Catalytic promiscuity in biocatalysis: using old enzymes to form new bonds and follow new pathways

Biocatalysis has expanded rapidly in the last decades with the discoveries of highly stereoselective enzymes with broad substrate specificity. A new frontier for biocatalysis is broad reaction specificity, where enzymes catalyze alternate reactions. Although often under-appreciated, catalytic promiscuity has a natural role in evolution and occasionally in the biosynthesis of secondary metabolites. Examples of catalytic promiscuity with current or potential applications in synthesis are reviewed here. Combined with protein engineering, the catalytic promiscuity of enzymes may broadly extend their usefulness in organic synthesis.     

Mirror-image packing in enantiomer discrimination molecular basis for the enantioselectivity of B.cepacia lipase toward 2-methyl-3-phenyl-1-propanol

Synthetic chemists often exploit the high enantioselectivity of lipases to prepare pure enantiomers of primary alcohols, but the molecular basis for this enantioselectivity is unknown. The crystal structures of two phosphonate transition-state analogs bound to Burkholderia cepacia lipase reveal this molecular basis for a typical primary alcohol: 2-methyl-3-phenyl-1-propanol. The enantiomeric alcohol moieties adopt surprisingly similar orientations, with only subtle differences that make it difficult to predict how to alter enantioselectivity. These structures, along with a survey of previous structures of enzyme bound enantiomers, reveal that binding of enantiomers does not involve an exchange of two substituent positions as most researchers assumed. Instead, the enantiomers adopt mirror-image packing, where three of the four substituents at the stereocenter lie in similar positions. The fourth substituent, hydrogen, points in opposite directions.     

Hydrolase-catalyzed biotransformations in deep eutectic solvents

Hydrolases show good catalytic activity in deep eutectic solvents, despite the presence of urea, which can denature enzymes, or alcohols, which can interfere with hydrolase-catalyzed reactions.     

Receptor-assisted combinatorial chemistry: thermodynamics and kinetics in drug discovery

Current drug discovery using combinatorial chemistry involves synthesis followed by screening, but emerging methods involve receptor-assistance to combine these steps. Adding stoichiometric amounts of receptor during library synthesis alters the kinetics or thermodynamics of the synthesis in a way that identifies the best-binding library members. Three main methods have emerged thus far in receptor-assisted combinatorial chemistry: dynamic combinatorial libraries, receptor-accelerated synthesis, and a new method, pseudo-dynamic libraries. Pseudo-dynamic libraries apply both thermodynamics and kinetics to amplify library members to easily observable levels, and attain selectivity heretofore unseen in receptor-assisted systems.     

Synthesis of a co-cross-linked nanocomposite hydrogels from poly(methyl vinyl ether-co-maleic acid)-polyethylene glycol and nanofibrillated cellulose

This study demonstrates the preparation of a renewable and biocompatible co-cross-linked nanocomposite hydrogel from poly(methyl vinyl ether-co-maleic acid), poly(ethylene glycol) and nanofibrillated cellulose (NFC). The cross-linking reaction was favored by the formation of ester linkages as evidenced by Fourier transform infrared spectroscopy. The increase in gel fraction content of the treated NFC varied from 22 to 85 % which exhibited an increase in degree of chemical cross-linking to form a rigid network with the addition of varying amount of NFC (20–60 %). This increase in gel rigidity influenced gel swelling, showing relatively reduced water uptake ability above 40 % NFC. Rheological measurements indicated the formation of gels with superior mechanical properties.     

Dipole magnet of the injection system for the SALO recirculator

A design of a dipole magnet was developed for the injection system of the SALO recirculator. The constructed prototype magnet has a C-shaped structure with rectangular sections. The pole section is 100 × 100 mm; the aperture is 25 mm. Field distributions in the magnet at a gap induction of 0.05 and 0.2 T were measured. The effective length of the magnet and multipole components of the magnetic field were calculated.     

Determination of EDTA in used fixing solutions by capillary electrophoresis

A capillary electrophoretic method for the determination of EDTA has been developed. EDTA was converted to Ni(II)-EDTA prior to separation, separated from Fe(III)-EDTA, thiosulphate, bromide and polythionates using a fused silica capillary (57 cm × 75 μm I.D.) filled with a borate buffer (50 mmol L^–1; pH 8.5; applied voltage, 30 kV) and detected at 214 nm. The separation time is about 6 min. The detection limit achieved is 2 × 10^–6 mol L^–1 for EDTA. This method was applied for the determination of free EDTA in used fixing solutions. Received: 27 February 1998 / Revised: 28 April 1998 / Accepted: 20 May 1998     

Laccase-generated quinones in naphthoquinone synthesis via Diels-Alder reaction

The tandem synthesis of naphthoquinones was conducted from the reaction of laccase-generated quinones and acyclic dienes via Diels-Alder reaction. This reaction was carried out under mild condition in aqueous medium and yielded naphthoquinones up to 80%. In addition, the effect of solvent was also investigated and water was shown to be optimal for this reaction.     

The preparation and characterization of chemically deuterium incorporated cotton fibers

Cellulose - Preparation of deuterium incorporated cellulose is a vital tool to investigate cellulose internal structure and to expand the application fields of cellulose materials. In this study,...