Binding behavior of calcium to polyuronates: Comparison of pectin with alginate

For polyuronates, such as pectin and alginate, the ability to bind calcium and to form gels is the basis of their biological functions and technological applications. In a previous paper [Fang, Y. P., Al-Assaf, S., Phillips, G. O., Nishinari, K., Funami, T., Williams, P. A., & Li, L. B. (2007). Multiple steps and critical behaviors of the binding of calcium to alginate. Journal of Physical Chemistry B, 111, 2456–2462], we investigated the binding of calcium to alginate by using isothermal titration calorimetry and viscometry, and proposed a multi-step model which involves several steps: monocomplexation, dimerization, and lateral association. The present study examines two pectins of low and high methoxyl contents, and compares their behavior with alginate. In contrast to alginate, low methoxyl pectin has a less demarked dimerization step, which starts even when the stoichiometry of the egg-box structure is not achieved. Moreover, low methoxyl pectin shows less dimerization, and no significant lateral association. This behavior can be interpreted in terms of the structural features of the pectin. The random distribution of ester and amide groups along the pectin chain introduces much more defects into the formation of egg-box dimers and even hinders the subsequent lateral association of the dimers. The high methoxyl pectin shows a negligible chain–chain association upon binding with calcium, and its behavior can be simply depicted as a conventional polyelectrolyte without strong specific interactions.