Surface ester linkages

Goodenough first proposed that carboxylic acids would dehydratively couple with titanol groups to form surface ester linkages (Figure 12.4).42 We have reported spectroscopic evidence for this and for the more commonly observed carboxylate linkage in reactions of Ru(dcb)(bpy)2(PF6)2 and Ru(bpy)2(ina)2(PF6)2, where deb is 4,4 -(C02H)2-bpy and ina is isonicotinic acid, with nanocrystalline Ti02 and colloidal... 

Polyesters are eneountered in many forms. They are important as laminating resins, moulding compositions, fibres, films, surface coating resins, rubbers and plasticisers. The common factor in these widely different materials is that they all contain a number of ester linkages in the main chain. (There are also a number of polymers such as poly(vinyl acetate) which contain a number of ester groups in side chains but these are not generally considered within the term polyester resins.)... 

A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and sometimes to serine or threonine residues within a polypeptide chain (Figure 9.18). This type of fatty acyl chain linkage has a broader fatty acid specificity than A myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the Cjg and Cjg chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein-coupled receptors, the surface glycoproteins of several viruses, and the transferrin receptor protein. 

Moisture acts as a debonding agent through one of or a combination of the following mechanisms 1) attack of the metallic surface to form a weak, hydrated oxide interface, 2) moisture assisted chemical bond breakdown, or 3) attack of the adhesive. (2 ) A primary drawback to good durability of metal/adhesive bonds in wet environments is the ever present substrate surface oxide. Under normal circumstances, the oxide layer can be altered, but not entirely removed. Since both metal oxides and water are relatively polar, water will preferentially adsorb onto the oxide surface, and so create a weak boundary layer at the adhesive/metal interface. For the purposes of this work, the detrimental effects of moisture upon the adhesive itself will be neglected. The nitrile rubber modified adhesive used here contains few hydrolyzable ester linkages and therefore will be considered to remain essentially stable. 

The attached polymers undergo enzyme-catalyzed chemical alteration. In dermatan most of the glucu-ronate residues found in chondroitin have been epimer-ized to iduronate and sulfate groups in ester linkages have been added. Chondroitin sulfate is especially abundant in cartilage dermatan sulfate is concentrated in skin. Heparan sulfates are more heterogeneous than the other polymers of this group and have been described as "the most complex polysaccharides on the surface of mammalian cells."110... 

Using the polymerized viologen bilayers, the viologen units were removed from the outer surface to which they were attached by ester linkages by reaction with imino ethanol. Photolysis with [Ru(bipy)3]2+ in the bulk phase then led to viologen cation radical formation on the inner surface. Presumably the iminoethanol groups acted as electron donors.342... 

The classic sensitizer dye employed in DSC is a Ru(II) bipyridyl dye, cis-bis(isothiocyanato)-bis(2,2/-bipyridyl-4,4/-dicarboxylato)-Ru(II), often referred to as N3 , or in its partially deprotonated form (a di-tetrabutyl-ammonium salt) as N719. The structure of these dyes are shown in 2 and 26. The incorporation of carboxylate groups allows immobilization of sensitizer to the film surface via the formation of bidendate coordination and ester linkages, whilst the (- NCS) groups enhance the visible light absorption. Adsorption of the dye to the mesoporous film is achieved by simple immersion of the film in a solution of dye, which results in the adsorption of a dye monolayer to the film surface. The counter electrode is fabricated from FTO-coated glass, with the addition of a Pt catalyst to catalyze the reduc-... 

Changes in cholesterol content. A third type of intrinsic change involves alteration in the amount of cholesterol in a membrane (Robertson and Hazel, 1997). Cholesterol can be incorporated into a membrane up to an approximately one-to-one ratio with phospholipids. Most membrane-localized cholesterol is found in the plasma membrane. Cholesterol is an amphipathic molecule, that is, different regions of the molecule have affinities for either polar or nonpolar environments (figure 7.19). In a membrane, the flexible alkyl tip of the molecule penetrates into the bilayer the 3-/1-hydroxyl group remains near the surface of the membrane, near the ester linkages between the acyl chains and the glycerol moiety. 

The esterases are involved in the hydrolysis of ester linkages of various types. The products formed are acid and alcohol. These enzymes may hydrolyze triglycerides and include several lipases for instance, phospholipids are hydrolyzed by phospholipases, and cholesterol esters are hydrolyzed by cholesterol esterase. The carboxylesterases are enzymes that hydrolyze triglycerides such as tributyrin. They can be distinguished from lipases because they hydrolyze soluble substrates, whereas lipases only act at the water-lipid interfaces of emulsions. Therefore, any condition that results in increased surface area of the water-lipid interface will increase the activity of the enzyme. This is the reason that lipase activity is much greater in homogenized (not pasteurized) milk than in the non-homogenized product. Most of the lipolytic enzymes are specific for either the acid or the alcohol moiety of the substrate, and, in the case of esters of polyhydric alcohols, there may also be a positional specificity. 

Figure 21. Surface binding through the dehydrative coupling of carboxylic acid groups with surface hydroxyl groups to yield ester linkages and water.

Fig. 11. Orientation of ethyl palmi-tate molecules in a compressed mono-layer. The ethyl groups are pushed below the surface, forming a protective sheath below the ester linkages. The latter are only slowly hydrolyzed by alkali, k = 0.005 min. (Alexander and Schulman, 18).

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