Samuel, John W. B.

2019-02-15T14:19:55Z

2019-02-15T14:19:55Z

1967

Alginic acid is one of the main carbohydrates of the Phaeophycae, the brown seaweeds, its function not being clear. The proportion varies with the season, species of Laminaria containing 24% of alginic acid in February and only 14% in September (1). To isolate it1the cleaned weed is first steeped in dilute acid, washed and then extracted with sodium carbonate solution, when a solution of sodium alginate is obtained. This material finds many commercial uses (2). It finds uses in cold setting jellies, as a stabiliser in many foods, particularly ice cream, as a.thickener in textile printing, in the surface sizing of paper and in water purification. Alginic acid is a high molecular weight polysaccharide and until 1955 it was thought that it contained only residues of D-mannuronic acid. However, Fischer and Dorfel (3) showed that hydrolysis gave L-guluronic acid, the C-5 epimer of D-mannuronic, as well as D-mannuronic acid. Methylation analysis indicates that both these units are 1,4- linked (4), a conclusion that is substantiated by the application of other methods of structural investigation (5,6), and with which results of earlier work are consistent (7,8). Isolation of 4-0-ß-D-mannopyranosyl-D-mannopyranose (9) from a partial hydrolysate of the reduced polysaccharide indicates that the mannuronic acid residues in alginicacid are linked through their C-4 positions by a ß-linkage and that the linkage is 1,4-pyranosyl rather than 1,5-furanosyl. Partial fractionation into fractions rich in mannuronic and guluronic acids, respectively, has been achieved,(10,11) but repeated fractionation failed to separate a polymer which contained only mannuronic or guluronic acid residues. Proof that the two acids appear together in at least some of the alginic acid molecules was supplied by the isolation of oligouronic acids (12) containing both acids, and of a crystalline mannosyl- gulose (9) from partial acid hydrolysates of alginic acid and its reduction product respectively. A study of the constitution of alginic acid by partial acid hydrolysis has been carried out by Haug, Snidsrod and Larsen (13). Heterogeneous hydrolysis of the alginate was carried out with oxalic acid. Results showed that a certain amount of the alginate passed rapidly into solution, but even prolonged hydrolysis did not increase the concentration of carbohydrate in the solution to more than corresponding to 28% of the alginate. This clearly indicated that only part of the alginate sample was available for hydrolysis, while the rest of the sample was protected against hydrolysis or hydrolysed very slowly. The insoluble material could only be further degraded when it was washed, dissolved in dilute alkali and then retreated with oxalic acid. Even then there was a limit to the amount of hydrolysis taking place. The insoluble fraction could be fractionated into one fraction which contained predominantly guluronic acid residues and another which contained predominantly mannuronic acid residues. Significantly, no fraction with an intermediate uronic acid composition could be prepared. The number average length of the insoluble chains was 20 -30. The soluble fraction was shown tentatively to consist predominantly of the two monomers and a diuronide containing both the monomers. From these results HaugkandaLarsen deduced that alginic acid consists of blocks of 20 -30 monomer units with either predominantly mannuronic or guluronic acids and that these blocks are separated by regions with another sequence of uronic acid residues, probably with a large proportion of alternating mannuronic and guluronic acid residues. The blocks with a highly regular structure more easily form crystalline regions with a 3. much lower rate of hydrolysis than the more amorphous regions. It has still to be shown conclusively whether structural irregularities, such as branching or non -1,4-linking, ever occurs in the molecule. The incomplete oxidation of sodium alginate by periodate (6) would be explained if such irregularities were present. These questions are more fully examined in Section A. Another unsolved problem is the configuration of the guluronosyl linkage. It is noteworthy that a bacterial polysaccharide resembling alginic acid has been isolated from Azotobacter vinelandii (14) and Pseudomonas aeruginosa (15,16). studies are not yet so complete as on algal alginic acid and there would seem to be a close structural similarity except that the bacterial polymer is at least sometimes 0- acetylated. Degradation of alginate by a ß-elimination reaction has been achieved both enzynically (17,18) and chemically (19) (see Section C), and oxidative degradation by a free -radical mechanism by naturally - occuring phenolic compounds has also been shown to take place (20,21). In some ways, alginic acid has a more simple structure than pectic acid, a related uronic acid polymer from higher plants, in which the monomer is galacturonic acid. Pectin (22) is similar to alginic acid in that it contains a backbone of uronic acid residues, but blocks of polygalacturonic acid appear to be interrupted by occasional neutral residues (rhamnose). Neutral side chains are also present in varying degree, depending on the source of the pectin. Unlike pectic acid, alginic ei.cid does not occur in the esterified state%nor do neutral sugars ever seem to be part of the molecule. The chemical reactions of alginic acid are therefore of interest, not only for their own sake, but also because they might usefully be applied in the structure determination of the more complex pectic substances. One of the aims of the work which is reported in this Thesis, has been to use alginic acid to develop new approaches to the structure determination of uronic acid- containing polysaccharides. Details of these approaches are given in Sections B and :C. X -ray analysis of alginic acid gives well- developed diffraction patterns (23,24), but it has since been shown by Frei and Preston (25) that the sample examined was in fact a guluronic acid -rich sample. The data obtained therefore corresponded to polyguluronic acid and not to polymannuronic acid as was originally supposed. If we ignore the C -6 carbon atom then polyguluronic acid has the same carbon skeleton as cellulose (as well as xylan, mannan and polymannuronic acid) and these two polymers are indeed related in having a 2 -fold screw axis along the fibre axis although the fibre repeat distance is somewhat shorter for polyguluronic acid - 8.71 as opposed to 10.32 for cellulose. Frei and Preston reached the conclusion that the algal cell wall contains mainly material rich in guluronic acid whereas the intercellular alginic acid is primarily polymannuronic acid. The physical properties of alginate solutions are in many ways similar to those of pectins in the higher plant kingdom and to those of the sulphated polysaccharides from the red seaweeds e.g. K- carrageenan. For example, all form strong, reversible, cation -sensitive gels. This similarity, and the fact that all these polysaccharides occur at least partly in the intercellular parts of plant tissue, suggest that they might have similar biological functions. Accordingly, in seeking to understand the conformation, and the physical and biological properties of alginic acids, it would seem worthwhile to study all three types of polysaccharide together. Some progress has been made in this laboratory towards the determination of the conformation of K-carrageenan (26) by X-ray diffraction; this is described later in this Thesis (Section D) together with the results of an attempt wo develop the X-ray methods further and to apply them to alginic and pectic acids.

http://hdl.handle.net/1842/33812

The University of Edinburgh

Annexe Thesis Digitisation Project 2019 Block 22

The structure and chemistry of alginic acid

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy