Isothermal titration calorimetry to study the influence of citrus pectin degree and pattern of methylesterification on Zn2+ interaction

This work explored the influence of the citrus pectin degree and pattern of methylesterification on its interaction with Zn2+ 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, DBabs) were produced through enzymatic (blockwise pattern) or alkaline (random pattern) demethylesterification. The pectin-Zn2+ 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 Zn2+. Decreasing pectin DM or increasing DBabs promoted Zn2+ binding, with the estimated binding capacity (mol Zn2+/mol GalA) and binding constant (mM−1) being mainly determined by pectin DBabs, 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.