Conformations and interactions of pectins. I. Polymorphism between gel and solid states of calcium polygalacturonate.

Ca2+ binding to poly-d-galacturonate (the main backbone sequence of the polysaccharide pectin from plant cell walls) has been investigated by equilibrium dialysis and circular dichroism to elucidate the nature of conformational ordering and chain association in the sol-gel transition. Of the total stoichiometric requirement of bound calcium, only (50 ± 5)% is resistant to displacement by swamping concentrations of univalent counterions. Closely similar behaviour has been reported for poly-l-guluronate (derived from the polysaccharide alginate) and was attributed to site-binding of Ca2+ within dimers of chains of 21 helical symmetry. The circular dichroism changes that accompany Ca2+ binding are also closely similar for the two polymers when allowance is made for their near mirror-image stereochemistry by inversion of sign. When polygalacturonate gels, with calcium as sole or principal counterion, are dried to solid films, very profound circular dichroism changes occur, suggesting that the chain conformation and/or packing undergoes some modification during interconversion between gel and solid states. No such circular dichroism changes are seen for polyguluronate, consistent with the persistence of similar (21) chain conformations and packing for this polysaccharide in both the gel and solid state. On the basis of all this evidence, we propose: 1. (1) The co-operative binding of Ca2+ in polyguluronate and polygalacturonate gels, is through “egg-box” complexes with the polysaccharide chains in analogous 21 conformations. 2. (2) Drying of calcium polygalacturonate gels, but not polyguluronate gels, is associated with a polymorphic phase transition, and it is for this reason that diffraction studies on dried films show the 31 helix.