Bulk organic solvents

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). 

Exchange of Water Molecules between Enzyme Surface and Bulk Organic Solvent... 

Partitioning of the two substrates between the bulk organic solvent and the binding pocket of the enzyme active site can act as an explanation. It is known that any binding pocket of enzymes can be identified with a value for the hydrophobic-ity. A hydrophobic substrate in water partitions readily into the binding pocket, but... 

An example of coordinative extraction with solvating agents is shown in Eq. (4). The difference with physical extraction is that the capacity and extraction power of the liquid neutral ion exchanger is much higher than with any bulk organic solvent (toluene, xylene, butanol, etc.) used in physical extraction. Alkyl-substituted... 

Spectral changes with the water content are attributed to the existence of aggregates which induce different environments surrounding the por phyrin and not to dimer formation (Brochette et al., 1985). The observation of an isosbestic point implies the existence of two different species of porphyrin, one associated with the interface and the other corresponding to the free porphyrin in the bulk organic solvent or water. This equilibrium is expressed by... 

Supported aqueous phase catalyst (SAPC) involves generating a thin water film on a high surface area support surrounded by a bulk organic solvent.20 SAPC suffers from mass transfer problems despite the sharp increase in interfacial surface area and leaching problems of the water-soluble catalyst. The SAPC method has been effectively applied to cyclohexene oxidations using a simple water-soluble ammonium molybdate catalyst with the aid of supported surfactant molecules.21... 

In the case where Ps l, or the substrate is associated with the micelles not with the bulk organic solvent, Equations 3 and 4 become... 

Micellar systems are microheterogeneous hence, according to the distribution laws, in the equilibrium state local concentrations of the reagents are different in different phases. Enzymes are generally localized in the water-micellar phase (where their concentrations are higher than the total enzyme concentration and depend on the volume ratio of the whole system and the micellar phase). Hydrophobic substances concentrate in the bulk organic solvent therefore, in the vicinity of the enzyme their concentrations are lower. In... 

AOT which has two bulky alkyl chains can easily form reversed micelles, but a surfactant with a single alkyl chain cannot easily form reversed micelles in aliphatic solvents such as iso-octane. Therefore, it needs a cosurfactant such as octanol for single-chain surfactants to form reversed micelles in iso-octane [53]. Distribution of alkanols between the bulk organic solvent and the micellar pseudophase was measured based on their effect on the fluorescence intensity of indolacetate ions incorporated into AOT reversed micelles [60]. For all the alkanols, the extent of incorporation into the micelles decreased with increasing JVo from 4 to 20. At a given JVo, the incorporation of the n-alkanol increased when the length of the alkyl chain increased. The ILTJ technique revealed that the aliphatic chain alcohol induced an increase in the interfacial rigidity. 

Facile mechanochemical synthesis of thioureas was developed by Strukil et al. (Scheme 3.46) [32], Formation of new thioamide C—N bond was achieved by reaction of aromatic isothiocyanates 166 with aromatic amines (Table 3.20). Addition of small amount of methanol (LAG) was beneficial and the high yields were obtained regardless on the reactant physical state. The whole synthesis was carried out without the use of bulk organic solvents, that is solvent-free mechanosynthesis was quantitative, which voids the use of solvent for purification. Mechanochemical reaction... 

Kinetic analyses were performed for the inactivation of the immobilized enzyme onto Amberlite XAD-7 in ethyl acetate and tert-amyl alcohol at 70°C, varying the water content in the bulk organic solvent phase. The results are shown in Table 2 [59]. The kinetic parameter 2 reflects the effect of the conformational change of enzyme, that is, a change from the active to inactive... 

The bulk organic solvent phase was separated into organic and water phases. Source Ref. 59. 

With an increase in the water content in the bulk organic solvent, the A 2 [Eo] value appreciably increased, as shown in Table 2. The increased A 2 [Eo] values with an increase in the water content in the bulk organic solvent might be ascribed to the increased fraction of the aqueous phase in which the enzyme aggregates exist, which promoted autolysis. To test this speculation, the relative remaining activity after 5-h incubation at 70°C at 4% water content in the bulk organic solvent was plotted against the water content inside the support, for enzyme immobilized onto various supports in addition to Amberlite XAD-7 in ethyl acetate and in tert-amyl alcohol (Fig. 12) [59]. The values of... 

FIG. 12 Relationship between the relative remaining activity of thermolysins immobilized onto Amberlite XAD-7 [1,2], Amberlite XAD-8 [3], Amberlite XAD-5003 [4], and CPG-7.5 [5], and CPG-300 [6] and water content inside the immobilized enzyme after 5-h incubation at 70°C in ethyl acetate (open circles) and in ferf-amyl alcohol (closed circles). The figures in parentheses indicate the water content in the bulk organic solvent phase. (From Ref. 59.)... 

Solutions of water-containing reversed micelles are systems characterized by a multiplicity of domains apolar bulk solvent, oriented alkyl chains of the surfactant, hydrated surfactant headgroup region at the water/surfactant interface, and bulk water in the micellar core. Many polar, apolar, and amphiphilic substances, which are preferentially solubilized in the micellar core, in the bulk organic solvent, and in the domain comprising the alkyl chains and the hydrated surfactant polar heads, henceforth referred to as the palisade layer, respectively, may be solubilized in these systems at the same time. Moreover, it is possible that (1) local concentrations of solubilizate are very different from the overall concentration, (2) molecules solubilized in the palisade layer are forced to assume a certain orientation, (3) solubilizates are forced to reside for long times in a very small compartment (compartmentalization, quantum size effects), (4) the structure and dynamics of the reversed micelle hosting the solubilizate as well as those of the solubilizate itself are modified (personalization). 

Scarso and Strukul observed that anionic micelles based on sodium dodecyl-sulfate (SDS) efficiently solubilized Pt bis-cationic complexes and ketones in the apolar core of the micelles. This favored the contact between substrate and catalyst and, more importantly, led to a higher steric control of the asymmetric reaction thanks to the more ordered nano-environment present in the micelles compared to bulk organic solvents. Each substrate required the dedicated optimization of the catalyst, the surfactant and the experimental conditions since the distribution of the substrates and catalysts is greatly affected by the kind of surfactant and aggregate considered (Scheme 23.42). Overall, in all the cases tested, an increase in enantioselectivity was observed for the asymmetric BV oxidation of meso-4-substituted cyclohexanones with bis-diphenylphoshinobinaphthyl (BINAP) as the hgand and SDS as the surfactant. A different scenario was present in the kinetic resolution of chiral cyclobutanones or with meso cyclobutanones where an increase of enantioselectivity was observed with the neutral polyoxyethanyl-a-tocopheryl sebacate (PTS) surfactant (Scheme 23.42). 

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