Hydrophobic modification

Hydrophobic groups are polymerised into the backbone of the polymer chain so that they are pendant to the backbone along with the carboxyl. 

Diagrammatic Representation of Association by Hydrophobically Modified Polymer 

The formation of micelles results in a three dimensional transient network in which the latex particles serve as the branch points and the thickener molecules act as associative crosslinks. These crosslinks resist the stretching that must occur when shear is applied to the system. In practice, this means improved brush drag properties. The transient nature of the crosslinks, due to the dynamic equilibrium of the micelle which is continually forming, breaking and re-forming allows the system to flow under low shear resulting in excellent levelling. 

Because this type of thickener relies on the micelle effect, a critical micelle concentration must be achieved before full thickening is achieved. At normal paint viscosities, this critical micelle concentration will have been passed and the optimum associative action and benefit will be available. 

Conversely, in low shear conditions when the paint film is levelling, the state of dynamic equilibrium which exists in the micelles results in a wet paint film with good flow out characteristics. This helps to eliminate brush marks or roller ribbing as the paint film dries. 

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Hydrophobically Modified Alkali-Swellable Emulsions. The hydrophobe modification of acryHc acid represents an important class of... 

Klotzbach T, WattM, Ansari Y, Minteer SD. 2006. Effects of hydrophobic modification of chit-osan and Nafion on transport properties, ion-exchange capacities, and enzyme immobilization. J Membrane Sci 282 276-283. 

An increase in the amount of hydrophobic modification restricts segmental mobility by an increase of viscosity within the monolayer for the same molecular weight (300,000) and hydrophobe chain length (C g), the polymer monolayer with the higher amount of hydrophobe has a smaller relaxation rate constant (Experiments Numbers 3 and 7). 

All proteins of the Ras superfamily (Table 1) - with the exception of the Ran protein - undergo a posttranslational modification which introduces at least one hydrophobic modification. These lipid anchors qualify the members of the Ras family as peripheral membrane proteins, which stick into membrane structures from the cytoplasmic side only. 

The H- and N-isoforms of Ras support the first (isoprenoid) hydrophobic modification by additional thioester formation with palmitoylic acids [18]. At physiological temperature (37°C) the dissociation of doubly modified lipo-peptides with an isoprenyl thioether and a palmitoyl thioester is very slow and characterized by half-times in the order of 50 h. Here, the relative effect of the carboxymethylation is significantly reduced. Palmitoyl groups with their C16 alkane chain length contribute more efficiently to membrane anchoring than the farnesyl modification. 

New insights into the analysis of hydrophobically post-translational modified proteins could be achieved by the construction of lipidated proteins in a combination of bioorganic synthesis of activated lipopeptides and bacterial expression of the protein backbone (Fig. 19). The physico-chemical properties of such artificial lipoproteins differ substantially from those of the corresponding lipopeptides. The pronounced dominance of the hydrophilic protein moiety (e.g., for the Ras protein 181 amino acids) over a short lipopeptide with one or two hydrophobic modifications provides solubility up to 10 4 mol/1, while the biotinylated or fluorescence labeled lipopeptides exhibit low solubility in aqueous solutions and can be applied in the biophysical experiments only in vesicle integrated form or dissolved in organic solvent. 

Thus lipoproteins could be injected over the surface of a lipid covered SPR sensor in a detergent free buffer solution, and showed spontaneous insertion into the artificial membrane [234]. Again, two hydrophobic modifications are... 

The second class of stable membrane anchoring motives does not rely on electrostatic interactions but supports the first (often isoprenoid) hydrophobic modification by additional thioester formation with fatty acids (eg. the H- and N-isoforms of Ras or in the a subunits of heterotrimeric G-proteins) or a second isoprenoid moiety (eg. Rab proteins).1331... 

