Iduronate ring

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Previously, our molecular mechanics studies showed that the iduronate ring can adopt the (skew boat)... 

The asymmetric unit mentioned above, with the coordinates given by Mitra et al. (based on iduronate rings) was then surrounded, according to the crystallographic symmetry, by all the atoms up to a cutoff distance of 0.6 nm. All of the partial residues arising from the above selection were completed, and the sodium atoms needed to ensure neutrality were added. The resulting microcrystal contains about 5200 atoms, but due to symmetry, only 215 atoms have independent coordinates for a total of 645 variables in the minimization (see Figure 3). 

M. Ragazzi, D. R. Ferro, and A. Provasoli, A force-field studie of the conformational characteristics of the iduronate ring, J. Comput. Chem., 1 (1986) 105-112. 

In the condensed form, the configurational symbol and the letter denoting ring size are omitted. It is understood that the configuration is D (with the exception of fucose and iduronic acid which are usually L) and that the rings are in pyranose form unless otherwise specified. The anomeric descriptor is written in the parentheses with the locants. 

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

While data from n.m.r. indicated that the prevails in solutions of the iduronate-containing saccharides, periodate data was interpreted to mean that the is correct. The possibility that other ring forms might be present was seldom considered. In one case, a "distortion of the chair was invoked to explain the data from iduronate-containing compounds in solution that could not be rationalized with a mixture of the two forms (1-3). 

An increase in the energy of 10 kcal/mol was found for an intermediate ring structure, in agreement with the results obtained for the monomeric iduronate ( ). A structure with minimal energy for... 

In dermatan sulfate a vacuum UV circular dichroism study showed that the iduro-nate ring is present only in the C4 conformation [220]. This applies to the solution as well as to the dried films. The evidence obtained from X-ray diffraction on crystalline fibers is just the opposite. According to this study, iduronate should adopt a 4Cj conformation in the solid state [221]. 

Fig. 3 - Newman projections for ring protons of iduronic acid (I a,b,Ctd) and protons at Cg of aminosugar (A,e)...

Jones In the polymer cellulose, which is 1,4-8-linked anhydroglucose, there is intramolecular hydrogen bonding form (0H>3 in one glucose residue to the ring oxygen of the next residue. Is this possible ste-rically in iduronic acid and could this restriction affect mobility ... 

Gatti Ue have no experimental information on this but we have found that the ring conformation of the iduronic acid moiety is completely in the 4(0 form. Perhaps this is arising because of such bond. 

Data obtained from n.m.r. analysis and from potentiometric titration of variously desulphated and periodate-oxidized heparins have supported the interpretation that non-sulphated cx-L-iduronate residues may adopt two ring conformations. The non-sulphated L-iduronic acid residues in heparin and heparin sulphate have been shown to be resistant to periodate oxidation at pH 3.0, under conditions which will oxidize a large proportion of the D-gluc-uronic acid residues. ... 

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