Cosolvent substrates

Evaporation Retardants. Small molecule solvents that make up the most effective paint removers also have high vapor pressure and evaporate easily, sometimes before the remover has time to penetrate the finish. Low vapor pressure cosolvents are added to help reduce evaporation. The best approach has been to add a low melting point paraffin wax (mp = 46-57° C) to the paint remover formulation. When evaporation occurs the solvent is chilled and the wax is shocked-out forming a film on the surface of the remover that acts as a barrier to evaporation (5,6). The addition of certain esters enhances the effectiveness of the wax film. It is important not to break the wax film with excessive bmshing or scraping until the remover has penetrated and lifted the finish from the substrate. Likewise, it is important that the remover be used at warm temperatures, since at cool temperatures the wax film may not form, or if it does it will be brittle and fracture. Rapid evaporation occurs when the wax film is absent or broken. 

Dica.rboxyIic AcidMonoesters. Enzymatic synthesis of monoesters of dicarboxyUc acids by hydrolysis of the corresponding diesters is a widely used and thoroughly studied reaction. It is catalyzed by a number of esterases. Upases, and proteases and is usually carried out in an aqueous buffer, pH 6—8 at room temperature. Organic cosolvents may be added to increase solubiUty of the substrates. The pH is maintained at a constant level by the addition of aqueous hydroxide. After one equivalent of base is consumed the monoesters are isolated by conventional means. 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

BZOSO2CF3, —78°, CH2CI2, a few minutes. With acid-semsitive substrates pyridine is used as a cosolvent. This reagent also reacts with ketals, epoxides, and aldehydes. ... 

The reaction is usually carried out in the presence of an anhydrous cosolvent such as ether, dioxane or tetrahydrofuran, depending on the solubility of the substrate. Since the primary aim of this reaction is generally... 

The charged group introduced into products by the aldol donors (phosphate, carboxylate) facilitates product isolation and purification by salt precipitation and ion exchange techniques. Although many aldehydic substrates of interest for organic synthesis have low water solubility, at present only limited data is available on the stability of aldolases in organic cosolvents, thus in individual cases the optimal conditions must be chosen carefully. 

Hydrolysis of substrates is performed in water, buffered aqueous solutions or biphasic mixtures of water and an organic solvent. Hydrolases tolerate low levels of polar organic solvents such as DMSO, DMF, and acetone in aqueous media. These cosolvents help to dissolve hydrophobic substrates. Although most hydrolases require soluble substrates, lipases display weak activity on soluble compounds in aqueous solutions. Their activity markedly increases when the substrate reaches the critical micellar concentration where it forms a second phase. This interfacial activation at the lipid-water interface has been explained by the presence of a... 

Soluble organic solvents have often been used as cosolvents to solubilize miscible organic substrates. Since organic compounds including solvents are possibly incorporated inside of the enzyme, they may affect the stereoselectivity of enzymatic reactions. For example, dimethyl sulfoxide (DMSO) (10%) enhance not only chemical yield but also enantioselectivity of yeast reduction. Thus, the poor yield of 23% with 80% ee was increased to 65% yield with >99% ee (Figure 8.20) [17]. 

The solubility of a drug candidate may not always be sufficient for its intended pharmacological purpose, and in such cases the solubility may need to be modified (either increased or decreased). These concerns are especially important for parenterals when the active substance does not exhibit the desired level of solubility. The solubility of a given compound can be profoundly altered in a number of ways, such as through the formation of salt species or drug-substrate complexes, or through the use of surfactants or cosolvents [43], In the majority... 

Use of benzene suspensions containing a neutral rhodium(I)-DIOP complex supported on a cross-linked polystyrene (50) (cf. 13 in Section III,A) for hydrogenation of a-ethylstyrene (to 1.5% ee) and methyl atro-pate (2.5% ee) was less effective than the homogeneous system, as the ethanol cosolvent required for substrate solubility caused a collapse of the resin (296). 

Intramolecular coupling Some aromatic diketones have been stereoselectively cy-clized under various electrolysis conditions, which, together with the substrate structure, strongly influence the stereochemistry of the formed cyclic diol. Reductive cyclization of 1,8-diaroylnaphthalenes led to trans-diols, 2,2 -diaroylbiphenyls and a, )-diaroylalkanes yielded cis-diols with different stereoselectivities depending on substrate structure and electrolysis conditions (pH, cosolvent) (Fig. 57) [310-312]. 

Since use of increasing amounts of cosolvents as antifreeze could perturb the conformation and thus the activity of lysozyme, a number of experiments were carried out to try to determine conditions for investigating lysozyme reactions and lysozyme-substrate intermediates in cooled mixed solvents as a preliminary to similar investigation by X-ray diffraction on crystals. Work began with an estimate of the solubility of... 

Under conditions of saturating substrate concentrations, other compounds initiate similar shifts of the high spin low spin equilibrium monovalent cations, especially K+, cause a significant shift. Cosolvents, such as primary alcohol and polyols, induce a similar shift, shown in Table II. Thus, there appears to be a unifying similarity between conformational changes induced by physiological modifiers and by organic... 

While it is tempting to explain regulatory and cosolvent effects on the basis of conformational changes favorable or unfavorable to enzyme activity, it is much more difficult to demonstrate the actual involvement, amount, and structural details of such changes. Experimental evidence consists in most cases of bits and pieces provided by techniques such as absorption and fluorescence spectroscopy, circular dichroism, and magnetic circular dichroism. These tools work in solution (and, when desired, at subzero temperatures) to investigate not simply empty enzymes but enzyme—substrate intermediates. However, even with this information, the conformational basis of enzyme activity remains more postulated than demonstrated at the ball and stick level, and in spite of data about the number and sequence of intermediates, definition of their approximate nature, rate constants, and identification of the types of catalysis involved, full explanation of any particular reaction cannot be given and rests on speculative hypothesis. 

Thus, any change in the independent variables P, T, j8p, j3w, and 8 would produce a change in the volume and surface area of the protein substrates which either bind on the surface of the protein or bind within the protein and change j3p, as well as cosolvents which have an effect on Py, can be included in this treatment. 

The data in Table 1 indicate several attempts made at PS polymerization under various conditions on Si wafers and Au substrate. Immediately noticeable is the large distribution of film thickness and contact angles even with similar concentrations and polymerization conditions, e.g., cosolvent and reaction time. These results are comparable to those obtained by Ul-... 

When the reactions of alkyl bromides (n-Q-Cg) with phenoxide were carried out in the presence of cosolvent catalyst 51 (n = 1 or 2,17 % RS) under triphase conditions without stirring, rates increased with decreased chain length of the alkyl halide 82). The substrate selectivity between 1-bromobutane and 1-bromooctane approached 60-fold. Lesser selectivity was observed for polymer-supported HMPA analogue 44 (5-fold), whereas the selectivity was only 1,4-fold for polymer-supported phosphonium ion catalyst 1. This large substrate selectivity was suggested to arise from differences in the effective concentration of the substrates at the active sites. In practice, absorption data showed that polymer-supported polyethylene glycol) 51 and HMPA analogues 44 absorbed 1-bromobutane in preference to 1-bromooctane (6-7 % excess), while polymer-supported phosphonium ion catalyst 1 absorbed both bromides to nearly the same extent. 

To clarify mechanisms of substrate selectivity, studies on elementary reaction steps with polymer-supported cosolvent catalysts must be carried out in detail. 

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