Pectic acid occurrence

L-arabinose molecules combine together in the furanose form to produce an araban. Since enzymes capable of hydrolysing pectic acid to n-galacturonic acid are known to be present in certain plant juices, and since the comparatively low temperature and slightly acid condition of plant materials would tend to favor the transformation of arabinose into the furanose form, the occurrence of the furanose structure in the polysaccharide derived from arabinose is not altogether unexpected. 

Modified aldoses, of wide natural occurrence, are the uronic acids. In these molecules the terminal CH2OH group of a hexose has been oxidised to carboxyl. Polysaccharides consisting almost entirely of uronic acid units are pectic acid (D-galacturonic acid) (LXXVIII), the main component of pectin found in plant tissues, and alginic acid (D-mannuronic (LXXIX) and L-guluronic acid (LXXX)) synthesised by the brown algae. For a full description of these acids see Chapter II, p. 73. 

As investigations with polysaccharides of microbiological, plant, and animal origin revealed the presence in these substances of uronic acids, it became necessary to know more about the acids. Moreover, the occurrence of uronic acids in plant gums and pectic substances, and the structures of urinary... 

There is the further, although remote, possibility that the only substantial variation in the composition of pectinic acids obtained from fruits and beets, and from some other sources, might be the result of differences in the occurrence in these of natural enzymes acting upon pectic substances. Pectinic acids isolated from beets have been shown to contain acetyl groups,8 the mode of attachment of which to the poly-... 

Polyuronides may be defined as polysaccharides that contain one or more uronic acid units in their molecular structures. They have a frequent and wide occurrence in nature. Much of the carbohydrate material in plants belongs to the group. It includes all pectic materials and plant gums and many plant mucilages, hemicelluloses and gel-forming substances and some microbial polysaccharides. These substances are to be looked for in water-soluble plant exudates and mucilages, as well as in water and alkaline extracts of most plant materials. This review will be limited to the polyuronides occurring in plants. 

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