X-ray diffraction studies of fibers and crystals of deoxygenated sickle cell hemoglobin.

Abstract

Paracrystalline fibers of deoxygenated sickle Hb in [human] erythrocytes or concentrated solutions exhibited a phase transformation to a fully crystalline state. X-ray diffraction patterns of the fiber and crystallites were similar except in 2 respects: the equatorial spacings of the fibers suggested that they pack into a square lattice with a = 220 .ANG., whereas those of the crystals can be indexed on the basis of a net of 187 .ANG. by 54 .ANG., and the 2nd-order near-meridional reflections were strong on the fiber pattern but weak on that of the crystallites. The crystallites were isomorphous with single crystals grown in polyethylene glycol solution at pH 4.5, whose structure was determined at near-atomic resolution. Double filaments of molecules with an axial repeat of 64 .ANG. comprised the basic unit of the crystal and fiber structures. Each filament of the pair was translated with respect to its neighbor by half a molecular diameter along the fiber axis. The 2 filaments were held together by contacts made by Val 6.beta. in the molecules of strand with hydrophobic side chains of the molecule in the neighboring strand. This interaction was probably the cause of the aggregation of filaments into fibers that leads to the sickling of erythrocytes.