Organic solvents system

In the third example of the previous paragraph [9], the reaction conditions described are similar to those used for the enzymatic transformations in bulk organic media, an area that was pioneered by Klibanov and coworkers in the 1980s [12] and later investigated and synthetically exploited worldwide [13]. 

In these systems, solid enzyme preparations (e.g. lyophilized or immobilized on a support) are suspended in an organic solvent in the presence of enough aqueous buffers to ensure catalytic activity. Although the amount of water added to the solvent (as a rule of thumb 5% v/v) may exceed its solubility in that solvent, a visible discrete aqueous phase is not apparent because part of it is adsorbed by the enzyme. Therefore, the two phases involved in an organic solvent system are a liquid (bulk organic solvent and reagents dissolved in it) and a solid (hydrated enzyme particles). 

The interest and success of the enzyme-catalyzed reactions in this kind of media is due to several advantages such as (i) solubilization of hydrophobic substrates (ii) ease of recovery of some products (iii) catalysis of reactions that are unfavorable in water (e.g. reversal of hydrolysis reactions in favor of synthesis) (iv) ease of recovery of insoluble biocatalysts (v) increased biocatalyst thermostability (vi) suppression of water-induced side reactions. Furthermore, as already said, enzyme selectivity can be markedly influenced, and even reversed, by the solvent. 

However, in most cases enzymes show lower activity in organic media than in water. This behavior has been ascribed to different causes such as diffusional limitations, high saturating substrate concentrations, restricted protein flexibility, low stabilization of the enzyme-substrate intermediate, partial enzyme denaturation by lyophilization that becomes irreversible in anhydrous organic media, and, last but not least, nonoptimal hydration of the biocatalyst [12d]. Numerous methods have been developed to activate enzymes for optimal use in organic media [13]. 

Before discussing the medium engineering phenomenon and its synthetic relevance in details, it is useful to offer a brief overview of the fundamentals of biocatalysis in organic media. 

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Table 1. A2eotropic Boiling Points of Allyl Alcohol—Water—Organic Solvent Systems...

ChLorofluorocarbene generated by the thermal decomposition of dichlorofluoro-methylphenylmercury reacts with 2,3-dimethylindole to give 3-fluoro-2,4-di-methylquinoline, 3-chloro-2,4-dimethylqumoline and 3-(chlorofluoromethyl)-2,3-dimethylindole [f] (equation 1). Similar results are obtained when the chloro-fluorocarbene is generated from dichlorofluoromethane by base-catalyzed de-hydrohalogenation using a phase-transfer catalyst in an aqueous-organic solvent system [21 (equation 1). 

ILs have also been used as inert additives to stabilize transition metal catalysts during evaporative workup of reactions in organic solvent systems [35,36]. The non-... 

Graft copolymerization of methylmethacrylate onto paper wood pulp using ceric ammonium nitrate as the initiator has been studied. Different experimental conditions have been used, including both water and water-organic solvent systems. The effects of composition of the grafting medium and grafting temperature on the grafting process are examined. 

Investigations in aqueous systems have established many of the fundamental principles of ion exchange as well as providing useful applications. The scope of the ion exchange process has, however, been extended by the use of both organic and mixed aqueous-organic solvent systems.32,33... 

C. M. Criss and M. Salomon, in Physical Chemistry of Organic Solvent Systems, A. K. 

Fortunately, as the reaction is transferred from a purely organic solvent system to mixed organic-aqueous media, which are employed in most RP-HPLC separations, the apparent multiplicity of maxima in the time profile of the intensity dependence seems to be suppressed or to collapse to a reasonably simple biexponential-like dependence. As shown in Figure 11, simply changing the solvent from ethyl acetate to 95% aqueous acetonitrile and the catalyst from triethylamlne to imidazole produces a single maximum profile, one that is more easily modeled mathematically, as defined in Equation 4 ... 

