Hydrophobic products

Most biphasic systems use sparingly water-soluble substrates and yield hydrophobic products. Therefore, the aqueous phase serves as a biocatalyst container [34,35]. However, in some systems, one of the reactants (substrate or product) can be soluble in the aqueous phase [23,36-38]. 

Biocatalysis localization in the biphasic medium depends on physicochemical properties of the reactants. When all the chemical species involved in the reaction are hydro-phobic, catalysis occurs at the liquid-liquid interface. However, when the substrate is hydrophobic (initially dissolved in the apolar phase) and the product is hydrophilic (remains in the aqueous phase), the reaction occurs in the aqueous phase [25]. The majority of biphasic systems use sparingly water-soluble substrates and yield hydrophobic products therefore, the aqueous phase serves as a biocatalyst container [34,35] [Fig. 2(a)]. Nevertheless, in some systems, one of the reactants (substrate or product) can be soluble in the aqueous phase [23,36-38] (Fig. 2(b), (c)). 

The hydrophobic products were isolated by TLC and characterized by means of NMR, ESI-CID-MS/MS, and FAB-CID-MS/MS as lipophilic long-chain alkyl diethanol amines, which are shown in Figure 7. In the literature these unusual lipophilic amines were not detected as constituents of any bacterial cells. Although the mode of linkage in the whole glycolipid is not clear at this moment, we speculate that these lipophilic amines that commonly existent in the cytokine-inducing gly-colipids from E. hirae are essential moieties for the cytokine-inducing activity. 

Figure 8.1 Schematic representation of some hydrophobic product, (d) and (e) hydrophobic...

A more promising approach for the synthesis of hydrophobic substances with ADHs is published by Kruse et al. [159, 238], They use a continuously operating reactor where the enzyme containing water phase is separated from the hydrophobic substrate-containing organic phase by a membrane. The hydrophobic product is extracted continuously via a hydrophobic membrane into an hexane phase, whereas the coenzyme is regenerated in a separate cycle, that consists of a hydrophilic buffer system. This method decouples advantageously the residence time of the cofactor from the residence time of the substrate. Several hydrophobic alcohols were prepared in this way with (S)-ADH from Rhodococcus erythropolis (Table 16). 

The small droplets act as microreactors when they contain the water-soluble catalysts. For hydroformylation reactions with water-soluble Rh/TPPTS in the droplets, the alkene, carbon monoxide and hydrogen approach the micelle surface where the reaction occurs, as is illustrated in Fig. 5.13. After the reaction is completed, phase separation can be achieved by changing the temperature of the reaction mixture. When the mixture is cooled down an aqueous bottom phase, containing most of the surfactant and the water-soluble catalyst separates from the organic upper phase, which contains the hydrophobic products and unconverted reactants. In case of incomplete catalyst recovery the micelle remaining in the product phase can be separated by means of ultrafiltration. 

Figure 8.22 Radiographic assay. Chloramphenicol Acetyl Transferase (CAT) assay system. Butyrylation of Chloramphenicol generates a more hydrophobic product that is separated from substrate by extraction into organic solvent.

Several of the nine products tested complied with all requirements and some test results are shown in Table 14.2. Good performance was obtained with hydrophobic products that were either ... 

Table 14.2 Test results of hydrophobic products on trowel-finished surfaces of Portland cement concrete (wjc = 0.50) and compliance to The Netherlands Ministry of Public Works and Vtetenways (Rijkswaterstaat) requirements [18]...

Unlike polarity, the solubility of ionic liquids depends strongly on the structure of the associated anion. In fact, the miscibiUty of ionic liquids with traditional solvents is one of their most interesting features and is clearly observed during the anion metathesis of imidazolium derivatives. The starting chlorine salt, for example [bmim][Cl], is completely soluble in water. After exchange of the anion, [bmim][PF6] or [bmim][NTf2] salts are obtained as hydrophobic products and therefore form a separate layer. Table 2.7 summarizes the solubility of l-butyl-3-methylimidazolium salts as function of the anion. ... 

In order to investigate the protective effectiveness of a silane-based hydrophobic agent used as admixture to the fresh concrete, an experimental program was carried out, some results of which are briefly reported in the following. For comparison, some field investigations on the effectiveness of a traditional surface hydrophobation of two concrete harbor structures based on similar hydrophobic products are also briefly reported. 

For some of the samples based on the concrete mixtures PO O, P2.0 and P4.0, the chloride diffusivity was also tested by use of the so-called bulk-diffusion method [8]. These samples were first cured in water for 45 days and then exposed to a 2.8 M NaCl solution for 45 days. A few of the concrete samples from mixture PO.O were also surface treated with the silane-based hydrophobation product of Type A and later on subjected to bulk-diffusivity testing (PO.S in Table 5). 

A simple and inexpensive assay of the P-o-galactocerebrosidase from leucocytes relies on separation of the tritiated sphingolipid (substrate) from the tritiated hydrophobic product (ceramide) by t.l.c. on silica gel impregnated with sodium tetraborate. ... 

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