Lipases in lipophilization reactions

Lipases are used in various sectors, as pharmaceutical, food or detergency industry. Their advantage versus classical chemical catalysts is that they exhibit a better selectivity and operate in milder reaction conditions. Theses enzymes can also be used in lipophilization reactions corresponding to the grafting of a lipophilic moiety to a hydrophilic one such as sugar, amino acids and proteins, or phenolic compounds. The major difficulty to overcome in such enzyme-catalyzed reaction resides in the fact that the two involved substrates greatly differ in term of polarity and solvent affinity. Therefore, several key parameters are to be considered in order to achieve the reaction in satisfactory kinetics and yields. The present review discusses the nature of such parameters (eg solvent nature, water activity, chemical modification of substrates) and illustrates their effect with examples of lipase-catalyzed lipophilization reactions of various sugar, amino acids or phenolic derivatives.     

Lipases have similar water activity profiles in different reactions

The shape of the profiles of enzyme activity versus water activity for four different lipases were independent of the reaction used to determine the activity. The profile for each lipase (Rhizopus arrhizus, Pseudomonas sp., Candida rugosa and Lipozyme) in esterification, hydrolysis and transesterifications profiles were the same. In transesterification the yield was unaffected by the water activity but the hydrolysis yield increased with increasing aW .     

Preparation of cyclodextrin chiral stationary phases by organic soluble catalytic 'click' chemistry

We describe an effective and simple protocol that uses click chemistry to attach native β-cyclodextrin (β-CD) to silica particles, resulting in a chiral stationary phase (CCNCSP) that can be used for the enantioseparation of chiral drugs by high-performance liquid chromatography (HPLC). Starting from β-CD, the CCNCSP is prepared in several steps: (i) reaction of β-CD with 1-(p-toluenesulfonyl)-imidazole to afford mono-6-toluenesulfonyl-β-CD; (ii) azidolysis of mono-6-toluenesulfonyl-β-CD in dimethylformamide to give mono-6-azido-β-CD (N(3)-CD); (iii) reaction of cuprous iodide with triphenylphosphine to form an organic soluble catalyst CuI(PPh(3)); (iv) preparation of alkynyl-modified silica particles; and (v) click chemistry immobilization of N(3)-CD onto alkynyl-modified silica to afford the desired chiral stationary phase. Synthesis of the stationary phase and column packing takes ～1 week.     

Lipases in organic solvents: The fatty acid chain length profile

Summary Competitive lipase-catalysed hydrolyses of triacylglycerols were used for the quantitative determination of the fatty acid profile of several lipases. In contrast to previously described methods a complete and reliable activity profile for triacylglycerols containing fatty acid residues with chain lengths from C₄ to C₁₈₁ can be obtained in one single experiment.     

Lipases and (R)-oxynitrilases: useful tools in organic synthesis

The use of enzymes in organic solvents is currently of special relevance for the preparation of products of high added value. Lipases are the enzymes that have shown the greatest utility through enzymatic transesterification reactions. Over the last few years, we have shown the value of the enzymatic aminolysis and ammonolysis reactions for the preparation of amides and for the resolution of esters and amines. We have shown that the enzymatic alkoxycarbonylation is also of great utility in chemoselective reactions of natural products. Lyases, enzymes much less exploited in organic synthesis, are proving increasingly interesting, especially the use of (R)-oxynitrilases for the synthesis of optically active cyanohydrins.     

Lipases from different sources vary widely in dependence of catalytic activity on water activity.

We have measured the rates of esterification in hexane catalysed by suspended immobilised lipases (triacylglycerol acylhydrolase, EC 3.1.1.3), with pre-equilibration to known thermodynamic water activity (a(w)). There were important differences between the enzymes from five different microbes in their retention of activity at low a(w). That from Rhizomucor miehei showed over 40% maximal activity at an a(w) of 0.12, and that from Rhizopus niveus was also fairly active at low a(w). Lipases from other sources required higher a(w) values to show good activity, increasing in the sequence Humicola sp., Candida rugosa and Pseudomonas cepacia. The behaviour was generally similar to two very different support materials, anion-exchange resin and macroporous polypropylene. Comparison of the sequences of the homologous enzymes from Rh. miehei, Rh. niveus and Humicola sp. suggests that changes in charged residues in the 'hinge and lid' region of the structure may be significant in low a(w) tolerance.     

