Iduronic acid skew conformation

Finally, there are intermediary conformations between the chair and the skew. Their symbolic descriptions will be given by using the most important example, L-iduronic acid (see Section 2.8). 

The ido configuration is present, isolated in the middle of other monosaccharide residues, in the polycondensed chains called glycosaminoglycans of natural polysaccharides dermatan sulfate, heparan sulfate, and heparin. The derivative in question belonging to the L-series is 2-0-sulfo-L-iduronic acid. It is represented (2.34, R = R = H) in a non-conformational manner with the a-L-idopyrano configuration present in these polysaccharides. The L-ido residue is isolated in the sequence, in the centre of the chains attached to 0-1 and 0-4, respectively (Section 17.3). It is found as a mixture of the l- C4 (2.35) and l- Sq (2.36) conformations. The proportion of the skew form varies from 40 to 60% according to the attached oligosaccharide sequences, R and R (Casu et al. 1986). 

The conformation of dermatan 4-sulfate, in which C5 of the glucuronic acid has been epimerised, is still unsettled. Fibre X-ray data fitted models in which the a-L-IdoA residue was in the conformation better than those in which it was in the 4 conformation. Models containing skew conformers were not considered and NMR studies have since established that in D2O solutions of dermatan 4-sulfate, the a-L-IdoA residues are in the C4 or So conformations. Molecular dynamics simulations have also supported a Sq conformation for the iduronate in dermatan sulfate. ... 

Dichroic Behavior of Carboxyl and Amide Chromophores in Polysaccharide It is evident that the configurational differences of the uronic acid moiety are well reflected in the dichroic behavior of the polymer, and the carboxyl chromophore thus plays a significant role in the chiroptical properties of the molecules. Certain facts regarding carboxyl chromophores are apparent from this study a) monomeric CD properties are well reflected in the Cotton effect of the polymer b) similarities in A0 values (Table 11) between monomers and polymers containing similar uronic acids indicate the additivity of monomer contributions to polymer CD c) iduronic acid shows considerably larger CD than glucuronic acid, which may have some origin in the observed difference in the NMR behavior (29) of methyl a-D-idopyranosiduronic acid from uronic acid with normal C-1 conformation. This has been interpreted in terms of either an equilibrium between the C-1 and 1-C chair forms or adoption of a hybrid skew boat structure. 

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