Stereochemistry of Polysaccharides

Hyaluronan (HA) is a component of every tissue or tissue fluid in higher animals. The highest concentrations are found in cartilage, vitreous humor, and umbilical cord and even blood contains some HA. At physiologic pH, the molecule has been shown to adopt a helical structure and therefore the molecule can be modeled as a series of helical segments that are connected with flexible segments. However, based on results of solution studies, the molecular structure of HA appears to be more complicated. 

Plots of fully allowed (inner solid lines) and partially allowed (outer solid lines) conformations of ( and ip for (A) D-glucuronic acid, which is (3-(l 3)-lin keel to IV-acetyl glucosamine, and (B) /V-acctyl glucosamine, which is P-(l-4)-linkcd to D-glucuronic acid. Note that the allowed conformations center around 0°,0° and show that hyaluronan has flexibility. 

---

In both their industrial and biological functions, the 3-dimensional characteristics of carbohydrates are important. Many of these stereochemical features are described for carbohydrates in the classic text by Stoddart (2). The inqportance of stereochemistry is underscored by the unique chemical and physical properties of the individual sugars, many of which are configurational isomers. Stereochemistry also plays a role in detentlining the properties of polysaccharides. Molecular shape is as significant for the properties of an industrially modified starch as it is for the recognition of one particular blood type and the rejection of others. 

Interactions can also be studied at the surface of a coated capillary wall. One binding partner is first immobilized on the capillary wall. As a result of the affinity of the second binding partner, the analyte will be delayed, compared with migration times observed in an untreated capillary. Based on this approach, modified capillaries have been prepared and used successfully to study polysaccharide-protein interactions as well as affinity separations. Coating of the capillary wall with heparin and heparan sulfate has been used to determine the affinity of these polysaccharides for synthetic heparin-binding peptides different only in the stereochemistry of a single... 

It is a remarkable fact that the main energy-storage polysaccharides and the main structural polysaccharides found in nature both have a primary structure of (l,4)-linked polyglucose. Why should two such closely related compounds be used in totally different roles A closer look at the stereochemistry of the a and /3 glycosidic linkage for polyglucose indicates why this is so. 

Yashima E, Okamoto Y, Chiral recognition mechanism of polysaccharides chiral stationary phase in The Impact of Stereochemistry on Drugs Development and Use (Aboul-Enein, HY, Wainer IW, Eds.), John Wiley Sons, New York, p. 345 (1997). 

This symposium presented an unusual opportunity in that we discussed the methods used to determine polymer structures from fiber diffraction data, rather than concentrating on the actual structures derived and their possible implications. At Case Western Reserve University we have been involved in determination of the structures of cellulose and chitin. This paper describes our analyses (1-6) of the structures of cellulose I and II and a- and 6-chitin, emphasizing the manner in which structural decisions were taken in each case. Efforts to determine these structures have a history of over 60 years, and it has only been with the advent of least squares techniques for the refinement of polymer structures (7) [notably the LALS method (8)], and the development of our present knowledge of polysaccharide stereochemistry, that solutions have become possible. In what follows we will look first at our methods for measuring intensities and thereafter will review the work on each of the four structures. 

This chapter begins with a discussion of the structure and stereochemistry of monosaccharides. Then the formation of cyclic structures front monosaccharides is discussed. This is followed by the presentation of a small number of reactions of these compounds. The classic series of experiments that was used to establish the structure of glucose is presented next. Finally, the structures of disaccharides, polysaccharides, and a few other types of carbohydrate-containing compounds are introduced. 

In the field of natural red algal galactans, MS was used for the first time in the structural analysis of odonthalan.353 Galactose and 6-O-methylgalactose, the hydrolysis products of the polysaccharide, were separated by column chromatography and identified by MS in the form of their di-O-isopropylidene derivatives. Cyclic derivatives of this type had been shown previously to be especially suitable for mass-spectrometric characterization of the structure and (in certain instances) stereochemistry of monosaccharides.349,354 A combination of GLC and MS was also used for identification of methyl 4.6-O-0 -methoxycarbonylethylidene)-D-galactosides (as their trifluoroacetates) in the methanolysis products of Ji-carrageenan from Petrocelis middendorfii,355... 

Suggest structural formulas for the following polysaccharides, neglecting the stereochemistry of the glycoside linkages ... 

---