Polysaccharides optical rotation

A large number of polyfructosans that have been reported from time to time by different authors have been investigated by Schlubach and his associates. In order to obtain polysaccharides of constant optical rotation, 100 to 300 precipitations from aqueous solution by the addition of alcohol were necessary. Fifty to 150 precipitations from chloroform solution with petroleum ether were required for purification of the acetate derivatives. These were methylated according to the procedure of Haworth and Straight,24 and upon hydrolysis partially methylated fructoses were obtained. 

Xylan has the general properties of insolubility in water, solubility in alkaline solutions, ease of acid hydrolysis, high negative optical rotation, and non-reducing action toward Fehling s solution. It can be placed in three general polysaccharide classes (1) pentosan, (2) glycan, and (3) hemicellulose. It is classed as a pentosan because it is principally a polymer of a pentose. It is by far the most abundant pentosan. 

On treatment with periodatd ion, straw and beechwood xylan oxidize rapidly.104 As with other polysaccharides, the oxidation comes to a more definite end point in solutions which are buffered to pH 4-5. While somewhat more than the theoretical amount of periodate ion is consumed, the reaction apparently proceeds uniformly with oxidative cleavage of the 2,3 carbon bond to produce the structure indicated in Figure 4. In the course of the reaction the xylan passes into solution. The optical rotation of the oxidized product is surprisingly high (co. 100°). 

N 142 "Conformational Analysis of Polysaccharides. Part V. The Characterization of Linkage Conformations (Chain Conformations) by Optical Rotation at a... 

Gelation and liquefaction of these polysaccharides may be readily monitored by observation of the optical rotation.183 A typical graph of... 

Gelling mixtures of Xanthomonas polysaccharide with locust-bean gum or tara gum do not give reproducible optical rotation results, possibly because of gel strain, but examination of non-gelling... 

Fig. 10. — Comparison of the Changes in Viscosity (—) and Optical Rotation (—) when a Solution of Xanthomonas Polysaccharide (1%) is Heated and Cooled.

The specific optical rotation of these polymers ([a]f> + 91° =b 5°) led Schuerch and coworkers 1 to suggest that they contain a minor preponderance of a-D-glucoeidic linkages. Only pyranoid forms were assumed to be present. In view of the finding of furanoid residues in other synthetic polysaccharides,15 1 2 this supposition is no longer valid. Additional experimental data, such as the results of methylation studies, must be acquired before the chemical structure of these complicated polymers can be formulated. 

These are some examples of the use of i.r. spectra in the analysis and identification of carbohydrates in foods and natural products. Very often, these spectroscopic techniques are complementary to others, such as the study of aldobiouronic acids obtained by hydrolysis of peach-gum polysaccharides by their optical rotations and their i.r. spectra.100 However, the i.r. results appear to be sufficiently reliable to be used in the detection of traces of fructose and glucose, and to determine the d.e. (dextrose equivalent) of corn syrups, as well as the quantitative carbohydrate content in different products.101... 

The first successful experiments were reported by Schwab [16] Cu, Ni and Pt on quartz HI were used to dehydrogenate racemic 2-butanol 23. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 23 had reacted preferentially (eeright-handed quartz gave the opposite optical rotation it was deduced that the chiral arrangement of the crystal was indeed responsible for this kinetic resolution (for a review see [8]). Later, natural fibres like silk fibroin H5 (Akabori [21]), polysaccharides H8 (Balandin [23]) and cellulose H12 (Harada [29]) were employed as chiral carriers or as protective polymer for several metals. With the exception of Pd/silk fibroin HS, where ee s up to 66% were reported, the optical yields observed for catalysts from natural or synthetic (H8, Hll. H13) chiral supports were very low and it was later found that the results observed with HS were not reproducible [4],... 

In 1954, Jeanes et al.5 reported the formation of glucan(s) by 96 strains of bacteria that were primarily Leuconostoc strains. (There is a question here as to whether they are strains or species, particularly with regard to the formation of different kinds of polysaccharides. The classification of the time was to place them into one species, mesenteroides, that had several different strains. This classification stands today.) The polysaccharides were characterized by various properties such as optical rotation, viscosity, periodate oxidation profile, and physical appearance after alcohol precipitation. The latter were observed to have different appearances, which were described by Jeanes et al.5 in various qualitative terms such as pasty, fluid, stringy, tough, long, short, flocculent, and so on. These differences in appearance provided an early suggestion of differences in structure (see Table I). Both water-soluble and water-insoluble polysaccharides were formed, and some strains seemed to form more than one kind of polysaccharide, as judged by their water solubility and by differences in the amount of alcohol needed to precipitate them. 

Conformational changes are easily followed by optical rotation (Hui and Neukom, 1964). Circular dichroism spectroscopy (CD) of polysaccharides (Morris, 1994) exploits optical anisotropy. In a CD instrumental design, the clockwise and counterclockwise rotation of two polarized beams of equal intensity, traversing a 180° path through a chiroptical medium, display a molar ellipticity maximum and minimum. CD is the differential measurement as a function of X. By CD spectroscopy, mixed interchain association rather than nonspecific incompatibility or exclusion was identified as the molecular basis of alginate-polyguluronate interaction (Thom et al., 1982). 

Rees DA (1970) Conformational analysis of polysaccharides. Part V. The characterization of linkage conformations (chain conformations) by optical rotation at a single wavelength. Evidence for a distortion of cyclohexaamylose in aqueous solution. Optical rotation and the amylose conformation. J Chem Soc B 877-884... 

---