Ionic links

Cell wall were isolated (2) and the pectic polysaccharides were extracted according with their solubility on water (highly esterified polysaccharides), imidazolium (ionically linked polysaccharides) and sodium carbonate (covalent linked polysaccharides), as shows figure 1. 

Cations can be seen as acting as ionic crosslinks between polyanion chains. Although this may appear a naive concept, crosslinking can be seen as equivalent to attractions between polyions resulting from the fluctuation of the counterion distribution (Section 4.2.13). Moreover, it relates to the classical theory of gelation associated with Flory (1953). Divalent cations (Zn and Ca +) have the potential to link two polyanion chains. Of course, unlike covalent crosslinks, ionic links are easily broken and re-formed under stress there could therefore be chain slipping and this may explain the plastic nature of zinc polycarboxylate cement. 

Many enzymes require additional substances in order to function effectively. Conjugated enzymes require a prosthetic group before they are catalytically active, such groups being covalently or ionically linked to the protein molecule and remaining unaltered at the end of the reaction. Catalase (EC 1.11.1.6), for instance, contains a haem group while ascorbate oxidase (EC 1.10.3.3) contains a copper atom. 

The subsequent heating treatment of thus obtained ionically-linked, pseudonetwork products at 120°C could convert the ionic bond to the covalent one to produce permanent gel products, which were no more soluble but swallen in THF. The ringopening reaction occurring during the heating was evidenced by IR spectroscopic... 

Replacement of the proximal histidyl residue (87 in the a chain and 92 in the chain), which occupies the fifth coordinate position of the iron of heme, by a tyrosyl residue (Hb-M-Iwate and Hb-M-Hyde Park, respectively) results in the formation of Met( = ferri) hemoglobin. It seems likely that the iron atoms in these two hemoglobins are covalently linked to the distal histidyl residue (a-58 and j8-63, respectively) whereas an ionic link is present between the iron atom and the OH group of the tyrosyl residue in position a-87 or 8-92. Replacement of the distal histidyl residue, a-5S or j8-63, by a tyrosyl residue is found in the methemo-globins M-Boston and M-Saskatoon. This substitution also results in an ionic link between the phenolic oxygen of this residue and the ferric iron. The fifth methemoglobin, Hb-M-Milwaukee, has similar properties be-... 

Block Copolymers With Ionic Linking Groups... 

An alternative approach to synthesizing block copolymers with Ionic linking groups was later developed. Polystyrene with tertiary amine and groups was prepared by the reaction of living polystyrene with a, w-chloroamlnes such as 3-(dlmethylamlno)propyl chloride (20) (Equation (16). This reaction was shown to take place with at least... 

In favourable cases we can form a quantitative estimate of the contribution of the ionic component in a bond by comparing the observed dipole moment with that which would be expected on the basis of a purely ionic link. In HF, for example, the ratio of these quantities is about o 45, from which we may say that the bond has 45 per cent ionic character. In HC1, HBr and HI the corresponding figures are about 17, 14 and 5 per cent, respectively. 

A comparison of the lattice energies given in table 6.02 with those of ionic structures (tables 3.04 and 3.05) reveals that the van der Waals bond has a strength one or more orders of magnitude smaller than that of the ionic link. Its weakness compared with the covalent... 

FIGURE 5 Representation of the two coiled-coil polypeptide chains of TM in cross section (i) and as the linear amino acid sequence (ii). Letters a to g represent the amino acid positions in the pseudoheptapeptide repeat. The a, a and d, d positions are nonpolar (N) and interact to form the core of the two-stranded structure. Acidic (A) and basic (B) amino acids occur frequently in e, e and, g positions. These can form ionic links. The outer b, c, / and b, c, f positions are usually but not always ionic or polar. 

We have seen how ordinary structural theory enhanced by the Lewis interpretation of the bond and of the distinction of the ionic link permitted a very good representation of a large group of compounds. We have also seen that for another large group the Lewis theory was... 

A number of chiral stationary phase (CSP) materials are commercially available for the resolution of enantiomeric mixtures. One of the first was based on derivatives of chiral amino acids, such as phenyl glycine and leucine, which are covalently or ionically linked to a 5 pm aminopropylsilica (Figure 6.4). These phases, introduced by Pirkle [11], have been used for the separation of amino acids and hydroxy acids. Separation in this instance is dependent on at least three points of interaction between the enantiomers and the stationary phase. 

For the synthesis of cyclic PSs, telechelic precursors having quinudidinium salt groups carrying a difunctional terephtha-late counterion were used. This ionically linked precydized polymer precursor was then subjected to heat treatment in diluted conditions to convert the ionic ends into a stable covalent linkage. 

---