Of ester linkage

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.)... 

Enzymes that react with a specific type of ester linkage are known as general hydrolysing enzymes. Thus lipases hydrolyse a wide range of organic esters. Generally, phosphatases will break down phosphate esters into phosphoric acid and an alcohol. 

Fibres reacted with pre-heated MAPP always exhibited larger contact angles compared with fibres reacted with as-received MAPP. FTIR studies showed that there was a higher level of ester linkages when the fibres were reacted with the pre-heated MAPP. Wood polymer composites were made by shear mixing in a twin screw extmder, and the mechanical properties of these composites were found to be superior when pre-heated MAPP was used as the coupling agent. 

This indicates the possibility of making addition polymers biodegradable by the introduction of ester linkages in to the backbone. Since the free radical ring-opening polymerization of cyclic ketene acetals, such as 2-methylene-1,3-dioxepane (1, Scheme I), made possible the introduction of ester groups into the backbone of addition polymers, this appeared to be an attractive method for the synthesis of biodegradable addition polymers. 

The experiments presented above definitively established the fact the Lipson s HPD-forming procedure gives dimeric and trimeric porphyrin esters as the high-molecular weight component which is eminently better localized and retained in tumor cells and is responsible for most of the photodynamic activity associated with HPD. Mechanistically, the formation of ester linkage is not only understandable but expected as well. Under the alkaline condition employed in the synthesis of HPD, the nucleophilic substitution reaction illustrated in the following scheme is very similar to the... 

One of the more important and promising groups is comprised of the pendant polyesters, which can be fluorinated as in example a and preserve up to 80% of the ester linkages. Of ester linkages that do decompose, most end up as acid fluorides directly. The perfluoroesters are extremely hydrolytically unstable and are easily hydrolyzed to give the perfluoro acids. Specific examples of... 

To confirm the importance of ester linkage for chiral formation, we conducted vibrational circular dichroism (VCD) measurements [64]. FT-IR spectrum of this molecule shows many C=0,0-C-O related peaks in addition to peaks of N=N and phenyl and CH2 vibrational modes, as shown in Fig. 22a. It is noted that strong VCD signals are observed in C=0 and O-C-O vibrations, as shown in Fig. 22b. This implies that ester linkage is strongly related to the chiral structure. 

Strength loss is caused by the rupture of ester linkages in the polymers. 

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. 

The position of the esterified phosphates at C-6 can be determined using acid hydrolysis followed by enzymatic methods, but 31P NMR has been used to determine the position of ester linkages at other positions, such as C-3 and C-2 (Lim and Seib, 1993 Kasemsuwan and Jane, 1994). 

Lipase—a hydrolase that catalyzes the breakup of ester linkages between a fatty acid and glycerol, within triglycerides and phospholipids. 

Lithium borohydride is very widely used for the specific cleavage of ester linkages. It is generally thought not to reduce free carboxyl groups or amide groups. On this basis it was introduced into protein chemistry as a reagent to be used in the determination of COOH-terminal amino acids (Chibnall and Rees, 1951). 

The basis of the method for the determination of ester linkages by hydroxylamine is set out in the following scheme. 

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