Molecular binding between anthocyanins and pectic polysaccharides – Unveiling the role of pectic polysaccharides structure

The interaction between cyanidin-3-O-glucoside and four citrus pectic fractions was investigated using a combined molecular approach, including isothermal titration calorimetry (ITC), magnetic nuclear resonance (NMR) and UV–Visible spectrophotometry. These pectic fractions differed on their type and degree of esterification (amidated (AHG 30%), low (HG 30%) and high methyl esterified fractions (HG 70%)) and on the pattern of esterification (blockwise (HG-B 70%) vrs random (HG-R 70%)). The binding constant (Ka) and the associated thermodynamic parameters determined showed weak noncovalent interactions, particularly due to electrostatic interactions, hydrogen bonding and hydrophobic effect. The degree and the type of esterification of each pectic fractions had a significant effect on the binding constants determined by ITC and NMR experiments. The binding constants ranged from 102 to 104 M−1, with the highest Ka value observed for the interaction between cy3glc and low methylesterified fraction, followed by the amidated fraction. Pectic fractions with a higher degree of methyl esterification, resulted on lower binding affinities, with these interactions being mostly driven by hydrophobic effect compared to enthalpy. The different binding affinities could be correlated with anthocyanins colour impact, as a higher red colour intensity could be observed for a cy3glc model solution fortified with a low methylated fraction. However, this colour improvement was not observed for amidated fraction, presumably due to charge repulsive forces, lowering the stability of the flavylium cation.