Carboxylates terms Links

Polymers with a sizable number of ionic groups and a relatively nonpolar backbone are known as ionomers. The term was first used for copolymers of ethylene with carboxylated monomers (such as methacrylic acid) present as salts, and cross-linked thermoreversibly by divalent metal ions. Such polymers are useful as transparent packaging and coating materials. Their fluorinated forms have been made into very interesting ion-exchange membranes (considered further below). 

Amino acids become linked by reactions that occur between the amine group of one amino acid and the carboxyl group of another. As shown in Figure 22.23, this polymerization produces a molecule of water and, hence, is a condensation reaction. Naturally occurring polypeptides with molecular weights in excess of 10,000 daltons are termed proteins. These biomolecules are ubiquitous in marine organisms and are not specific to particular species. Proteins are important components of enzymes as well as of structural parts and connective tissues. 

A peptide is any compound produced by amide formation between a carboxyl group of one amino acid and an amino group of another. The amide bonds in peptides are called peptide bonds. The word peptide is usually applied to compounds whose amide bonds (sometimes called eupeptide bonds) are formed between C-1 of one amino acid and N-2 of another, but it includes compounds with residues linked by other amide bonds (sometimes called isopeptide bonds). Peptides with fewer than about 10-20 residues may also be called oligopeptides those with more residues are called polypeptides. Polypeptides of specific sequence of more than about 50 residues are usually known as proteins, but authors differ greatly on where they start to apply this term. 

In a protein the amino acids are joined together in a linear order by amide linkages, also known as peptide bonds. The sequence of the covalently linked amino acids is referred to as the primary level of structure for the protein. In writing the sequence of amino acids in a chain, it is conventional to orient the chain so that the amino acid on the left is the one with a free amino group on its a-carbon, while the last amino acid on the right is the one whose a-carbon carboxylate is free. In other words, the amino- or N-terminus of the peptide chain is written on the left, and the carboxyl-or C-terminus is written on the right. One more convention the term backbone for a protein refers to the series of covalent bonds joining one a-carbon in a chain to the next a-carbon. 

Amides are the derivatives of the carboxylic acids. In amides the carboxylic acid is linked with ammonia or an amine. Like esters, the parent carboxylic acid is identified. This is then termed an alkanamide and includes the nitrogen atom. For example, linking ethanoic acid with ammonia gives ethanamide ... 

Any polymer of amino acids linked by amide bonds between the amino group of each amino acid and the carboxyl group of the neighboring amino acid. The terms dipeptide, tripeptide, etc. may specify the number of amino acids in the peptide, (p. 1174)... 

The values of 3.3 to 3.6 obtained for the pK pp value to be assigned to the C-25 gel is close to the pK value of 3.31 (I = O.lCNaClO )) published for methoxyacetic acid [58], the weak carboxylic acid that most closely resembles the weak-acid functionality repeated in the cross-linked carboxymethyldextran. It can be concluded, on the basis of this observation, that Eq. (25) provides an adequate resolution of pKjn,. Indeed, the resemblance between pKjpp and pKjp, has led to the use of statistical arguments for the rationali2ation of the absence of nonideality in the a/(l - a) term. The pKjpp values for the highly swollen gel of C-50 at Cs = 0.01 mol dm, on the other hand, increases with increasing a. The extrapolated pK pp values of 3.3-3.6 at a = 0 are consistent with the values of 3.3-3.6 obtained for the C-25 gel over the a range studied. 

As this position is fixed in space isomeric compounds are possible in which the position of two of the elements or groups linked to the doubly bound carbon atoms are reversed, as in (A) and (B) above. Two stereo-isomeric compounds should therefore be possible according to such a space arrangement and the two isomeric crotonic acids may thus be explained. This kind of stereo-isomerism is termed geometric isomerism. Without taking up in detail the proofs as to which of the two stereo-chemical formulas applies to each of the two crotonic acids, we may simply state the fact, that the properties of the solid or ordinary crotonic acid prove that it must be represented by formula (A), above, in which the methyl and carboxyl groups are 12... 

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