Polysaccharides molecular-weight determinations

Data from f.a.b.-m.s., and also f.d.-m.s., revealed the existence of naturally occurring, large cyclic polysaccharides. The first indication that a molecule may be cyclic comes from its precise molecular-weight determination. Cyclic molecules are 18 mass units less than their linear counterparts. Loss of water may, of course, occur in a number of ways, for example, by dehydration or lactonization, and conclusive evidence for the presence of a cyclic molecule can only be obtained from f.a.b.-m.s. of suitable derivatives, such as the permethyl derivative. Cyclic and dehydrated linear polymers are distinguishable after permethylation, as the cyclic polymer will incorporate one methyl group less than the linear molecule. 

The polysaccharide was acetylated with acetic anhydride in the presence of pyridine. Purification of the acetate consisted in precipitation of a 10% boiling benzene solution with petroleum ether. One hundred and fifty such precipitations were necessary before a constant-rotating product resulted [a]D20 = — 20.1° (c = 1.0, chloroform). Cryoscopic molecular weight determinations in benzene solution gave an average value of 3918. 

Molecular weight cutoff, 15 829 Molecular weight determination, for polysaccharides, 20 551 Molecular weight distribution (MWD), 20 530-531... 

The hydrogenation of 5 was carried out in an experimental manner similar to that of 2 (Scheme 8). No observation of the aromatic peak in the NMR spectra of the product indicated the occurrence of the perfect deben-zylation, giving rise to polysaccharide 7. The molecular weight determined by GPC with water eluent was 5,600, which is in good agreement with the calculated value (6,200). The solubility of 7 in water, DMSO, and DMF is higher than that of 3. 

Yu, L. -P, Rollangs, J. E. (1987). Low-angle laser light scattering-aqueous size exclusion chromatography of polysaccharides molecular weight distribution and polymer branching determination. Journal of Applied Polymer Science, 33, 1909-1921. 

Although most polysaccharides may be acylated without degradation, acyl derivatives of xylans29 are often unsatisfactory for molecular-weight determinations owing to their poor solubility characteristics. Mixed esters of xylans143 are often superior in this respect, but there are obvious difficulties in the differential analysis of acyl groups. Hemicellulose nitrates have been used for measurements of molecular size,168 but xylan nitrates,29 like xylan acetates, are reported to have poor solubility characteristics. 

The results of molecular-weight determinations on hemicelluloses need to be interpreted in the light of comments made earlier (see p. 461) regarding the degrees of heterogeneity possible in polysaccharides of this type. 

Harding, S. E., Varum, K. M., Stokke, Bjorn, T., and Smidsrod, O. (1991a). Molecular weight determination of polysaccharides. Advances in Carbohydrate Analysis, White, C. A. (Ed.), Vol. 1. JAI Press, Greenwich, CT. 

Molecular weight determinations on both groups of polysaccharides are not extensive. 

S. Geresh, I. Adin, E. Yarmolinsky, and M. Karpasas, Characterization of the extracellular polysaccharide of Porphyridium sp. Molecular weight determination and rheological properties, Carbohydr. Polym., 50 (2002) 183-189. 

Capillary gel electrophoresis (CGE), Size determination Protein molecular weight determination, aggregates, oligonucleotides, DNA fragments, polysaccharides... 

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