Salt bonds/bridges

Chemically, keratin is formed from polypeptide chains formed from 20 different amino acids. The relative proportion of which vary for different keratins. Some possess an acid side group (12 Z of which are glutamic acid) others a amine function (30 Z of which are lysine), others a hydroxyl (10 Z of which are serine). But the keratin is above all characterized by an important quantity of sulfur due to the presence of cystine units which form a disulfide bridge between two chains greatly contributing to the stability of the proteins. In conclusion, the chains are connected by a nuid>er of diverse interactions including hydrogen bonds, salt bonds and covalent bonds. 

These favourable results can probably be traced back to the stable chelates formed by the lithium salts of the aldol adducts. In the hydrolysis product the nitrogen of the aldimine group is linked through an intramolecular hydrogen bond bridge, as was established by the IR spectrum. Since the aldimine adduct could finally be converted by treatment with acid in a nearly quantitative yield to 3-phenylcinnamaldehyde, the preparative problem of subjecting aromatic ketones to the directed aldol condensation was solved. 

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

If the protein of interest is a heteromultimer (composed of more than one type of polypeptide chain), then the protein must be dissociated and its component polypeptide subunits must be separated from one another and sequenced individually. Subunit associations in multimeric proteins are typically maintained solely by noncovalent forces, and therefore most multimeric proteins can usually be dissociated by exposure to pEI extremes, 8 M urea, 6 M guanidinium hydrochloride, or high salt concentrations. (All of these treatments disrupt polar interactions such as hydrogen bonds both within the protein molecule and between the protein and the aqueous solvent.) Once dissociated, the individual polypeptides can be isolated from one another on the basis of differences in size and/or charge. Occasionally, heteromultimers are linked together by interchain S—S bridges. In such instances, these cross-links must be cleaved prior to dissociation and isolation of the individual chains. The methods described under step 2 are applicable for this purpose. 

The underlying cau.se of DNA binding to nitrocellulo.se is not clear, but probably involve.s a combination of hydrogen bonding, hydrophobic interactions, and salt bridges. 

Also important for stabilizing a protein s tertiary stmcture are the formation of disulfide bridges between cysteine residues, the formation of hydrogen bonds between nearby amino acid residues, and the presence of ionic attractions, called salt bridges, between positively and negatively charged sites on various amino acid side chains within the protein. 

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