By careful definition of polymer environment, heparin i.r. spectra were examined in a region (750-950 cm-1) in which sulphate half-ester absorptions occur. Changes seen in this region when metal ion-heparin complexes are converted into heparinic acid, when heparin is carboxy-group-reduced and when various concentrations of Li(+)-heparin are examined are tentatively interpreted in terms of changes in the ring conformation of iduronate residues.
