Solubilization of reactants

Sihcone surfactants are used to assist in controlling cell size and uniformity through reduced surface tension and, in some cases, to assist in the solubilization of the various reactants (52,53). 

The solubilization of enzymes and proteins in water-containing reversed micelles has attracted a great deal of interest for their selective separation, purification, and efficient refolding and for bioreactions involving a wide class of polar, apolar, and amphiphilic reactants and products [13,44,162-164]. 

Water-soluble l,3-bis(di(hydroxyalkyl)phosphino)propane derivatives were thoroughly studied as components of Pd-catalysts for CO/ethene (or other a-olefins) copolymerization and for the terpolymerization of CO and ethene with various a-olefins in aqueous solution (Scheme 7.17) [59], The ligands with long hydroxyalkyl chains consistently gave catalysts with higher activity than sulfonated DPPP and this was even more expressed in copolymerization of CO with a-olefins other than ethene (e.g. propene or 1-hexene). Addition of anionic surfactants, such as dodecyl sulfate (potassium salt) resulted in about doubling the productivity of the CO/ethene copolymerization in a water/methanol (30/2) solvent (1.7 kg vs. 0.9 kg copolymer (g Pd)" h" under conditions of [59]) probably due to the concentration of the cationic Pd-catalyst at the interphase region or around the micelles which solubilize the reactants and products. Unfortunately under such conditions stable emulsions are formed which prevent the re-use... 

Conventional extraction with chemical reaction is used for solutes that are insoluble in the organic phase unless they react with a reagent present in that phase. An example of this is the extraction of metal ions described by Eq. (15.8). In this case, if the organic phase is replaced by a W/O microemulsion containing the reactant, there is usually extraction enhancement due to the solubilization of the metal complex in the microphase. There are two possible ways of forming a W/O microemulsion in the solvent phase ... 

A threefold excess of a 10 mM soln of H2NOCH2CO-Lys(FITC)-OH (10) in H20 containing 20% MeCN was added to the oxidized interleukin-8 soln (2 mg - mL 1 in 0.1 M NaOAc buffer, pH 4.6). MeCN was added to a final concentration of 10% to ensure solubilization of the reagent, and the pH of the soln was adjusted to 4.6 with glacial AcOH. After incubation for 15 h at rt in the dark, the product was purified from reactants by RP-HPLC on a C8 column. The coupling yield was estimated at 80% from the recovery of coupled protein. The conjugate had the expected molecular mass mlz calcd, 8941.6 found, 8940.6T1.0. 

In 1968, Stober et al. (18) reported that, under basic conditions, the hydrolytic reaction of tetraethoxysilane (TEOS) in alcoholic solutions can be controlled to produce monodisperse spherical particles of amorphous silica. Details of this silicon alkoxide sol-gel process, based on homogeneous alcoholic solutions, are presented in Chapter 2.1. The first attempt to extend the alkoxide sol-gel process to microemul-sion systems was reported by Yanagi et al. in 1986 (19). Since then, additional contributions have appeared (20-53), as summarized in Table 2.2.1. In the microe-mulsion-mediated sol-gel process, the microheterogeneous nature (i.e., the polar-nonpolar character) of the microemulsion fluid phase permits the simultaneous solubilization of the relatively hydrophobic alkoxide precursor and the reactant water molecules. The alkoxide molecules encounter water molecules in the polar domains of the microemulsions, and, as illustrated schematically in Figure 2.2.1, the resulting hydrolysis and condensation reactions can lead to the formation of nanosize silica particles. 

The effect of micelles on organic reactions can be attributed to both electrostatic and hydrophobic interactions (Rosen, 1979). Electrostatic interaction is important because it may affect the transition state of a reaction orthe concentration of reactant in the vicinity ofthe reaction site. The hydrophobic interactions are important because they determine the extent and the locus of solubilization in the micelle. 

