The exudate gums and their structural relationship to other groups of plant polysaccharides.

The exudate gums are complex acidic polysaccharides produced by trees either spontaneously or after mechanical injury. Each polysaccharide contains three or more constituent sugars including D-gluCurOfliC acid (or its 4-methyl ether) and/or D-galacturOniC acid. Highly branched structures are invariably encountered and since each constituent sugar may be present in furanose or pyranose ring forms may have different configurations at its glycosidic linkages, and may be involved in a variety of types of linkage, the elucidation of the detailed molecular structure of the exudate gums provides one of the most challenging problems in organic chemistry. Many plant gums consist almost entirely of polysaccharides and consequently the problem of isolating these substances free from other materials, especially from other natural macromolecules, is often relatively simple compared with that of the isolation of other plant polysaccharides which are present as cell-wall components. The additional problem of obtaining polysaccharides in a sufficiently homogeneous form for detailed chemical examination cannot be considered at length on this occasion. Suffice it to say that three types of polymer heterogeneity have been recognized amongst the polysaccharide constituents of the exudate gums. First, there are those gums, such as gum tragacanth1 and Khaya senegalensis gum2'3, in which two polysaccharide components of entirely different structural type are present. Secondly, at the other end of the scale, there is the kind of micro-heterogeneity found in Combretum leonense gum4 in which polysaccharide subfractions show small differences in composition but no differences in the nature of the structural units or in the linkages between them. Thirdly, heterogeneity of an intermediate type has been encountered recently in Anogeissus leiocarpus (formerly A. schimperi) gum5'6. This gum contains two discrete polysaccharide components which are sufficiently different to permit fractionation on a preparative scale. Structural investigations so far carried out indicate that the two polysaccharides contain many of the same structural units but in markedly different relative proportions. To an increasing extent complex plant polysaccharides may be classified in terms of families of related molecular species in which the members of each group contain common structural units, most frequently the sequences of sugar residues in the interior chains, but differ considerably in the nature and number of other units attached as side-chains. This type of structural classification of polysaccharides, often of diverse origins, has been applied to many of the cell-wall polysaccharides of higher plants78, the xylan group