Monosaccharides Constituents of Glycans

For example, a-o-glucose (axial hydroxyl at anomeric carbon) has a specific rotation, [a]d° of 113.2° and that of P-D-glucose (equatorial hydroxyl at anomeric carbon) has 

TABLE 2.3 Optical rotations of some naturally occurring hexoses and derivatives 

For six-member pyranose rings, six conformations, P (planar), E (envelope), B (boat), S (skew, twisted, twist boat), H (half chair) and C (chair) are possible. The chair conformation C is by far the most stable of the six conformations. Two chair conformations are possible for pyranoses. To distinguish the two conformations, a reference plane is drawn through four atoms (e.g. C2, C3, C5 and 05) of the ring so that the C-atom with the lowest position niunber (i.e. Cl) lies out of the plane. A ring atom that lies above this reference plane is placed before the abbreviation of the ring conformation as a superscript. The one below the plane is written as a subscript following this letter as Ci (C4 above and Cl below) and C4 (Cl above and C4 below). 

Starting with the assumption that all pyranoses exist in a chair form, a hexopyranose will try to adopt a conformation, which allows an equatorial rather than an axial position for the hydroxymethyl (primary hydroxyl) group. Thus in the D-series, a Ci conformation will be preferred, except idopyranose in which an equatorial position for the hydroxymethyl group, Cl, must be at the expense of three axial hydroxyl groups. 

Five-member furanose rings are also not planar. The furanose ring can have a common envelope conformation (E), with four atoms coplanar and the fifth out of the plane (any of the four carbons or the oxygen). For the envelop, the conformation is defined by the one atom that is above or below the plane, such as E and E3 for C3 above and below the plane (Cl—C2—C4—04) respectively. It can also have a less common twist 

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