Calcium binding and swelling behaviour of a high methoxyl pectin gel

Progress in understanding the structural role of pectic polysaccharides in plant cell walls is currently restricted by a lack of information on the molecular properties of undegraded cell wall pectins. We have examined, in solution, the calcium ion binding behaviour of a high methoxyl cell wall pectin from unripe tomato pericarp, and found it to be comparable to other pectins. After gelation through calcium addition, the affinity of the pectin for calcium ions was increased by at least an order of magnitude, with an estimated stability constant, K, of ∼8000. At pH 6 calcium binding is directly related to crosslink formation and gel stiffness. The swelling of the gel in aqueous salt solution was also examined; the kinetics of swelling were comparable to other polyelectrolyte gel systems. A fraction of the galacturonate residues in the gel does not participate in crosslinking but can contribute to gel swelling through a general polyelectrolyte effect at low ionic strengths. With decreasing concentration of free calcium ions, further marked swelling, and eventually dissolution, occurs as a result of dissociation of calcium crosslinks.