FT-IR study of pectate and pectinate gels formed by divalent cations

The interaction of divalent cations with potassium pectate and three potassium pectinate samples with degrees of esterification E of 23.8, 59.1 and 93.4% were studied by FT-IR spectroscopy. Characteristic shifts occurred in C–O and ring vibrations in the 1200–900 cm−1 region as well as in the νas and νs carboxylate bands at around 1617 and 1420 cm−1, indicating a metal coordination by the pectate chains in accordance with the `egg-boxʹ hypothesis [Grant et al. FEBS Lett., 32 (1973) 195–198]. The FT-IR spectra showed some interaction between pectate and K+ and Mg2+ even when no gels were formed. Ca2+ and Sr2+ interacted strongly with pectate and low methoxyl pectinates. Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ also showed complex formation with the pectinates at E=59.1%, and Pb2+ and Cu2+ to some extent even with very highly esterified pectinate (E=93.4%).