Eukaryotic cells utilize an efficient transport system that delivers macromolecules fast and secure to their destination. In the case of the small GTP binding proteins of the Ras family the modified C-terminus seems to be sufficient for addressing the polypeptide to its target membrane (in the case of Ras itself the plasma membrane). Lipopeptides with the C-terminal structure of N-Ras (either a pen-tamer with a C-terminal carboxymethylation and farnesylation or a heptapeptide with a palmitoyl thioester in addition) and a N-terminal 7-nitrobenz-2-oxa-l,3-diazolyl (NBD) fluorophore were microin-jected into NIH3T3 fibroblast cells and the distribution of the fluorophore was monitored by confocal laser fluorescence microscopy. Enrichment of the protein in the plasma membrane was efficient only for peptides with two hydrophobic modification sites, while the farnesylated but not palmitoylated peptide was distributed in the cytosol.1121... 

Limitations of molecular biology become obvious if hydrophobic modifications do not match the... 

The chain overlap parameter has been very successful at superimposing the data from the systems without hydrophobic modification, producing the continuous curve. However, it is clear from Flynn s work that once the hydrophobes are introduced into the polymer the viscosity rapidly increases at lower values of the chain overlap parameter. Increasing the mole percentage of hydrophobes also increases the viscosity at lower values of the chain overlap parameter. The position and number of the hydrophobes on a chain are important in determining the structure that forms and the onset of the increase in viscosity. The addition of side chains to hydroxyethyl cellulose modifies the network modulus as a function of concentration. This is discussed further in Section 2.3.4. 

The hydrophobe modification of acrylic acid represents an important class of hydrophobe-modified thickeners prepared by a chain-growth free-radical process. They differ slightly from other examples in that these products are generally cross-linked. 

Hydrophobe-modification of hydroxyethylcellulose produces w hat should be considered model associative thickeners, for the distribution of hydroxycthyl units has been characterized. The commercial material, a Hercules product, contains three hydrophobes per chain. 

Fig. 6 Schematic representation of the hydrophobic modification of a hydrophilic polymer in a solvent, which is selectively poor for hydrophobic modifier and modified chain segments. The modifying agent and hydrophobic monomers are shown in gray...

Fig. 20. The surface of a hydrophobic SPR-sensor (covered with long chain alkanethiols) was enlarged to an artificial membrane by application of lipid vesicles with a defined size distribution. Application of a Ras-lipopeptide construct with both a farnesyl- and a palmitoyl-modification leads to increase in resonance signal assumed to indicate membrane insertion (grey trace). Washing with buffer induces the slow decrease in signal. A Ras protein without hydrophobic modification (black trace) does not lead to signal increase...

The interfacial properties of HM - PNIPAM, including the formation and the compression - expansion reversibility of the monolayers, at different subphase temperatures were more recently studied by using the Langmuir film balance technique [90], The stability and dynamic nature of the HM - PNIPAM monolayers were also further studied by the time - dependent surface pressure measurements. All results have suggested a compression - promoted temperature - and rate - dependent conformational rearrangement of the polymer on the water surface. Increasing the level of hydrophobic modifications progressively improved the monolayer compressibility and stability, and reduced the hysteresis. 

The conversion of dextran with 1,2-epoxy-3-phenoxypropane, epoxyoctane or epoxydodecane may be exploited for the preparation of amphiphilic dextran derivatives. Polymeric surfactants prepared by hydrophobic modification of polysaccharides have been widely studied, starting with the pioneering work of Landoll [261]. Neutral water-soluble polymeric surfactants can be obtained by reaction of dextran with 1,2-epoxy-3-phenoxypropane in 1 M aqueous NaOH at ambient temperature (Fig. 35, [229,233]). The number n of hydrophobic groups per 100 Glcp units varies between 7 and 22 depending on the reaction conditions. 2-Hydroxy-3-phenoxy propyl dextran ethers (DexP) behave like classical associative polymers in aqueous solution. In dilute solution, the intrinsic viscosity decreases significantly whereas... 

One approach to introduce an anionic moiety onto dextran is sulfopropylation. In combination with a hydrophobic modification, sulfopropylation was utilised for manufacture of anionic amphiphilic dextran ethers. The derivatives were synthesised in two steps. Dextran was reacted with 1,2-epoxy-3-phenoxy propane in aqueous NaOH solution (Fig. 35) or in DMSO using TBAH instead of NaOH. The subsequent step is the conversion with 1,3-propane sultone in DMSO (Fig. 37). 

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