtained initial reaction rates in granules were in good agreement with the theoretically obtained values from extrapolation to infinitely lar ge areas. However, the still low initial reaction rates in the hydrogels co mpared to those in microemulsion-based gels cannot be explained by diffusio n limitations. This finding was supported by the similar activation energie s in both gels in granulated form. Changes in apparent molar standard entha lpy, entropy, and Gibbs energy for the activated complex formation were als o estimated. The low reaction rate in hydrogels might thus be due to partia l denaturation of the enzyme during the preparation step, to higher surface energy, or to the influence of a different solvent environment on the enzy me in these gels than in the microemulsion-based gels. (C) 2001 Academic Pr ess.