NMR investigation on oligo‐ and poly(galacturonic acid)s; gel formation in the presence of Ca²⁺ counterions

Abstract

Abstract In this paper, the assignment of the 1 H and 13 C NMR spectra of poly(galacturonic acid)s is proposed; the 4 C 1 chair conformation of the monomeric unit in poly(galacturonic acid) is confirmed and the composition of the anomeric mixture at the reducing end (α−D)/(β−D) = 35 : 65 mol/mol independent of the degree of polymerization DP (2 ⩽ DP ⩽ 5) is given. It is demonstrated on oligomers that both 13 C and 1 H chemical shifts and coupling constants are nearly unaffected by exchanging Na counterions with Ca; the osmotic coefficient of Ca depends on the DP just as predicted by an electrostatic mechanism. The intrinsic viscosity of the polymer increases and the intensity of 13 C NMR signals decreases suddenly for 0,3 equivalents Ca 2+ per carboxylic site. These results confirm a multichain association followed by a rigid gel formation; the crosslink is proposed to correspond to a cooperative “egg‐box” mechanism on chain segments randomly distributed in the gel.