Interactions of mono‐ and divalent counterions with alkali‐ and enzyme‐deesterified pectins in salt‐free solutions

Abstract

SynopsisThe free fractions of monovalent and divalent counterions were determined on saltfree solutions of pectins. The effects of charge density, distribution of the carboxyl groups, polymer concentration, and the nature of the counterion were investigated by determinating the calcium and sodium activity coefficients (with specific electrodes) and by measuring the transport parameters (by conductimetry). Poor agreement for calcium ions was found with the Manning theory. The strong binding of these ions to highly charged polymers, which is ascribed to a dimerization process was demonstrated in very dilute solutions. INTRODUCTIONPectins are ionic polysaccharides extracted from plant cell walls used as gelling po1ymers.l They are composed mainly of a linear 1,4-linked a-D-galacturonic acid unit backbone, with some interruptions by 1,2-linked L-rhamnose residues and some neutral sugars, typically galactose and arabinose, as side chains2 In nature, they generally occur as the partial methyl ester with a degree of esterification, the fraction of galacturonate residues that are methyl-esterified being of about 70%.l These "high-methoxyl" pectins form gels in an acidic medium on addition of sucrose. Pectins of lower ester content are prepared by controlled deesterification, and the resulting pectins can gel in the presence of calcium ions. ' The nature of conformational ordering and calcium binding was investigated by Rees and coworker^,^-^ mainly by equilibrium dialysis and CD. They proposed a mechanism of calcium binding similar to the "egg-box" model given for alginates7 and postulated a two-stage process: initial dimerization of the molecules and subsequent aggregation of these preformed dimers. This intermolecular binding of the calcium counterion was confirmed for polygalacturonic acid by Kohn and LuknarB and by Ravanat and Rinaudog using methods such as measurement of calcium activity coefficient. THIBAULT AND RINAUDOThe purpose of this paper is to study the binding of cations, especially calcium ions, to pectin molecules with different levels and patterns of esterification. Well-characterized samples of pectin were prepared, and the degree of binding of the counterions was determined from conductimetric and potentiometric data in salt-free solutions. This study should contribute to an understanding of the polyelectrolyte behavior of pectins and may help to complete our knowledge of gel formation of these polysaccharides. EXPERIMENTAL MaterialsThe starting pectin was a commercial apple pectin ("RS" pectin, Unipectine, Redon, France). It was purified by precipitation with cupric ions.IO A pectin sample from apple juice was also used.ll Purity was tested as described elsewhere.12 DGalacturonic acid was a Baker sample. All the hydroxide solutions were carbonate-free. Methods Chemical Characterization of the SamplesPectin samples were characterized by their galacturonic acid content and by their degree of esterification, as determined by c o n d~c t i m e t r y .~~ This method involves three or four steps on 10 mL...

Conductometric analysis of polyelectrolytes in solution

“…( 2 5 ) ) the condensation theory predicts a slight increase of X with an for systems without added salt. Such a dependence was indeed found for pectins at different values of f : X increasing from 45 to 60 S.mol-'.cm2 from f increasing for 1.0 to 1.6 [26], For humic acids, mainly bearing carboxylic groups, X It is remarkable that most X values reported for different types of polycarboxylic acids are rather close, about 50 S.mol-l.cm2, not too far from the value of X for monomeric entities such as the acetate ion: 41 S.mol-l.cm . For H-CMC it was reported that [32] X -48 S.mol-'.cm An important feature of the condensation theory is the valence dependency of Ecrit.…”

“…[29,37,56,57,58,59,60,61,62], polyvinyl ether/maleic anhydride copolymers [51], fulvic acids (FA) [63], kaolinitic clays and clay-humic acid complexes [64], carboxymethylcellulose (H-CMC) [32,38], poly(acry1ic acid) (H-PAA), poly(methacry1ic acid) (H-PMA) [37], chondroitin sulfuric acid, heparin [65], polystyrene latices [55,66], and pectins [26].…”

“…We finally remark that one of the main assumptions results in the phenomenological approach, i.e., X being independent of the nature of the counterion, is largely in line with eq. (26). The term 0.134Hlln n ' a l in the denominator is of second order and usually less than 10 % of the term lzclXg (compare eq.…”

Difficultés expérimentales de l'étude des macromolécules pectiques

“…Si les valeurs expérimen-tales sont très proches de la théorie pour les ions monovalents (Thibault et Rinaudo, 1985 ;Racapé et al, 1989), il n'en est pas de même pour tous les ions divalents, notamment le calcium. En effet, on obtient pour l'acide polygalacturonique, pour des pectines désesterifiées ou pour des pectines ayant des distributions groupées des fonctions acides (Kohn et Luknar, 1977 ;Thibault et Rinaudo, 1985) des valeurs inférieures de moitié ; ce résultat indique une dimérisation des chaînes sous l'influence du calcium et a été interprété par une complexation coopérative de type "egg-hox" (Grant et al, 1973) liée à la stéréo-régularité de la chaîne. Ce phénomène n'existe pas pour le magnésium car sa taille est trop petite pour occuper la cavité entre acides galacturoniques ; son comportement est ainsi parfaitement décrit par la théorie (Thibault et Rinaudo, 1985).…”

“…En effet, on obtient pour l'acide polygalacturonique, pour des pectines désesterifiées ou pour des pectines ayant des distributions groupées des fonctions acides (Kohn et Luknar, 1977 ;Thibault et Rinaudo, 1985) des valeurs inférieures de moitié ; ce résultat indique une dimérisation des chaînes sous l'influence du calcium et a été interprété par une complexation coopérative de type "egg-hox" (Grant et al, 1973) liée à la stéréo-régularité de la chaîne. Ce phénomène n'existe pas pour le magnésium car sa taille est trop petite pour occuper la cavité entre acides galacturoniques ; son comportement est ainsi parfaitement décrit par la théorie (Thibault et Rinaudo, 1985). Cette forte sélectivité pour le calcium rend difficile la purification des pectines sous une autre forme ionique ct peut expliquer des divergences entre des résultats obtenus dans différents laboratoires.…”