Pectin neutral sugar composition

This review will describe the analytical methods available to determine the structural components of pectin. These features determine the important physical, chemical, and biological properties of pectin. Included will be discussion of galacturonic acid determinations, degree of esterification with methyl groups, the neutral-sugar composition, and analysis of some less-common entities, such as 0-acetyl and 0-feruloyl linkages. 

Table I shows the neutral sugar composition as estimated for pectin types A to E from ripe apples expressed as moles sugar per mole arabinose. For the pectins from unripe apples similar results were obtained. It can be seen that the neutral sugar composition for all pectin types is fairly constant with the exception of galactose. The same is true for pools a to 6 of Fig. 4 here galactose is practically only present in pool a. From these results the conclusion was derived that within the neutral sugar side chain blocks a repeating pattern is present.

Table I. Neutral sugar composition of pectins extracted from ripe and unripe apples (AIS). The capitals A-E refer to Fig. 2. The neutral sugar composition is expressed as moles sugar per mole arabinose. The values are averages of those for pectins with about the same neutral sugar content in the four extracts. Mannose was absent in all cases (1).

The materials obtained are analysed for pectin content by uronlc acid assay. The method of Blumenkrantz and Asboe-Hansen (1973) Is employed. Further characterization of the pectlc material Included assay for neutral sugar composition by the alditol acetate method of Albershelm et al (1967), which entails derlvltlzatlon, and... 

Aphids have flexible, stylet-like mouthparts adapted for probing of plant tissues. By this means, the aphid must use chemical cues from the plant to determine if the plant is a suitable host (host plant quality), where the aphid is on the plant, the location of the stylet within the plant tissues, and the direction to probe to locate the plant phloem (Campbell and Dreyer, 1990 Dreyer and Campbell, 1987). Aphids avoid many of the toxic compounds stored in plant cells by probing between the cell walls. These insects are able to penetrate the intercellular spaces by producing a variety of digestive enzymes which depolymerize the pectins and hemicelluloses forming the intercellular-cell wall matrix. Aphid-host plant compatibility is associated to the extent that these enzymes depolymerize their respective substrates, and a reduced rate of depolymerization is often associated with host plant resistance. Differences in methoxylation, acetylation, or neutral sugar composition in the matrix polysaccharides are often involved in this reduced depolymerization. Further, specific breakdown products from depolymeri-... 

Table II. Carbohydrate compositions (weight percentage) of individual oligomer peaks purified (QAE-Sephadex or HPLC ion-exchange separation, respectively) from mixtures of citrus pectin oligomers or B fruit extracts Compositions shown are for peaks whose biological activity is described in Figure 4. Uronic acid values are based on colorimetric assay. Proportions of neutral sugars were determined by GC and adjusted so that totals equal 100%. In fact, some oligomers (G7 peaks 8, 9 and 10. B extract peak 10) produced small (less than 1 % of the total integrated area), unknown peaks in the GC chromatograms.

Characteristics of fraction I. The carbohydrate content of Fraction I was 84.4 %, in which the main component is D-galacturonic acid (71 %). Consequently, the polysaccharide is of a pectic type. The neutral sugars accounted for 13.4 % and according to their qualitative composition (Table 2) they correspond to the composition of pectin, isolated from sunflower heads (18). It is worth noting the high content of L-arabinose and D-galactose, compared with the other monosaccharides. The protein content was 7.8 %. 

It was not possible to distinguish between the transfer of neutral sugar residue-by-residue or its transfer as blocks. The difference in composition between the neutral blocks and the arabinogalactan is not inconsistent with either process. Methylation analysis could provide a means of separating the mechanisms since differences of sequence in the arabinan components of the pectinic acid and the arabinogalactan would imply the transfer of single sugars or very small saccharides. Identity of sequence would not separate the two mechanisms. 

The composition of the neutral sugars comprising the pectins of Phaseolus vulgaris has been shown to fluctuate during the culture cycle of suspensions of cotyledon cells, whereas only small changes were observed in the monosaccharide composition of the neutral polysaccharides. The pectin from the bark of the white willow Salix alba has been partially depolymerized with acid. Neutral disaccharides [(16) and (17)] and acidic oligosaccharides [(18)—(20)] isolated from the hydrolysates were characterized by g.l.c.-m.s. of their methylated derivatives. 

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