Iduronate, iduronic acid conformation

In the disaccharide with a-L-iduronic-acid-2-sulfate linked to the 4-OH of 2,5-anhydro-mannose, which is obtained from heparin by nitrous acid degradation, the conformation of the iduronic acid was found to be C4 and to be independent of temperature in the range from 20 °C to 90 °C [217]. 

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). 

A novel H NMR study suggested that a-L-iduronic acid in dermatan sulfate exists predominantly in a slightly distorted l- C4 conformation (Rao 1995). 

Just as the primary structure of heparin and heparan sulfate has a wealth of fine detail, depending on its exact provenance and function, so the exact conformation of a stretch of polysaccharide depends on its exact location in the chain. The key to this conformational flexibility lies in iduronic acid residues, which can adopt either the C4 or the conformation (the glucosamine-derived residues are firmly in the C conformation, as are glucuronic acid residues). Interpretation of vicinal proton proton coupling constants of IdoA residues in terms of an equilibrium between just and conformations suggests the equilibrium changes from 60 40 for internal IdoA residues to 40 60 for terminal residues (Figure 4.84(a)). " ... 

Ferro DR, Provasoli A, Ragazzi M, Casu B, Torri G, Bossennec V, Perly B, Sinay P, Petitou M, Choay J. Conformer populations of L-iduronic acid residues in glycosaminoglycan sequences. Carbohydr Res 1990 195 157-167. 

X-ray studies (20) favor the 1-C conformation of iduronic acid in heparin as opposed to the C-1 in dermatan sulfate, but NMR studies (21) in solution do not suggest such a difference (Figure 3). The conformation of the L-iduronic add in dermatan sulfate has been suggested (22) as C-1, based on analysis of optical rotatory dispersbn curves. iTie striking dissimilarities in the dichroic properties of heparin and dermatan sulfate in... 

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