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Investigation on oligo‐ and polygalacturonic acids by potentiometry and circular dichroism
Author(s) -
Ravanat G.,
Rinaudo M.
Publication year - 1980
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1980.360191206
Subject(s) - chemistry , counterion , potentiometric titration , polymer , stoichiometry , circular dichroism , crystallography , analytical chemistry (journal) , ion , chromatography , organic chemistry
The potentiometric data concerning the dependence of the apparent p K 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 λ app 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 (ϕ Ca ) 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 Ca 2+ is ever fixed in excess of stoichiometry. The dependence of CD spectra obtained under the same conditions as the p K measurements shows analogies between the two sets of results. The CD data, in agreement with 13 C‐nmr 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.