Lemon-peel pectin. III. Fractionation of pectic acids from lemon-peel and lucerne

Abstract

The pectic acids formed on saponification of lemon-peel pectins C and L have been fractionated by graded precipitation with sodium acetate. The results of analytical and electrophoretic studies on the pectic acid fractions have been interpreted as being typical of those of the fractionation of chemically homogeneous polydisperse systems. Similar fractionations of lucerne polysaccharides have indicated the presence of a single component in pectic acid A and of two components in pectic acid B. The nature of polysaccharide heterogeneity in pectic acid preparations is discussed.Canadian Journal of Chemistry, 48, 1290 (1970) Previous investigations (1, 2) have shown that two homogeneous pectins may be isolated from dried lemon-peel. The two pectins show slight differences in uronic acid content and in degrees of esterification and it has been suggested that the higher proportion of non-esterified galacturonic acid residues in pectin L than in pectin C accounts for the differences in ease of extraction from the peel (2). In order to compare the two polysaccharides and to assess the homogeneity of each in respects other than the degree ofesterification of galacturonic acid residues, the pectins have been saponified under the mildest possible conditions (3) to give acid-insoluble pectic acids C and L. Similar pectic acids have been prepared from pectins A and B isolated from lucerne (alfalfa) by extraction with disodium ethylenediaminetetraacetate (4) and with water alone (5).As indicated by the compositions of the parent lemon-peel pectins, analysis showed that pectic acid L contained a higher proportion of galacturonic acid and a correspondingly lower proportion of neutral sugar residues than pectic acid C (see Table 1). Bhattacharjee and Time11 (6) have isolated from Amabilis fir bark a polysaccharide preparation which may be separated into a galacturonan and a pectic acid with galacturonic acid as the dominant sugar but with neutral sugars as integral constituents. Zitko and Bishop (7) have similarly interpreted the results of fractionations of various pectic acids by graded precipitation with sodium acetate as indicating the presence of two such polysaccharides in other pectins. Lemon-peel pectic acids C and L were therefore fractionated in the same way in order to see if the differences in composition of the two pectic acids could be accounted for by the presence in each of two polysaccharides, but in different proportions. Similar fractionations have been performed on the lucerne pectic acids.Lemon-peel pectic acid C was electrophoretically homogeneous in both borate and pyridineacetic acid buffers. Graded precipitation with sodium acetate gave four polysaccharide fractions and a fifth fraction was obtained by precipitation with ethanol. Table 1 shows some of the properties of these fractions and those from pectic acid L, and Figs. 1-8 show some typical electrophoretic patterns. Pectic acid fractions C 1-5 contained varying proportions of galacturonic acid residues, but all gave neutral sugars on hydrolysis. P...

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