Conformation Secondary and Tertiary Structures of Polysaccharide Chains

3 CONFORMATION SECONDARY AND TERTIARY STRUCTURES OF POLYSACCHARIDE CHAINS 

There are two possible types of strainless six-member pyranose rings, the boat (B) and the chair (C) forms and of these, the chair is usually preferred as there are fewer interactions across the ring between snbstituents. The rings can be defined as 1 if C4 is above the plane (described by C2, C3, C5 and 05) and Cl is below it, or as C4 for the alternative ring. P-D-Glncopyranose has all substituents (OH) that are equatorial in the 01 conformation, a possible reason why D-Glc is a common monosaccharide in nature. 

For the furanose ring, which is adopted by some hexoses and pentoses, the five-member ring is only slightly puckered and exists in three forms, the more common envelope (E) and the less common twist (T). For the more stable envelope forms, and E3, the conformation is defined by the C3 atom that is above or below, the plane (described by C1,C2, C4 and 04). 

Secondary structures of D-glucopyranose units connected by the a-1,4-linkage (A) and P-1,4-linkage (B) respectively are schematically represented (not scaled to the actually dimension) to show the directionality (helical versus linear) of the structures. 

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