Surface-modified polymeric nanogranules containing entrapped enzymes: A novel biocatalyst for use in organic media

Multi-scale features in recent development of enzymic biocatalyst systems

Functional relation among elements of different size scales in a system is probably a main challenge across the areas of the science of engineering ever since their emergence. Multi-scale time and size correlation for description and prediction of complex systems, however, has been systematically examined only recently with the aid of new computational tools. In the pursuit of efficient and sustainable chemical processing technologies, people have seen a growing emphasis on synthetic biotechnology in recent R&D efforts. In particular, industrial enzyme technologies are attracting enormous attention. Having been traditionally developed for food and detergent applications, industrial enzyme technologies are being re-examined and tested to their limits to keep abreast of the challenges in drug, biochemical, and the emerging biorenewable energy industries. Toward that, enzymes are required to function in non-conventional conditions, such as organic solvents, extreme pH, and temperatures; they also have to compete against alternative chemical technologies in terms of costs and efficiency. Accordingly, enzymic biocatalyst systems are being tackled dynamically at all size levels through efforts ranging from molecular level protein engineering and modification, nanoscale structure fabrication, and microenvironment manipulation to the construction of microchip devices and macroscopic industrial bioreactors and devices. These efforts are probably still on a case-to-case trial basis without much consideration of cross-scale correlations. Discovering, understanding, and controlling of the common features that relate functions of biocatalysts at different size scales may eventually be realized in future.

Nanoscale biocatalyst systems

Since the large-scale application of immobilized enzymes in the 1960s, substantial research efforts have aimed to optimize the structure of carrier materials for better catalytic efficiency. In this regard, nanoscale materials provide the upper limits in balancing the key factors that determine the efficiency of biocatalysts, including surface area, mass transfer resistance, and effective enzyme loading. Various nanomaterials, such as nanoparticles, nanofibers, nanotubes and nanoporous matrices, have shown potential for revolutionizing the preparation and use of biocatalysts. Beyond their high surface area:volume ratios, nanoscale biocatalyst systems exhibit unique behaviors that distinguish them from traditional immobilized systems. The Brownian motion of nanoparticles, confining effect of nanopores and self-assembling behaviors of discrete nanostructures are providing exciting opportunities in this field. The development of catalyst systems that are highly stable and efficient, capable of self-targeting or that function as molecular machines to catalyze multiple reactions is rapidly reshaping our vision of biocatalysts.

Synthetic polymers as solubilizing vehicles for enzymes in water-poor media

A recent method for exposing enzymes to organic solvents is reviewed. By complex formation between the enzyme and polymers that per se are soluble in organic solvents it is possible to disperse the enzyme in the organic medium in such a way that an optically transparent (in the visible region) solution is obtained. After reaction, the separation of the enzyme from the organic medium can be obtained simply by addition of water. The enzyme can be recovered from the water phase. Physicochemical studies have revealed that the enzyme is more stable in the complex-bound form.

Catalysis by alpha-chymotrypsin entrapped into surface-modified polymeric nanogranules in organic solvent

Physicochemical characteristics of previously suggested surface-modified polymeric nanogranules (SMPN) and catalytic and stability properties of alpha-chymotrypsin entrapped into such nanogranules in a nonpolar solvent were investigated in more details. SMPN were obtained by polymerization of an acrylamide/N,N'-methylene-bisacrylamide mixture in a mixed reversed micellar system composed of Aerosol OT [sodium di(2-ethylhexyl)sulfosuccinate] and the polymeric surfactant Pluronic F-108 modified with polymerizable groups, followed by the chromatographic removal of the auxiliary surfactant, Aerosol OT. An optimal solvent system was found providing the required orientation of the polymeric surfactant in starting mixed micelles, i.e. with polar fragments immersed into the micellar interior and apolar fragments protruding into organic solvent. The hydrodynamic diameter of SMPN in benzene solution was estimated by means of quasi-elastic light scattering to be 84 +/- 1 nm. Catalytic and stability properties of alpha-chymotrypsin entrapped into SMPN strongly depended on conditions of preparation of SMPN. The optimal concentration of acrylamide monomers in the micellar interior and hydration degree of starting reversed micelles were found to be 20% by mass and wo = 15, respectively. alpha-Chymotrypsin-containing SMPN were used as a catalyst in the synthesis of N-acetyl-L-tyrosine ethyl ester from N-acetyl-L-tyrosine and ethanol, performed in a membrane reactor.

Reversed micelles of polymeric surfactants in nonpolar organic solvents. A new microheterogeneous medium for enzymatic reactions

A new microheterogeneous non-aqueous medium for enzymatic reactions, based on reversed micelles of a polymeric surfactant, was suggested. The surfactant termed CEPEI, was synthesized by successive alkylation of poly(ethyleneimine) with cetyl bromide and ethyl bromide and was found to be able to solubilize considerable amounts of water in benzene/n-butanol mixtures. The hydrodynamic radius of polymeric-reversed micelles was estimated to be in the range 22-51 nm, depending on the water content of the system, as determined by means of the quasi-elastic laser-light scattering. Polymeric reversed micelles were capable of solubilizing enzymes (alpha-chymotrypsin and laccase) in nonpolar solvents with retention of catalytic activity. Due to the strong buffering properties of CEPEI over a wide pH range, it could maintain any adjusted pH inside hydrated reversed micelles. It was found that catalytic behavior of enzymes entrapped in polymeric reversed micelles was rather insensitive to the pH of the buffer solution introduced into the system as an aqueous component, but determined mostly by acid-base properties of the polymeric surfactant itself. Both catalytic activity and stability of entrapped alpha-chymotrypsin and laccase were found to increase with increasing water content of the system. Under certain conditions, the entrapment of alpha-chymotrypsin into CEPEI reversed micelles resulted in a considerable increase in catalytic activity and stability as compared to aqueous solution. CEPEI reversed micelles were demonstrated to be promising enzyme carriers for use in membrane reactors. Owing to the large dimensions of CEPEI reversed micelles, they are effectively kept back by a semipermeable membrane, thus allowing an easy separation of the reaction product and convenient recovery of the enzyme.