Structure of Sickled Erythrocytes and of Sickle-Cell Hemoglobin Fibers

Deoxyhemoglobin from patients homozygous for sickle-cell anemia (deoxyhb S) aggregates into long straight fibers. These may extend through most of the length of the sickled cell, forming either square or hexagonally packed bundles with lattice constants of 170-180 A. Each fiber is a tube made up of six thin filaments, which are wound around the tubular surface with a helical pitch of about 3000 A. Each filament is a string of single hemoglobin molecules linked end to end at intervals of 62 A in dry and 64 A in wet fibers. Molecules in neighboring filaments are in longitudinal register so that they form flat hexagonal rings; these rings are stacked so that successive ones are rotated about the fiber axis by 7.3 degrees . The whole structure repeats after about eight rings. In this structure each molecule makes contact with four neighbors. The likely orientation of the molecules and points of contact between them are discussed. Similar filaments are also observed in normal deoxygenated erythrocytes, but in much lower concentration and aggregated into fibers of irregular diameter. No filaments appear in oxygenated sickle, or normal, adult cells, nor in oxygenated or deoxygenated fetal cells.