Hydrophobic activators

Hydrophobic Activated carbon Carbon molecular sieves... 

The improvement of its activity and stability has been approach by the use of GE tools (see Refs. [398] and [399], respectively). A process drawback is the fact that the oxidation of hydrophobic compounds in an organic solvent becomes limited by substrate partition between the active site of the enzyme and the bulk solvent [398], To provide the biocatalyst soluble with a hydrophobic active site access, keeping its solubility in organic solvents, a double chemical modification on horse heart cytochrome c has been performed [400,401], First, to increase the active-site hydrophobicity, a methyl esterification on the heme propionates was performed. Then, polyethylene glycol (PEG) was used for a surface modification of the protein, yielding a protein-polymer conjugates that are soluble in organic solvents. 

A comparison of the activity levels of 7V-acetoxy-/V-butoxybenzamides 26, Ar-acetoxy-A -benzyloxybenzamides 27 and A-benzoyloxy-A-benzyloxybenzam-ides 28 with common para substituents showed increasing mutagenicity with size or hydrophobicity. Activities increased with increasing aromatic substitution. 

In a measurement of a hydrophobic activation volume, Kettman et al. (1966) found a large positive volume change when ribonuclease molecules with attached polyvalyl chains were made to aggregate under pressure. Work is also in progress on the interaction kinetics... 

To solubilize insoluble lypophilic or hydrophobic active substances in an aqueous medium, BASF Pharmaceutical... 

The soluble Kollidon products form reversible complexes with many hydrophobic active substances, and clear solutions in water are thus obtained. This may be affected by the molecular weight. The longer the chains or the higher the -value of the Kollidon type are, the stronger the solubility effect is, and thus the greater the solubility that can be obtained by the active substance. In practice, this effect was mostly exploited for the solubilization of antibiotics in human and veterinary medicine. There are also restrictions on the use of this substance in human parenter-als. In many countries the -value must not exceed 18, and there is also a restriction on the amount to be used for each dose administered in intramuscular application. 

The wetting properties of the particles play a crucial role in flotation. We have already discussed the equilibrium position of a particle in the water-air interface (Section 7.2.2). The higher the contact angle the more stably a particle is attached to the bubble (Eq. 7.19) and the more likely it will be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficiency. For other minerals surfactants are used to improve the separation. These are called collectors, which adsorb selectively on the mineral and render its surface hydrophobic. Activators support the collectors. Depressants reduce the collector s effect. Frothing agents increase the stability of the foam. 

The actual pKa value of an active site catalytic group will be influenced by the particular microenvironment of the active site, which could raise or lower the pKa. For example, the enzyme acetoacetate decarboxylase contains an active site lysine residue that forms an imine link with its substrate its pKa value was found to be 5.9, which is much less than the expected value of 9. Adjacent to this residue in the active site is a second lysine residue, which in protonated form destabflizes the protonated amine and, therefore, reduces the pKa. Conversely, aspartic acid or glutamic acid residues that are positioned in hydrophobic active sites can have increased pKa values near 7 because the anionic form of the side chain is destabilized. 

Both the charged species are initially solvated by water, and to form the salt bridge both ions must be desolvated. This comes at some enthalpic cost, but the freeing of water molecules leads to a concomitant, favorable increase in entropy. The strength of the ion pair will depend on the stability of the salt bridge vs. that of the individual solvated ions. If the salt bridge is buried in a relatively hydrophobic active site, it is less solvated and will be more favored than the same interaction in a solvent-exposed active site. 

Experiments with liposomes, which are more complex stmctures than micelles, point in the same direction. Liposomes consist of one (or several) lipid double layers (see Sect. 10.2), ordered concentrically around an aqueous interior. Blocher et al. (2000) carried out polycondensation reactions of amino acids and peptides using l-palmitoyl-2-oleoyl-.vn-glycero-3-phosphocholine liposomes hydrophobically activated amino acids gave chain lengths of up to 29. The linkage of dipeptides (e.g., H-Trp-Trp-OH) to give Trps-OH, i.e., an octapeptide, was also carried out using liposomes. Thus, a second possibility, as well as adsorption on mineral surfaces, was available for polymer formation via micelles and liposomes. 

The presence of a nucleophilic serine residue means that water is not required in the initial stages of the mechanism. This is important since water is a poor nucleophile and may also find it difficult to penetrate a hydrophobic active site. Secondly, a water molecule would have to drift into the active site, and search out the carboxyl group before it could attack it. This would be something similar to a game of blind man s buff. The enzyme, on the other hand, can provide a serine OH group, positioned in exactly the right spot to react with the substrate. Therefore, the nucleophile has no need to search for its substrate. The substrate has been delivered to it. 

They found that out of the two ILs studied, namely, [bmim][PF ] and [bmim] [BF ], [bmim][PFJ gave better results presumably due to its hydrophobic activation activity. It is postulated that water formed in situ from the condensation process is miscible with hydrophilic [bmim][BF ] and thus detained, which prevents the reaction from completion. In contrast, the hydrophobic nature of [bmim][PFg] would create a microenvironment to drive the equilibrium by extruding water out of the ionic liquid phase and thus results in a higher conversion. 

Dodecyl methacrylate (DMA) and stearyl methacrylate (SMA) are efficiently reactive coemulsifiers in the St mini-emulsion polymerization using SDS as the emulsifier [ 13]. The high coemulsifier (hydrophobe) activity of these unsaturated compounds results from the fact that the Rp data are almost the same in the mini-emulsion polymerizations stabilized by SDS/CA, SDS/DM A, SDS/SMA and SDS/HD. The average particle size (D) of the reaction mixture comprising both the monomer droplets and latex particles in the SDS/CA or SDS/DMA stabilized polymerization decreases rapidly from ca. 300-350 nm to a minimum (ca. 115— 125 nm), followed by a gradual increase to a plateau (ca. 125-150 nm). The initial abrupt decrease in D is attributed to the generation of particle nuclei by ho-... 

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