Activation of immobilized lipase in non-aqueous systems by hydrophobic poly-DL-tryptophan tethers

Many industrially important reactions use immobilized enzymes in non-aqueous, organic systems, particularly for the production of chiral compounds such as pharmaceutical precursors. The addition of a spacer molecule ("tether") between a supporting surface and enzyme often substantially improves the activity and stability of enzymes in aqueous solution. Most "long" linkers (e.g., polyethylene oxide derivatives) are relatively hydrophilic, improving the solubility of the linker-enzyme conjugate in polar environments, but this provides little benefit in non-polar environments such as organic solvents. We present a novel method for the covalent immobilization of enzymes on solid surfaces using a long, hydrophobic polytryptophan tether. Candida antarctica lipase B (CALB) was covalently immobilized on non-porous, functionalized 1-microm silica microspheres, with and without an intervening hydrophobic poly-DL-tryptophan tether (n approximately 78). The polytryptophan-tethered enzyme exhibited 35 times greater esterification of n-propanol with lauric acid in the organic phase and five times the hydrolytic activity against p-nitrophenol palmitate, compared to the activity of the same enzyme immobilized without tethers. In addition, the hydrophobic tethers caused the silica microspheres to disperse more readily in the organic phase, while the surface-immobilized control treatment was less lipophilic and quickly settled out of the organic phase when the suspensions were not vigorously mixed.