Isothermal titration calorimetry to study the influence of citrus pectin degree and pattern of methylesterification on Zn interaction.

This work explored the influence of the citrus pectin degree and pattern of methylesterification on its interaction with Zn using isothermal titration calorimetry (ITC). Pectin samples with a comparable degree of methylesterification (DM) but distinct distribution patterns of non-methylesterified carboxylic groups (absolute degree of blockiness, DB) were produced through enzymatic (blockwise pattern) or alkaline (random pattern) demethylesterification. The pectin-Zn interaction was found to be endothermic, in which a positive entropy change compensated for the unfavorable endothermic enthalpy change, driving the interaction between pectin and Zn. Decreasing pectin DM or increasing DB promoted Zn binding, with the estimated binding capacity (mol Zn/mol GalA) and binding constant (mM) being mainly determined by pectin DB, rather than DM. ITC was found to be a useful technique to study the pectin-cation interaction, however, low DM pectin samples are sensitive to gelling and therefore more difficult to study.