X-RAY FIBRE DIFFRACTION OF HEPARIN By I. A. NIEDUSZYNSKI and E. D. T. ATKINS

Oriented films of the sodium salt of heparin crystallize in a triclinic unit cell. The X-ray diffraction results favour a tetrasaccharide covalent repeating sequence but the measured molecularrepeat of1.59nm is inconsistent with all a-(1 -*4)(lax -->4eq)-glycosidic linkages as generally accepted. Three models are proposed for the molecular shape of heparin, which depend on the chair conformation adopted by the hexuronic acid moieties. Model I has both uronic acids in the Cl chair form, model II has one uronic acid in each ofthe Cl and 1C chairs and model III has both in the IC chair form. The merits ofthese models are discussed.