Amides linkage

Mammalian bile contains sodium salts of conjugated bile acids, e.g. glycocholic acid and taurocholic acid, in which cholic acid is combined (amide linkage) with glycine and taurine respectively. 

Several chemical methods have been devised for identifying the N terminal ammo acid They all take advantage of the fact that the N terminal ammo group is free and can act as a nucleophile The a ammo groups of all the other ammo acids are part of amide linkages are not free and are much less nucleophilic Sanger s method for N terminal residue analysis involves treating a peptide with 1 fluoro 2 4 dimtrobenzene which is very reactive toward nucleophilic aromatic substitution (Chapter 23)... 

An amide linkage between two a ammo acids is called a peptide bond By convention peptides are named and written beginning at the N terminus... 

Copoly(amide-imides) comprise an important class of copolyimides that have been developed into a commercial product. Incorporating the amide linkage into the PI makes the polymer more tractable than simple Pis, but involves a loss in thermal stabiUty. However, copoly(amide—imides) still possess quite good thermal stabiUties, intermediate between those of polyamides and Pis (12). They are relatively inexpensive to synthesize. 

However, significant exceptions to all these attributes occur. The regularity of the amide linkages along the polymer chain defines two classes of polyamides AB and AABB. 

The amide linkage characterizes nylons. In the first commercial nylon, nylon-6,6, R = (CH2 )g and R = (CH2 )4. Nylon-6,6 is familiar as a textile fiber (nylon stockings) and a molded plastic (see Polyamides). 

The metabohc rate of poly(ester—amide) where x = Q has been studied in rats using carbon-14 labeled polymer. This study indicates that polymer degradation occurs as a result of hydrolysis of the ester linkages whereas the amide linkages remain relatively stable in vivo. Most of the radioactivity is excreted by urine in the form of unchanged amidediol monomer, the polymer hydrolysis product (51). 

Amines with Amide Linka.ges, Representatives of this group are prepared from carboxyUc acids and di- and polyamines. The amide linkage connects the amine to relatively inexpensive hydrophobes. Eormulas for typical amide amines are as foUow ... 

The amide linkage between monomer units in a protein is called a peptide bond. Peptides and polypeptides, which often exhibit biological activity (see Antibiotics, peptides Neuroregulators), are smaller than proteins. Although the differentiation between polypeptide and protein is somewhat arbitrary, the usual distinction is drawn around 100 monomer units. Proteins are also characterized by higher levels of stmcture resulting from internal interactions. 

Polyfunctional 2-hydtoxyalk5iamides can serve as cross-linkers for carboxyHc acid-terrninated polyester or acryHc resins (65). The hydroxyl group is activated by the neighboring amide linkage (66). SoHd grades of hydroxyamides are finding use as cross-linkers for powder coatings (67). 

In the course of developing the Polacolor and SX-70 processes many insulated dye developers were synthesized and investigated. An extensive review of this work is available (21). The insulating linkage, chromophore, and developer moiety can each be varied. Substituents on the developer modify development and solubility characteristics substituents on the chromophore modify the spectral characteristics in terms of both color and tight stability. The attachment of two dyes to a single developer by amide linkage has also been described (22). 

Polyamide Imides. Polyamide imides (PAIs) are formed from the condensation of trimellitic anhydride and aromatic diamines (33). The polymer is called amide—imide because the polymer chain comprises amide linkages alternating with imide linkages, with the general chemical stmcture ... 

The chloroisocyanurates can be used in the bleaching of cotton, synthetics, and their blends they do, however, attack proteinaceous fibers, such as silk or wool, presumably via active chlorine reaction with the peptide (amide) linkage. However, the chloroisocyanurates can be used as shrink-proofing agents in wool finishing (131), (see Textiles Wool). The same action of chlorine upon proteins contributes to the effectiveness of chloroisocyanurates in automatic dishwashers. 

Nylon resins are made by numerous methods (53) ranging from ester amidation (54) to the Schotten-Baumann synthesis (55). The most commonly used method for making nylon-6,6 and related resins is the heat-induced condensation of monomeric salt complexes (56). In this process, stoichiometric amounts of diacid and diamine react in water to form salts. Water is removed and further heating converts the carboxylate functions to amide linkages. Chain lengths are controlled by small amounts of monofunctional reagents. The molten finished nylon resin can be dkectly extmded to pellets. 

In the ordered smectic or nematic phase, the rigid rods are arranged in parallel arrays that allow for close packing. The nematic phase is the most common type found with synthetic polymer molecules. The molecules long axes are parallel, but there is no layering. Aromatic polymer chains that have stiff ester or amide linkages are ideal. 

In recent years there has been considerable interest in aromatic polyamide fibres, better known as aramid fibres. These are defined by the US Federal Trade Commission as a manufactured fibre in which the fibre-forming substance is a long chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings. ... 

The in-out bicyclic amines prepared by Simmons and Park bear a remarkable semblance to the cryptands but lack the binding sites in the bridges. As a result, these molecules interact with electrophiles in a fashion similar to other tertiary amines and generally do not exhibit strong interactions with alkali or alkaline earth metal ions. The in-out bicyclic amines are prepared by reaction of the appropriate acid chlorides and amines in two stages to yield the macrobicyclic amine after reduction of the amidic linkages. A typical amine is shown above as compound 18. 

Chemically, proteins are unbranched polymers of amino acids linked head to tail, from carboxyl group to amino group, through formation of covalent peptide bonds, a type of amide linkage (Figure 5.1). 

FIGURE8.il Formation of an amide linkage between a fatty acid and sphingosine produces a ceramide. 

Lipoic acid exists as a mixture of two structures a closed-ring disulfide form and an open-chain reduced form (Figure 18.33). Oxidation-reduction cycles interconvert these two species. As is the case for biotin, lipoic acid does not often occur free in nature, but rather is covalently attached in amide linkage with lysine residues on enzymes. The enzyme that catalyzes the formation of the lipoamide nk.2Lg c requires ATP and produces lipoamide-enzyme conjugates, AMP, and pyrophosphate as products of the reaction. 

FIGURE 18.35 Formation of THF from folic acid by the dihydrofolate reductase reaction. The R group on these folate molecules symbolizes the one to seven (or more) glutamate units that folates characteristically contain. All of these glutamates are bound in y-carboxyl amide linkages (as in the folic acid structure shown in the box A Deeper Look Folic Acid, Pterins, and Insect VFingis). The one-carbon units carried by THF are bound at N, or at or as a single carbon attached to both... 

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