Progress in understanding the structural role of pectic polysaccharide
s 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 met
hoxyl 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 fbr calcium ions was increased by at least
an order of magnitude, with an estimated stability constant, K, of si
milar to 8000. At pH 6 calcium binding is directly related to crosslin
k 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 contr
ibute to gel swelling through a general polyelectrolyte effect at low
ionic strengths. With decreasing concentration of free calcium ions, f
urther marked swelling, and eventually dissolution, occurs as a result
of dissociation of calcium crosslinks. (C) 1998 Elsevier Science Ltd.
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