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Evidence for conformational equilibrium of the sulfated l iduronate residue in heparin and in synthetic heparin monosaccharides and oligosaccharides nmr and force field studies

, : Evidence for conformational equilibrium of the sulfated l iduronate residue in heparin and in synthetic heparin monosaccharides and oligosaccharides nmr and force field studies. Journal of the American Chemical Society 108(21): 6773-6778

The conformation of sulfated L-iduronic acid (I2S) in different heparin sequences, including the specific pentasaccharide sequence representing the binding site to antithrombin III (AT-III), was investigated by 1H NMR spectroscopy on suitable synthetic mono- and oligosaccharides. For the monomer methyl 2-O-sulfo-.alpha.-L-iduronate vicinal interproton coupling constants are all small (1.8-3.4 Hz), accounting for a predominant contribution of a 1C4 chair. By contrast, some couplings (especially J2,3) become larger (up to 6 Hz) when I2S is inserted between two N,6-disulfated D-glucosamine residues as occurring in the regular sequences of heparin, and even larger (up to 7.5 Hz) when I2S is glycosylated by the N,3,6-trisulfated D-glucosamine residues typical of the binding site to AT-III. The interproton coupling constants of the individual, nearly isoenergetic conformers of I2S (1C4, 2S0, and 4C1) were evaluated for different heparin sequences by using molecular geometries obtained by a force field method. Other conformations were discarded on the basis of energy considerations. The conformer populations were obtained by least-squares fitting the average computed coupling constants of the conformer mixture to the observed values. When I2S is part of regular heparin sequences, the skew-boat form 2So becomes an important contributor (.apprx. 40%) to the conformation of the sulfated iduronate residues. When the amino sugar residues glycosylating the I2S is trisulfated (as in the binding site to AT-III), 2S0 becomes predominant (> 60%). Force field calculations suggest that such a drive toward the 2S0 conformation is associated with electrostatic effects of the unique 3-sulfate group.


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