X-RAY-DIFFRACTION STUDIES OF FIBERS AND CRYSTALS OF DEOXYGENATED SICKLE-CELL HEMOGLOBIN

Paracrystalline fibers of deoxygenated sickle hemoglobin in erythrocytes or concentrated solutions exhibit a phase transformation to a fully crystalline state. X-ray diffraction patterns of the fiber and crystallites are similar except in two respects: the equatorial spacings of the fibers suggest that they pack into a square lattice with a=220 Å, whereas those of the crystals can be indexed on the basis of a net of 187 Å by 54 Å, and the second-order near-meridional reflections are strong on the fiber pattern but weak on that of the crystallites. The crystallites are isomorphous with single crystals grown in polyethylene glycol solution at pH 4.5 whose structure has been determined at near-atomic resolution. Double filaments of molecules with an axial repeat of 64 ∅ comprise the basic unit of both the crystal and fiber structures. Each filament of the pair is translated with respect to its neighbor by half a molecular diameter along the fiber axis. The two filaments are held together by contacts made by Val 6β in the molecules of one strand with hydrophobic side chains of the molecule in the neighboring strand. This interaction is probably the cause of the aggregation of filaments into fibers that leads to the sickling of erythrocytes.