Cyclodiphosphazane appended with thioether functionality: Synthesis, transition metal chemistry and catalytic application in Suzuki-Miyaura cross-coupling reactions

The coordination chemistry of thioether functionalized cyclodiphosphazane ligand, cis-{^tBuNP(OCH₂CH₂SCH₃)}₂ (1) is described. The reactions of 1 with [Pd (COD)Cl₂] in 1:1, 1:2 and 2:1 M ratios afforded cis-[PdCl₂{^tBuNP(OCH₂CH₂SCH₃)}₂] (2), cis-[{PdCl₂}₂{^tBuNP(OCH₂CH₂SCH₃)}₂] (3) and trans-[PdCl₂{(^tBuNP(OCH₂CH₂SCH₃))₂}₂] (4), respectively. Treatment of 1 with [Pd(PEt₃)Cl₂]₂ or [PdCl(η³-C₃H₅)]₂ in appropriate molar ratios produce the mono- and binuclear complexes [PdCl₂(PEt₃{^tBuNP(OCH₂CH₂SCH₃)}₂] (5) and [{PdCl(η³-C₃H₅)}₂{^tBuNP(OCH₂CH₂SCH₃)}₂] (6) in good yield. The reaction of 1 with [{Ru(p-cymene)Cl₂}₂] afforded the mononuclear cationic complex, [{(p-cymene)RuCl{^tBuNP(OCH₂CH₂SCH₃)}₂]Cl (7), whereas the reactions of [Rh(COD)Cl]₂, [Pt(COD)Cl₂] and [Au(SMe₂)Cl] with 1 yielded the corresponding P-coordinated neutral complexes, [RhCl(COD){^tBuNP(OCH₂CH₂SCH₃)}₂] (8)cis-[PtCl₂{^tBuNP(OCH₂CH₂SCH₃)}₂] (9), respectively. The binuclear palladium(II) complex 3 was found to be an effective catalyst for the Suzuki-Miyaura cross-coupling reactions.     

Glutaraldehyde fixation chemistry: oxygen-consuming reactions

In tissue fixed with glutaraldehyde, dissolved O2 is rapidly consumed by two processes: residual respiration and glutaraldehyde-induced chemical uptake. The nature of the chemistry which consumes O2 during tissue fixation was investigated by studying model reactions of glutaraldehyde with amines and with homogenized tissue suspensions. The addition of glutaraldehyde to solutions of most primary amines and ammonia stimulated rapid O2 consumption. The reaction of glutaraldehyde with primary amines (e.g., 25 mM ethanolamine, glycine, or methylamine) consumed 50% of the dissolved O2 in 15 to 20 s at 37 degrees C. The initial rate of O2 uptake followed second-order kinetics with respect to the primary amine concentration. The total amount of O2 consumed was sufficient to account for the stoichiometric conversion of the primary amines to pyridines. These data are consistent with the synthesis of pyridine derivatives from glutaraldehyde-amine precursors in which the last step is an irreversible oxidation of dihydropyridines to pyridines. The addition of glutaraldehyde to homogenized muscle suspensions, in which respiration was chemically inhibited, significantly increased the rate of O2 uptake. Thus, in tissue O2 is rapidly depleted both by respiration and the chemical demands of the glutaraldehyde-amine reactions during the cross-linking process. Since these experiments were done under conditions commonly used for tissue fixation, hypoxia should be assumed to exist in biological preparations fixed with glutaraldehyde.     

Improving the catalytic performance of peroxidases in organic synthesis

Peroxidases are ubiquitous enzymes that catalyze a variety of enantioselective oxygen-transfer reactions with hydrogen peroxide (H2O2). Although they have enormous potential, their industrial application is hampered by their high price and low operational stability. Recent developments, such as the controlled addition and in situ formation of the oxidant, protein engineering and the rational design of semi-synthetic peroxidases, aim to improve the operational stability of peroxidases.