The selection of proper mobile phase in TLC exerts a decisive influence on the separation of inorganic ions. With a particular stationary phase, the possibility of separation of a complex mixture is greatly improved by the selection of an appropriate mobile phase system. In general, the mixed aqueous-organic solvent systems containing an acid, a base, or a buffer have been the most favored mobile phases for the separation of ionic species. The mobile phases used as developers in inorganic PLC include ... 

King, E. J., Acid-base behaviour, in Physical Chemistry of Organic Solvent Systems (Eds A. K. Covington and T. Dickinson), Chap. 3, Plenum Press, New York, 1973. 

To study the structural behaviour of MAPs, an adequate membrane model (see Fig. 5) is essential. Simple organic solvent systems, such as DMSO, MeOH/H20 or TFE/PLO mixtures, present a similar dielectric environment as a membrane on... 

Different procedures of this dehydrogenation are thoroughly discussed in the monograph (4). It is only necessary to note here that the process is carried out mainly as halogenation-dehydrohalogenation. The intermediate hydroximoyl halide is frequently not isolated (Scheme 1.3). The reaction is convenient for both the generation of unstable nitrile oxides (in the presence of a dipolarophile) and the preparation of stable nitrile oxides. It is usually carried out in a two-phase water-organic solvent system with methylene dichloride as the preferred solvent. 

The direct current and alternating current polarographic response of aspirin in an a aprotic organic solvent system (acetonitrile - 0.1M tetrabutyl ammonium perchlorate) has been studied. 00 The following values were obtained ... 

YV Mitin. An effective organic solvent system for the dissolution of amino acids. Int J Pept Prot Res 48, 374, 1996. 

Covington, A. K. Dickinson, T., Ed. "Physical Chemistry of Organic Solvent Systems" Plenum Press, New York,... 

Li, A., Doucette, W.J., and Andren, A.W. Solubility of polychlorinated biphenyls in binary water/organic solvent systems, Chemosphere, 24(9) 1347-1360, 1992. 

Under certain conditions, polymer incompatibility in aqueous solutions can lead to the formation of two phases with high water content. With such a system, it is possible to separate sensitive biological molecules, such as proteins, without denaturation, which would be the case for an ordinary aqueous-organic solvent system. 

The apparent pH value of an aqueous-organic solvent system an important factor in cryoenzymology experi-... 

Due to increasing demands for optically active compounds, many catalytic asymmetric reactions have been investigated in this decade. However, asymmetric catalysis in water or water/organic solvent systems is difficult because many chiral catalysts are not stable in the presence of water [19]. In particular, chiral Lewis acid catalysis in aqueous media is extremely difficult because most chiral Lewis acids decompose rapidly in the presence of water [20, 21]. To address this issue, catalytic asymmetric reactions using water-compatible Lewis acids with chiral ligands have been developed [22-29]. 

The syntheses described below allow such a cycle of operations to be circumvented. As reported previously,2 the electrochemical oxidation of indium metal at room temperature in an organic solvent system yields neutral, anionic, or cationic complexes of indium(III) within 2-3 hours, usually in good crystalline form. These syntheses are significantly quicker and more convenient than the procedures used in this laboratory in previous work. 

Semenov, A N., Khmelnitsky, Y.L., Berezin, I.V. and Martinek, K. (1987) Water-organic solvent systems as media for biocatalytic reactions the potential for shifting chemical equilibria towards higher yield of end products. Biocatalysis, 1, 3-8. 

The 3-pyridinecarboxyaldehyde 58 is highly water soluble, and so the spontaneous cyanide addition to give racemic cyanohydrin cannot be suppressed unless the aqueous pH is lowered below 3.5, which is not tolerated by the enzymes. The only available option is to operate in a 100% organic solvent system. This was recently made possible by the availability of the cross linked enzyme aggregate particles (CLEAs), which can tolerate organic solvents [64]. The individual precipitated protein molecules are chemically bonded to one another through the formation... 

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