Hydrophobicity surface

Properties of proteins such as solubility and chromatographic behavior depend on the residues that are accessible to the solvent. The nonpolar residues buried in the interior of the molecules should not directly affect these properties. Therefore, the surface hydrophobicity may be the more relevant parameter. Of course, the ideal model depicted by the polarity ratio is never attained. Because of the steric constraints and the inevitable adjacency of polar and nonpolar residues in the amino acid sequence, a proportion of nonpolar residues will be found at or close to the periphery of molecules. For proteins whose structures have been determined (e.g., by x-ray crystallography), it is possible to calculate the surface hydrophobicity. Some values are recorded in Table 9.1. Because of their heterogeneity, cereal proteins are not easily amenable to structure determination, so their surface hydrophobicities have not been calculated. 

Notes NFS = nonpolar side chain frequency HO = average hydrophobicity P = polarity ratio CHF = charged group frequency RSH = relative surface hydrophobidty. 

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Hydrophobic Interaction Chromatography. Hydrophobic interactions of solutes with a stationary phase result in thek adsorption on neutral or mildly hydrophobic stationary phases. The solutes are adsorbed at a high salt concentration, and then desorbed in order of increasing surface hydrophobicity, in a decreasing kosmotrope gradient. This characteristic follows the order of the lyotropic series for the anions ... 

Lesk and Chothia did find, however, that there is a striking preferential conservation of the hydrophobic character of the amino acids at the 59 buried positions, but that no such conservation occurs at positions exposed on the surface of the molecule. With a few exceptions on the surface, hydrophobic residues have replaced hydrophilic ones and vice versa. However, the case of sickle-cell hemoglobin, which is described below, shows that a charge balance must be preserved to avoid hydrophobic patches on the surface. In summary, the evolutionary divergence of these nine globins has been constrained primarily by an almost absolute conservation of the hydro-phobicity of the residues buried in the helix-to-helix and helix-to-heme contacts. 

Besides the spontaneous, complete wetting for some areas of application, e.g., washing and dishwashing, the rewetting of a hydrophobic component on a solid surface by an aqueous surfactant solution is of great importance. The oil film is thereby compressed to droplets which are released from the surface. Hydrophobic components on low-energy surfaces (e.g., most plastics) are only re wetted under critical conditions. For a complete re wetting of a hydrophobic oil on polytetrafluoroethylene (PTFE) by an aqueous solution, the aqueous solution-oil interface tension must be less than the PTFE-oil interface tension... 

The connection between hydrophobicity and tissue compatibility has been noted for classical organic polymers (19). A key feature of the polyphosphazene substitutive synthesis method is the ease with which the surface hydrophobicity or hydrophilicity can be fine-tuned by variations in the ratios of two or more different side groups. It can also be varied by chemical reactions carried out on the organo-phosphazene polymer molecules themselves or on the surfaces of the solid materials. 

MUller, R. H., Davis, S. S., Ilium, L., and Mak, E. (1986). Particle charge and surface hydrophobicity of colloidal drug carriers, in Targeting of Drugs with Synthetic Systems (G. Gregoriadis, J. Senior, and G. Poste, eds.), Plenum Press,... 

Xanthate compounds are widely used as collectors in flotation. Their function is to render the mineral surface hydrophobic and thus facilitate bubble attachment. The adsorption of xanthates onto sulfide minerals occurs via an electrochemical mechanism involving the reduction of oxygen and the anodic adsorption of xanthate. 

In this context, the esterification of 4-(l-pyrenyl)butyric acid with an alcohol to the corresponding ester was investigated [171]. Without the presence of sulfuric acid no reaction to the ester was foimd in the micro reactor. On activating the surface by a sulfuric acid/hydrogen peroxide mixture, however, a yield of 9% was achieved after 40 min at 50 °C. On making the surface hydrophobic by exposure to octadecyltrichlorosilane, no product formation was observed. Using silica gel in a laboratory-scale batch experiment resulted in conversion, but substantially lower than in the case of the micro reactor. The yield was no higher than 15% (40 min ... 

