Hydroxymethyl groups, rotational

Thus the trans relationship between the hydroxymethyl group and the C-l hydroxy group in a-D-glucopyranose, and the cis relationship between the methyl group and the C-l hydroxy group in P-L-fucopyranose, are clearly shown. Note that representation of ketoses may require a different modification of the Fischer projection, as shown in the fructofuranose example above. Here C-2 is rotated about the bond with C-3 to accommodate the long bond to C-2 from the oxygen at C-5. 

In any case, it is worthwhile to point out that the observation of such unusually prolonged 13C T, values or displacement of the 13C chemical shifts can be utilized as a characteristic means to be able to locate a possible site for the presence of restricted or ceased group rotations for either methyl or hydroxymethyl groups caused by short interatomic contact or strong interactions. 

Figure 10. Trajectory of the rotation of one of the C(6) hydroxymethyl groups in cellobiose, 400 K. 300°, 180°,...

Crystal structure refinement of a polysaccharide requires not only definition of the conformation of the backbone but also substituent orientation with respect to the backbone. For example, different dispositions of the hydroxymethyl group are possible by a rotation about the C-5-C-6 bond.10-17 A convenient notation28 for denoting this torsion angle by use of the terms tg, gt, and gg, as shown in Fig. 1, has been adopted. 

The conformational analysis of monosaccharides, disaccharides, and ohgosaccharides is reviewed. Conformational terms are introduced through examination of the conformations of cyclohexane and cyclopentane then applied to the pyranose, furanose, and septanose rings. Concepts such as the anomeric effect are discussed. Topics of current interest, such as hydroxymethyl group and hydroxyl group rotation and disaccharide conformations are summarized. Physical methods for studying conformation are outlined. 

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