Pectate aggregation

Figure 9. The cable model for the structure of concentrated calcium pectate gels. Egg-box dimers link single-chains segments (top left) and are themselves ed together by larger aggregates of either egg-box or 3i helical chains (lower right)...

Based on the same principle, there are monomeric / -helical proteins that carry at their extremities a cluster of helical or nonrepetitive structures that could act as a capping element covering their exposed ends (Emsley et al., 1996 Lietzke et al, 1994 Petersen et al, 1997 Steinbacher et al, 1994). For example, the last 40 residues of pectate lyase C form a large loop that partially covers the surface of the /Hielix (Yoder et al, 1993). The fibrous (or otherwise elongated) domain of these natural /f-stranded proteins is not stable in isolation, as for example in the case of the P22 tailspike where bacterially expressed isolated /Hielix domain, at high concentrations, forms fibrous aggregates that bind Congo red (Schuler et al, 1999). 

In the presence of anions such as phosphate and oxalate, the relaxivity of [Gd(Tex)]2+ is considerably reduced revealing that these anions compete with water for binding to the Gd3+ ion [167,171]. Most likely, texaphyrin complexes self-associate due to strong van der Waals interactions. UV-VIS studies suggest that the aggregates dissociate upon interaction with polyuronides (pectate, alginate), which probably act as polydentate polycarboxylate ligands for the [Gd(Tex)]2+ complexes [173]. 

The behaviour of pectic substances in solutions involves both short and long range interactions among all the constituents of the macromolecule as well as interactions with the solvent molecules and the ions in the solutions. The calculations reported here explain the aggregation of PGA chains in acidic media as due to the formation of a network of inter-intra chain hydrogen bonds. Also the gel formation of Na- and Ca-Pectate is well reproduced. 

The aggregation of pectate in the presence of copper and calcium ions has been repeatedly reported and is commonly used as a method for the quantitative precipitation of the polymer. The actual distribution of the precipitate and the increase of the apparent molecular weight has been studied as function of R by means of sedimentation and membrane osmometry, respectively. 

A series of O-acetyl derivatives of pectic acid has been prepared to study the intramolecular binding of Ca + ions to pectic acids. The electrostatic free enthalpy of dissociation of these polyacids and the rate of exchange of the counter-ion from Ca + to K+ were determined by potentiometric titration measurements. The binding of Ca + ions was characterized by the production of single ion affinity coefficients determined in solutions of the calcium salts of 0-acetyl pectic acids and by the circular dichroism measurements of calcium and potassium salts of the pectate derivatives. The intramolecular binding of Ca + ions to isolated pectate macromolecules is purely of electrostatic character in contrast to the intermolecular chelate binding of Ca + ions which takes place when the pectate macromolecules aggregate. 

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