The segment chemical potential ps(o)is also called the o-potential of a solvent It is a specific function expressing the affinity of a solvent S for solute surface of polarity a. Typical o-profiles and o-potentials are shown in Fig. 11.4. From the a-potentials it can clearly be seen that hexane Ukes nonpolar surfaces and increasingly dislikes polar surfaces, that water does notUke nonpolar surfaces (hydrophobic effect), but that it likes both H-bond donor and acceptor surfaces, that methanol likes donor surfaces more than does water, but acceptors less, and many other features. 

Texturization is not measured directly but is inferred from the degree of denaturation or decrease of solubility of proteins. The quantities are determined by the difference in rates of moisture uptake between the native protein and the texturized protein (Kilara, 1984), or by a dyebinding assay (Bradford, 1976). Protein denaturation may be measured by determining changes in heat capacity, but it is more practical to measure the amount of insoluble fractions and differences in solubility after physical treatment (Kilara, 1984). The different rates of water absorption are presumed to relate to the degree of texturization as texturized proteins absorb water at different rates. The insolubility test for denaturation is therefore sometimes used as substitute for direct measurement of texturization. Protein solubility is affected by surface hydrophobicity, which is directly related to the extent of protein-protein interactions, an intrinsic property of the denatured state of the proteins (Damodaran, 1989 Vojdani, 1996). 

Plate type packing to separate the phases is discussed by Carlsson et al. (1983) and by Hatziantoniu etal. (1986). De Vos et al. (1982,1986) describe use of a monolithic porous catalyst with vertical and horizontal channels. The liquid phase flows downward through an array of parallel channels in the monolith, while gas moves in cross flow through a separate set of channels. Another approach treats the catalyst to make part of the surface hydrophobic or lyophobic (Berruti et aL, 1984). The gas phase has direct access to the surface on these unwetted portions of the surface, resulting in partial, spatial segregation of the phases. 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

Differences in surface hydrophobicity of proteins Separates proteins on the basis of their isolectric points Complex interactions between proteins and the calcium phosphate-based media not fully understood... 

In order to verify that the adsorbed lipid membrane indeed forms a bilayer film, another experiment is conducted with an aim to detect the formation of a monolayer lipid. It starts with a piranha-cleaned micro-tube treated with silane to render its inner surface hydrophobic. POPC liposome is then injected into the microtube. It is known that POPC lipid will form a monolayer to such a surface by orienting their hydrophobic tails toward the hydrophobic wall. The experimental results using a mode with similar sensitivity as the previous experiment are shown in Fig. 8.39. The resonance shift in this case is 22 pm, which is about half of that observed for the adsorption of a lipid bilayer. These two experiments suggest that the microtube resonator is capable of accurately determining an adsorbed biomolecular layer down to a few nm thicknesses. 

Varian (see Catalog, SPE products) Bond Elut Plexa Highly cross-linked polymer with hydroxylated surface Hydrophobic retention of small molecules and hydrophilic exclusion of proteins See catalog... 

The method of Kato and Nakai (27) for determining protein surface hydrophobicity was adapted for evaluating procyanidin binding to BSA and Gl. The procedure is based on the fact that the fluorescence quantum yield of cis-parinaric acid increases 40-fold when cis-parinaric acid enters a hydrophobic environment from a hydrophilic environment. The digestion of BSA by trypsin in the presence of procyanidin dimer, procyanidin trimer and black bean procyanidin polymer was evaluated by discontinuous sodium dodecyl sulfate (SDS) slab gel electrophoresis and a picryl sulfonic acid (TNBS) assay (28). 

Trimeric procyanidin binds more tightly to BSA than dimeric procyanidin (Table II). Partition coefficients of dimeric and trimeric catechin between n-octanol and water indicate procyanidin trimer is more hydrophobic than procyanidin dimer. Increased binding constants of trimer relative to dimer agree with reported partition coefficients. Surface hydrophobicity assays with cis-parinaric acid confirm the thermodynamic analysis that binding of procyanidin to BSA is hydrophobic. 

Heat-denatured Gl exhibited a surface hydrophobicity greater than that of native Gl. The increase was not unexpected since hydrophobic groups are commonly oriented towards the center of proteins in aqueous solvents. Heat denaturation of protein exposes hydrophobic groups to the solvent. Binding of denatured Gl to bean procyanidin oligomer was predominantly hydrophobic. 

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