Pectate divalent ions

The effect on the conformational-aggrega-tlonal properties of pectate in aqueous solution brought by the addition of specific ions (H+, Ca +. Cu +) was studied by osmometric, microcalorimetric, dilato-metric, and rheological methods. Evidence is provided for the intramolecular nature of the pH induced conformational transition. The addition of divalent ions brings about at the same time a conformational change of the chain of pectate and chain-chain association. 

The results presented here on pectate solutions containing calcium or copper ions bring interesting evidence for an anomalous behavior of pectate properties in the range of ion to polymer ratio below that corresponding to the massive phase separation. The possibility that a conformational change proceeds along with the interaction with divalent ions is examined and discussed in comparison with the case of the proton-induced conformational transition. 

Pectate is known to interact very strongly with several divalent ions, although the concept of binding" may change among different authors, to include different modes of interaction. Nevertheless, our present investigation seems to confirm such an established picture. 

Figure 3. Dependence of Che corrected enthalpy of mixing (see text) sodium pectate with different counterions in 0.05 M aqueous NaC104 at 25 C. R Me+ and R Me + denote Che ion-to-polymer repeating unit molar ratio for monovalent and divalent ions, respectively.

Some divalent cations such as Cu and Pb form very stable complexes with pectate, but are unlikely to be present at sufiScient concentration in the apoplast of plants to form a major fraction of the counterions associated with the pectic fraction in vivo. The Al ion may deserve closer examination, as it is certainly able to displace Ca from cell walls and reaches substantial concentrations in plant roots under some conditions [60,61]. aluminium is not usually considered to be freely translocated, however. Basic peptides with their negative charges spaced at a similar interval to galacturonans (0.43 nm or a small multiple thereof) can in principle have a very high afiBnity for pectate [62,63], but the extensins that are associated with the most insoluble pectic fractions [M] do not appear to have this type of structure. The possibility that the non-extractable pectic polymers in most cell walls are very strongly complexed with some cation other than Ca " cannot be ruled out, but there is little evidence to support it at present. 

For low ester pectin to precipitate as pectate and clarify juice, divalent cations such as Ca must be available to participate in the coacervppion (36). The use of hexameta-phosphates to sequester Ca ions has been suggested as a means to block pectate precipitation and stabilize cloud (A3). 

One component of dietary fiber, pectin, has long been known to form chelates with a number of divalent metals. Haug and Smidsrod reported on the selectivity of some anionic polymers, including pectates, for divalent metal ions and Schweiger et al (23) have presented data in support of the formation of both inter-molecular and intramolecular chelates of pectin with divalent metals. Recently Camire and Clydesdale reported significant... 

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