Substrates, hydrophilic, hydrophobic

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)). 

After the cleaning process, other techniques are used to prepare the surface of the substrate for coating. Some techniques include drying, surface etching, and chemical surface preparation. Examples of chemical surface preparation include the formation of an oxide layer or the monolayer assembly of an adhesion promoter on the surface. These processes modify the surface of the substrates so as to facilitate the subsequent deposition process. In surface preparation, frequently, the hydrophilic/hydrophobic character of the surface is controlled to match the coating solution properties. For example, Van Driessche et al.19 reported on improving the wettability of Ni-4at%W tapes... 

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). 

Besides, different pH values lead to the formation of different reaction products [2, 31], due to modifications in the ionization state of the catalyst surface. Depending on the substrates, an increase of the pH will have a positive or negative effect on their reaction rate because the hydrophilic/hydrophobic character of the catalyst changes with the pH. When Ti02 is used as catalyst the strongest attractive interactions occur at pH values around the point of zero charge (PZC) (values of PZC for Ti02 are... 

Enzyme selectivity is usually limited because it depends on the interaction between the substrate and hydrophobic and hydrophilic amino acid residues at the active site, but here the degree of substrate immobilization is generally low. After the electron transfer process has occurred, the substrate is transformed into a radical compound that diffuses to the bulk of the solution and evolves according to its chemical properties, generally independently of the enzyme. This implies that the peroxidases rule the yield and the rate of radical formation but, once the latter species has been formed, the product composition and the stereoselectivity of the reaction are essentially dependent on the radical chemical structure and, to some extent, on the solvent and temperature of the reaction. 

The substrate 2 is assumed in this case to have 30 and 35 erg cm-2 as y and y, respectively. There is a controversy about expressing the work of adhesion in vacuum by Eq. (6) [13]. y is usually calculated by means of Eq. (6), without any strong theoretical support since there are only few methods available at present for estimating y. Therefore it should be noted that Eq. (10) is a very rough approximation for the work of adhesion in water. In addition, the curve in Fig. 1 was obtained from unsatisfactory experimental results. Nevertheless, it qualitatively represents the features that have been generally observed. In addition, Fig. 2 demonstrates that in both the extreme cases, where the polymer surface is extremely hydrophilic or hydrophobic, W]2,w is quite low but shows a maximum when the polymer surface has a certain hydrophilic-hydrophobic balance. Since the work of adhesion is directly... 

RND is a large ubiquitous superfamiiy of transporters with representations in all domains of life. Composed typicahy of about 1000 amino-acid residues, they are arranged as 12 transmembrane hehces proteins with two large hydrophilic extra-cytoplasmic loops between hehces 1 and 2 and hehces 7 and 8. It has been postulated that these proteins developed from an internal gene duplication event. The members of the RND family are also secondary active transporters that catalyze the proton-motive-force driven transport of a range of substrates, including hydrophobic drugs, bile salts, fatty acids, heavy metals, and more (22). 

Functional biomaterial surfaces have been created to change between being hydrophobic and hydrophilic, in response to external signals, such as differences in temperature, solvent environment, light, or electrical current. Temperature-sensitive poly(A/-isopropylacrylamide) (PNIPAm)-coated substrate is hydrophobic at a cell culture temperature of 37°C that favors cell adhesion and becomes hydrophilic at a lower temperature of 20°C, causing the detachment of cell sheets. These reversible responsive surfaces have been used to culture and harvest layers of endothelial, epithelial, lung, liver, cardiac, and kidney cells, and could eventually enable assembly of complex tissues and organs. 

Hydrophilicity/hydrophobicity of the carrier matrix, which influences type and strength of non-covalent protein-matrix interaction. In addition, it can influence the adsorption, distribution and availability of the substrate and product. 

We have initiated a series of investigations to study the catalytic effects of a class of cationic polyelectrolytes ranging in solution behavior from "normal" polyions to polysoaps upon the alkaline hydrolysis of neutral and anionic phenyl esters of varying chain lengths. Employing these catalysts of varying hydrophylic-hydrophobic character in reactions of neutral and anionic substrates of varied hydrophilic-hydrophobic character, it should be possible to elucidate the contributions of both the hydrophobic interactions and electrostatic interactions on the rate of reaction. 

By varying the number of methylene units on the neutral and charged substrate and by changing the length of the pendent group on the catalysts, it is possible to control the hydrophilic-hydrophobic nature of both substrate and catalyst. 

Several studies have been carried out on the immobilization of enzymes by radiation (ionizing and photochemical) induced polymerization reactions. (37-44) Most of these studies involved the use of combinations of hydrophilic or hydrophobic monomer/polymer substrates for the entrapment of the enzyme catalyst. A listing of typical hydrophilic/hydrophobic polymer materials is contained in Table II. The effects of hydrophilic/hydrophobic polymer properties in enzyme activity are... 

The stereoselectivity of the hydrogenation on a cyclic molecule containing a polar group can be controlled by the hydrophilic - hydrophobic interactions between solvent, substrate and catalyst support. 

Horbett, T., M. Schway, and B. Ratner, Hydrophilic-hydrophobic copolymers as cell substrates Effect on 3T3 cell growth rates. Journal of Colloid and Interface Science, 1985, 104, 28-39. 

Hydrophilic-Hydrophobic Copolymers as Cell Substrates Effect on 3T3 Cell Growth Rates... 

Amphiphilic molecules, when dissolved in organic solvents, are capable of self-assembly to form reversed micelles. The reversed micelles are structurally the reverse of normal micelles in that they have an external shell made up of the hydrocarbon chains of the amphiphilic molecules and the hydrophilic head-groups localized in the interior of the aggregate. Water molecules are readily solubilized in this polar core, forming a so-called water pool. This means that reversed micelles form microcompartments on a nanometer scale. The reversed micelles can host all kinds of substrate molecules whether hydrophilic, hydrophobic, or amphiphilic due to the dynamic structure of the water pool and the interface formed by the surfactant layer, in contrast with a liposome system. The properties of water molecules localized in the interior of reversed micelles are physicochemically different from those of bulk water, the difference becoming progressively smaller as the water content in the micellar system increases [1,2]. The anomalous water at low JVo =[water]/[surfactant] obviously influences the chemical behavior of host molecules in the water pools. 

Fig. 20 Multilayer deposition on a hydrophilically/hydrophobically patterned gold substrate. Upper AFM images height (left) and friction force (right) images of patterned methyl- and hydroxyl- alkanethiol self-assembled monolayers. Adsorption of poly(ferrocenylsilane) polyions (5/7, 12 bUayers) occms selectively on the broad methyl-terminated stripes (lower AFM images). Reprinted with permission from [94]. Copyright 2002, American Chemical Society...

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