Polyuronides structure

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. 

Previous to the last decade most of the research on the group dealt with the composition of the substances. There still remains an enormous amount of this work to be done. Now, however, emphasis is shifting to a study of the structure and molecular size of these bodies. The methods that were developed in the study of the simple sugars, the glycosides, starch and cellulose are being applied in the study of the polyuronides. 

Per-iodic acid is known to split carbon chains between carbons containing adjacent hydroxyl groups. This method has been used in studying the structure of some polyuronides and confirms the results obtained by methylation of these materials. ... 

Fulvic acids. Marine sedimentary humic substances soluble in base and acid (fulvic acids) have previously been examined by and NMR (12). The dominant structural components were postulated to be polysaccharide - like substances, probably polyuronic acids. Solid-state NMR spectra of fulvic acids isolated from a number of marine and estuarine sediments are shown in Figure 1. Major peaks at 72 and 106 ppm betray the overwhelming presence of polysaccharide -like substances, and, as shown by Hatcher and others (12.), the moderate peak for carboxyl or amide carbon at 175 ppm suggests that these polysaccharides are more like polyuronides. Aromatic carbons (110 to 160 ppm) are decidedly minor components. Aliphatic carbons (0-50 ppm) are also minor components. H NMR spectra shown by Hatcher and others (12) indicate that these aliphatic structures are highly branched. 

The classical definition of fulvic acids is not very specific. Many biochemical substances such as proteins, sugars, and fatty acids would fall under this classification. These substances, in many instances, can hardly be considered "humic" in nature. But, if one uses the classical definition of fulvic acids these substances are included. No doubt the observations made above that polysaccharidelike substances constitute the major components of sedimentary fulvic acids is partly attributable to the fact that the operational definition classes polyuronides as fulvic acids. It is not the intent of this paper to discuss the merits of using the classical operational definition for fulvic acids as opposed to one s perception of what true fulvic acids are. He know far too little about the composition of humic isolates and about their origin to begin discussions of whether they are humified or not. He therefore chose to use operational definition with recognition that well defined structural entities can sometimes be a part of what is isolated. 

Physical Methods. Infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectroscopic methods have been applied to structural analysis of polysaccharides such as pectin. These applications have been reviewed ( ), and reference IR spectra of pectic substances have been published ( ). Quantitative IR has been used to estimate acid dissociation constants of polyuronides from the ratio of -CO2H to -CO2- as a function of pH ( ). Also, by... 

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