Glucuronic acid conformations

Figure 2.3. Boat and chair forms of P-D-glucuronic acid, (a) Cj chair conformation (left) and planar projection (right) (b) Boat conformation and (c) 1C chair conformation of P-D-glucuronic acid.The latter two conformations are energetically less stable than the Ci chair conformation.

Plots of fully allowed (inner solid lines) and partially allowed (outer solid lines) conformations of ( > and ip for (A) D-glucuronic acid, which is (3-(l 3)-lin keel to IV-acetyl glucosamine, and (B) /V-acctyl glucosamine, which is P-(l-4)-linkcd to D-glucuronic acid. Note that the allowed conformations center around 0°,0° and show that hyaluronan has flexibility. 

Figure 2.33. Conformational plot of p (1—>3) and p (1—>4) linkages in hyaluronic acid plots of fully allowed (—) and partially allowed (—) conformations of (p and )/ for (a) D-glucuronic acid which is linked P (1—>4) to N-acetyl glucosamine and (b) N-acetyl glucosamine which is finked P (1—>3) to D-glucuronic acid. Allowed conformations center around (0°, 0°) and indicate that stereochemically the backbone of hyaluronan has some flexibility.

Figure 4.78 Conformational options for a sulfated glucuronic acid residue, as monomer and polymer. Sulfation of vicinal diequatorial residues is sufficiently disfavoured by electrostatics that it can alter conformation, but the effect is not large enough to overcome a methoxyl anomeric effect.

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. ... 

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)). " ... 

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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