(485at) Enantioselective Transesterification by Candida Antarctica Lipase B Immobilized On Fumed Silica

The exquisite ability of enzymes to efficiently catalyze complex reactions in organic solvents enables numerous synthetic schemes. However, when enzymes are suspended in non-denaturing organic solvents they tend to form poorly active clusters. Immobilization on solid supports can overcome this major hurdle. We have recently reported that by immobilizing Candida antarctica Lipase B (CALB) and subtilisin Carlsberg on fumed silica, the catalytic activity in hexane can be improved by three orders of magnitude with respect to suspending the native enzyme in the same solvent. The activity of the enzyme depends on surface coverage in an intricate and non-obvious manner. CALB activity increases as the surface coverage decreases (i.e., more fumed silica is added) reaching a maximum at an intermediate surface coverage followed by a precipitous decrease. S. Carlsberg activity, however, is constantly increasing as more surface area is provided for immobilization. We will discuss the application of CALB immobilized on fumed silica to the enantioselective transesterification of (RS)-1-phenylethanol. It appears that at high surface coverages most enzyme molecules are present in multi-layers and the active sites are not easily accessible. As more support area is added, the enzyme molecules start to be separated from each other and finally reach an optimum where protein-protein and surface-protein interactions are balanced to preserve the active protein structure while minimizing diffusion barriers. When even more surface area is provided, surface-protein interactions start to dominate and cause damage of the catalytic machinery of the enzyme molecules. We confirm concepts introduced previously by Gross et al. at Polytechnic University. We will also discuss the long-term, solvent, and thermal stability of our preparations for this model reaction system.