Conformation glycosides

FIGURE 11.12 Rotation around the glycosidic bond is sterically hindered syn versus anti conformations in nucleosides are shown. 

Most nucleosides contain D-ribose or 2-deoxy-D-ribose linked to N-1 of a pyrimidine or to N-9 of a purine by a P-glycosidic bond whose syn conformers predominate. 

Figure 2. Potential energy maps of a) GalA(l->2)Rha as a function of the glycosidic dihedrals, b) the same disaccharide with the galacturonic acid acetylated at 02 and 03. The conformation used to build integer helices is indicated.

Conformational data of this type can be obtained by solid-state C NMR spectrometry. The chemical shifts (peak positions) of the carbons on either side of any glycosidic linkage are sensitive to the conformation of the linkage. 

In Fig. 3 all conformers are plotted within 3 kcal/mol above, the global minimum found by CICADA for each of the glycosidic torsions, superimposed on the corresponding disaccharide relaxed map. Fig. 4 clearly indicates that most of the local minima present in the corresponding disaccharide fragments are also explored along the PES of the pentasaccharide by CICADA... 

Fig. 3. Projections on the (<1>, maps of the CICADA conformational search of the pentasaccharide. The dots indicate the values of all the optimized conformations determined by CICADA at each glycosidic linkange in 8 kcal/mol energy window For comparison, the isocontours, drawn in 1 Kcal/mol steps with an outer limit of 8 kcal/mol, represent the energy level of each disaccharide and calculated with the relaxed grid search approach. Dashed regions represent the locations of the low energy conformation of the pentasaccharide plotted on the potential energy surfaces of the constituting disaccharide segments...

Glucose occurs in the chair form in the C 1 conformation all functional groups and the glycosidic bonds have equatorial positions, a very stable conformation. The carbon-glycosidic oxygen bond makes an angle of approxi-... 

As illustrated in Fig. 7, the two chair conformations for a-L-idopyrano-syluronic residues are substantially different in terms of relative orientation of functional groups, as well in the geometry of the overall heparin chain, segments with I2S residues in the C4(l) conformation (as in a, having two axial glycosidic bonds) being shorter than residues in the 4Ci(l) conformation.341... 

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