In vitro interactions between polyamines and pectic substances.

Putrescine, spermidine and spermine induce a decrease in the pH value of 1 mM polygalacturonic acid or pectin solutions; spermidine and spermine also cause the precipitation of the polymers. The association constants between polyamines and polygalacturonic acid were in the order of 10(5) for putrescine and spermidine, and 10(6) for spermine. The number of galacturonic units per binding sites are proportional to the number of positive charges on the polyamine molecule. Low affinity binding sites appear at high polyamine concentrations. Calcium ions seem to compete weakly with spermine by lowering the association constant 4- to 6-fold. Two natural pectins tested, showed that methylation of the carboxylic groups influences only the number of galacturonic units per site but not the association constant.