SynopsisThe potentiometric data concerning the dependence of the apparent pK with the degree of neutralization and the nature of counterions (Na or Ca) of galacturonic oligomers and polymers are discussed. They are characterized by an apparent charge density A,, which predicts their electrostatic behavior. The calculated osmotic coefficients from Katchalsky and Manning's theories are in good agreement with the experimental data obtained by potentiometry. The dependence of the osmotic coefficient (&a) as a function of the polymer concentration is established by potentiometry and interpreted in terms of a multichain aggregation. In addition, it is proven that no Ca2+ is ever fixed in excess of stoichiometry. The dependence of CD spectra obtained under the same conditions as the pK measurements shows analogies between the two sets of results. The CD data, in agreement with W -n m r measurements, suggest that in dilute solutions the polymer adopts two conformations: one (acidic form) which may be represented by threefold screw symmetry and a second (Na or Ca form) which can be related to a twofold screw symmetry. The twofold screw symmetry is known to allow the formation of cooperative "egg-box'' fixation of Ca without conformation change.

In this paper, the assignment of the 'H and I3C NMR spectra of poly(ga1acturonic acid)s is proposed; the 4C, chair conformation of the monomeric unit in poly(ga1acturonic acid) is confirmed and the composition of the anomeric mixture at the reducing end (~-D)/@-D) = 35 : 65 mol/mol independent of the degree of polymerization DP (2 < DP < 5) is given. It is demonstrated on oligomers that both 13C and '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 I3C NMR signals decreases suddenly for 0,3 equivalents Ca2+ 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.