Influence of Diffusion on the Kinetics of an Enzyme-Catalyzed Reaction in Gelatin-Based Gels

Abstract

The influence of mass-transport limitations on the initial reaction rates of a lipase-catalyzed stereoselective esterification reaction has been investigated for two structurally different gelatin-based gels. The time to reach equilibrium is much longer in pelleted hydrogels (pseudo-solid aqueous gels; PAGs) than in pelleted microemulsion-based gels (MBGs). R/S-(±)-2-Octanol and hexanoic acid were used as substrates. The reaction takes place by imbibition of the substrate-containing organic solvent into pores of the pelleted gel. To minimize the diffusion distances, the macroscopic surface areas of the gels were increased by granulating the gel pellets. The experimentally obtained initial reaction rates in granules were in good agreement with the theoretically obtained values from extrapolation to infinitely large areas. However, the still low initial reaction rates in the hydrogels compared to those in microemulsion-based gels cannot be explained by diffusion limitations. This finding was supported by the similar activation energies in both gels in granulated form. Changes in apparent molar standard enthalpy, entropy, and Gibbs energy for the activated complex formation were also estimated. The low reaction rate in hydrogels might thus be due to partial denaturation of the enzyme during the preparation step, to higher surface energy, or to the influence of a different solvent environment on the enzyme in these gels than in the microemulsion-based gels.