Amino acids cross-link

A polymer is a macromolecule that is constructed by chemically linking together a sequent of molecular fragments. In simple synthetic polymers such as polyethylene or polystyrer all of the molecular fragments comprise the same basic unit (or monomer). Other poly me contain mixtures of monomers. Proteins, for example, are polypeptide chains in which eac unit is one of the twenty amino acids. Cross-linking between different chains gives rise to j-further variations in the constitution and structure of a polymer. All of these features me affect the overall properties of the molecule, sometimes in a dramatic way. Moreover, or... 

The differences in the amino acid chemistry of the hide coUagen and the hair keratin are the basis of the lime-sulfide unhairing system. Hair contains the amino acid cystine. This sulfur-containing amino acid cross-links the polypeptide chains of mature hair proteins. In modem production of bovine leathers the quantity of sulfide, as Na2S or NaSH, is normally 2—4% based on the weight of the hides. The lime is essentially an unhmited supply of alkah buffered to pH 12—12.5. The sulfide breaks the polypeptide S—S cross-links by reduction. Unhairing without sulfide may take several days or weeks. The keratin can be easily hydrolyzed once there is a breakdown in the hair fiber stmcture and the hair can be removed mechanically. The coUagen hydrolysis is not affected by the presence of the sulfides (1—4,7). 

Several experimental approaches have been used to study these protein complexes, such as kinetics experiments of electron transfer, chemical and genetic modifications of specific amino acids, cross-linking and spectroscopic experiments (NMR, EPR, CD). However, to our knowledge. 

Progress in the Insulin field has been rapid in these two years, particularly in the axea, of the blos thesis of the hormone. It is now well established that Insulin, which has two peptide chains (A, 21 amino acids and B, 30 amino acids) cross-linked by two disulfide bridges, is synthesized as a single peptide chain, prolnsulin. in which the A and B chains of inmUn are connected by a "connecting peptide" (C-peptide) chain of 33 (porcine) or 30 (bovine) amino acids. Work on prolnsulin has be reviewed, 1, 10 and the amino acid sequences of bovine and porcine proinsulins have been published. Hie amino acid composition of cod proinsulin has a o appeared Two different proinsulins have been demonstrated in the rat, 8 aod proinsulin has been Isolated from human islet cell tumor tissue cultures. The structures of porcine and bovine prolnsulins are as follows ... 

As previously mentioned, bridging groups other than glycine occur. These include alanine, serine, or threonine. In bacteria where the lysine is replaced by meso-2, 6-diaminopimelic acid, cross-linking is directly between the D-alanine of the tetrapeptide side chain and the 6-amino group of the pimelic acid residue.8 Other bridgings are also known. 

The peptidoglycan is conqtosed of glycan chains, which are linear strands of two alternating amino sugars (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by peptide chains. 

The variability in the pyrolysis profiles of the different classes of coatings is self-evident. The interpretation of the composition revealed is as follows The acrylic lacquer (General Motors) is a methyl methacrylate/methacrylic acid copolymer plasticized with dibutyl-, butyl cyclohexyl-, and butyl benzyl phthalates. The acrylic enamel (Ford) is a styrene/ethylhexyl acrylale/methyl methacrylate terpolymer. The alkyd enamel (Honda) pyrolysis profile indicates that the paint resin is an orthoph-thaUc alkyd containing a butylated-amino resin cross-linking component. 

It can be concluded that the beverage can coating is a methyl melhacrylate/eth-ylhexyl acrylate copolymer modified polyester. The polyester is an adipic acid-modified neopentyl glycol iso-/orthophthalic acid type. The electrical appliance coating is a butylated amino resin cross-linked pentaerythritol-orlhophthalic thermosetting alkyd enamel. 

A polypeptide consists of at least ten amino acids that are joined together by peptide bonds. Peptide bonds are formed by coupling the carboxyl group of one amino acid to the amino group of another (see Fig. 2.2.15). The cross-over from polypeptides to proteins is not clearly defined. If more than 100 amino acids are linked a protein is formed. However, a protein often refers to a fully functional form and can consist of several polypeptide chains. 

Use of dimethylaminoethyl (also rert-butylaminoethyl) methacrylate as a monomer permits the introduction of pendent amino groups which can serve as sites for secondary cross-linking, provide a way to make the copolymer acid-soluble, and provide anchoring sites for dyes and pigments. 

Hexamethylolmelamine can further condense in the presence of an acid catalyst ether linkages can also form (see Urea Eormaldehyde ). A wide variety of resins can be obtained by careful selection of pH, reaction temperature, reactant ratio, amino monomer, and extent of condensation. Eiquid coating resins are prepared by reacting methanol or butanol with the initial methylolated products. These can be used to produce hard, solvent-resistant coatings by heating with a variety of hydroxy, carboxyl, and amide functional polymers to produce a cross-linked film. 

Hydrogen bonding stabilizes some protein molecules in helical forms, and disulfide cross-links stabilize some protein molecules in globular forms. We shall consider helical structures in Sec. 1.11 and shall learn more about ellipsoidal globular proteins in the chapters concerned with the solution properties of polymers, especially Chap. 9. Both secondary and tertiary levels of structure are also influenced by the distribution of polar and nonpolar amino acid molecules relative to the aqueous environment of the protein molecules. Nonpolar amino acids are designated in Table 1.3. 

Esters. Most acryhc acid is used in the form of its methyl, ethyl, and butyl esters. Specialty monomeric esters with a hydroxyl, amino, or other functional group are used to provide adhesion, latent cross-linking capabihty, or different solubihty characteristics. The principal routes to esters are direct esterification with alcohols in the presence of a strong acid catalyst such as sulfuric acid, a soluble sulfonic acid, or sulfonic acid resins addition to alkylene oxides to give hydroxyalkyl acryhc esters and addition to the double bond of olefins in the presence of strong acid catalyst (19,20) to give ethyl or secondary alkyl acrylates. 

On curing, amino resins not only react with the nucleophilic sites (hydroxyl, carboxyl, amide) on the other film formers in the formulation, but also self-condense to some extent. Highly alkylated amino resins have less tendency to self-condense (33,34) and are therefore effective cross-linking agents, but may require the addition of a strong acid catalyst to obtain acceptable cure even at bake temperatures of 120—177°C. 

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