ABSTRACT
VI. Adsorption and Chemisorption Equilibrium for Proteins on Silica Surfaces 552
VII . Some Up-to-Date Investigations in the Field of Biologically Active Compounds Immobilized on Silicas 556
VIII. Conclusions 557
References 558
I. INTRODUCTION
Over the last few years inorganic carriers, especially those based on dispersed silicas, have been used widely not only for transfer of many organic ligands and metal complex compounds [1-4] and important analytical reagents [5,6] to a heterogeneous state, but also for immobilization of enzymes, coenzymes, hormones, antibodies, antigens, and other biologically active macromolecules [7-10]. As distinct from organic matrices, silica carriers offer a number of advantages owing to their mechanical, microbiological, chemical, and thermal stability, feasibility of regeneration, as well as rigidity of their skeleton, which does not change its structure at various values of pH and ionic strength of diverse solvents. Silicas lend themselves readily to geometrical modification, which allows one to produce matrices with the optimum structure characteristics, namely pore diameter, specific surface area, and particle size. Chemical stability of inorganic carriers makes it possible to effect addition of cross-linking agents in organic solvents, which extends substantially the range of feasible surface activation reactions. As is known, preparations immobilized on silica have typical advantages that are caused by their capability of transferring active compounds to a heterogeneous state, and manifest themselves in ease of separation from reaction mixtures, possibility of repeated use and automation of processes. In addition, such preparations are often characterized by higher stabilities in comparison with native enzymes, which obviously is a matter of interest for practical purposes. It is believed that the increased stability of enzymes is attributed to multiple-point bonding of protein with surface sites of an activated carrier, which prevents unfolding of polypeptide globules (for instance in a thermal treatment). Of no small importance is also the fact that enzymes immobilized on silica are suitable to study their kinetic characteristics at high concentrations of an organic solvent, which is impossible to do in the case of soluble enzymes. In view of this, when working with immobilized preparations in a number of instances it is possible to gather important data furnishing a better insight into the mechanism of action of biocatalysts.
