α-l-iduronate ring conformations in heparin and heparin derivatives. 13C Nuclear-magnetic-resonance analysis and titration data for variously desulphated and periodate-oxidized heparins

A heparin derivative that had been O/N-desulphated and re-N-acetylated was investigated by 13C n.m.r. spectroscopy and potentiometric titration. Three forms of uronic acid were observed, tentatively identified as beta-D-glucuronate, and two different forms of alpha-L-iduronate. A comparison of the n.m.r. spectra of heparin, an oligosaccharide (beta-D-glucuronate-2-acetamido-2-deoxy-alpha-D-glucose)n, and heparin that had been subjected to selective oxidation of beta-D-glucuronate, enabled the position of the anomeric carbon of the latter residue to be assigned [delta 102.9 (p.p.m.)]. Periodate oxidation of O/N-desulphated heparin destroyed in addition, approx. 40% of the alpha-L-iduronate content. The remainder of the alpha-L-iduronate residues displayed only one anomeric resonance, at delta 99.7 (p.p.m.). In another preparation, after sequential desulphation of heparin (N-desulphation, re-N-acetylation and O-desulphation) the anomeric resonance of the alpha-L-iduronate residue shifted downfield [from delta99.7 (p.p.m.) to delta 102.3]indicating a change in ring conformation. These data support the interpretation that the unsulphated alpha-L-iduronate residues may adopt two conformations. It was shown that the proportions of alpha-L-iduronate conformers are determined by the sequence of desulphation operations. Also minor components of heparin were assigned.