Reaction between neat reactants. The reaction between neat reactants may take place provided that there is present at least on polar molecule [19] as a liquid-liquid or liquid-solid systems. In case of liquid-solid systems the solubilization of solid in liquid phase or adsorption of liquid on solid surface as interfacial reaction takes place. In this type of reaction the effect of microwave irradiation is more pronounced due to the fact that in absence of solvent the microwaves are directly absorbed by the reagent. 

Aminolysis of active ester 91 can be effected by the amino group of an amino acid or peptide that is not protected at the terminal carboxy group (Scheme 27). Zwitterionic amino acid 92 or peptide is deprotonated by the addition of a base that generates the anionic form of the reactant. A partially aqueous milieu containing a water-miscible solvent such as acetone, dioxane, or acetonitrile is best for solubilizing the reactants. Esters that are hydrolyzed slowly or not at all are preferred because the acid produced is not readily separated from target molecule 66 (obtained from anion 93) because of their similar properties. Piperidino esters do not undergo hydrolysis but their aminolysis is slow.bl 4-Nitrophenyl esters react efficiently but the nitrophenol is difficult to remove. Succinimido esters are employed most frequently and are particularly useful in the preparation of N -protected dipeptides. Use of an excess of amino acid 92 favors complete consumption of active ester 91 by aminolysis before any of it can hydrolyze. 

In the swollen state the gel-type supports can best be described as polymeric solutions and the reactions take place on solubilized polymer chains within each bead, which act as small, individual reaction vessels. Depending on the degree of crosslinking and the degree of swelling, the polymer chains are more or less flexible, a fact that affects the reaction rates. The mass transport of reactants to the reactive sites inside the polymer beads is driven by... 

These heterogeneous reactions are performed between neat reactants in quasiequivalent amounts without any adduct. In the case of solid-liquid mixtures, the reaction implies either solubilization of solid in the liquid phase or adsorption of liquid on the solid surface as an interfacial reaction. 

Plots of rate constant versus surfactant concentration often show a maximum at some surfactant concentration above the CMC. There are a number of explanations for this. First, the number of micelles increases with increase in the surfactant concentration. When the number of micelles exceeds that required to solubilize all of the substrate, there is a dilution of the concentration of substrate per micelle as the surfactant concentration is increased further. This causes a reduction in the rate constant. Second, the charged surface of an ionic micelle in aqueous media may cause not only the concentration at the micelle-solution interface of an oppositely charged reactant in the solution phase, but the adsorption of that reactant on it, or even the solubilization of the reactant into the micelle. Such adsorption or solubilization of the reactant will result in a decrease in its activity in the solution phase. An increase in the concentration of surfactant over that required to effect substantially complete solubilization of the substrate may therefore result in a decrease in the rate constant, even in those cases where rate enhancement by micelles occurs. 

Reversed micellar systems have certain attributes which can be exploited when considering enzymatically-based synthesis reactions. These systems can solubilize hydrophilic and hydrophobic reactants and, If the reactants Interact with the surfactant layer, higher concentrations can be obtained than Is possible In either an aqueous or an organic environment. Partitioning of reactants between the bulk organic portion of a reversed micellar solution and the micellar core can result In localized high concentrations of polar reactants. This can be used to promote desired reactions, such as the synthesis of dlpeptldes. 

The concept of organometallic catalysis in micellar systems is compared by several writers [15, 33, 34] with the concept of heterogeneous catalysis on solid surfaces since the solubilization of the reactants in the core of the micelle containing catalytically active sites can be compared with adsorption on surfaces and the number of micelles to surface area (cf. Section 4.5). 

It is also noteworthy that micelle-forming surfactants may solubilize organic compounds sometimes in a very low concentration of the surfactant (still above the CMC). This embedding depends on the charge of surfactant and the charge of reactant. Only hydrophobic reactants may permeate into the hydrophobic core. Important for good solubilization properties is the hydrophile-lipophile balance (HLB) of the surfactant because sufficient water-solubility is required [12] (cf. Sec-... 

The solvent is present to solubilize all reactants, of course, and it also absorbs excess heat that may be liberated by the reaction. A very important property of a solvent is polarity, which largely determines its... 

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