Triclinic crystals associated with fibers of deoxygenated sickle hemoglobin

Triclinic crystals have been found in capillaries that initially contained deoxygenated sickled erythrocytes, and in solutions of sickle hemoglobin that were stirred during deoxygenation. In both cases these crystals occur as a phase transition from fibers. They have been observed only as twins; the a-axis of one member is related to that of its twin by 180 degrees rotation about the b* direction. The volume of the triclinic crystal unit cell is half that of the monoclinic crystals that have also transformed from fibers. Analysis of X-ray diffraction data indicates that the two molecules in the triclinic unit cell that repeat at an interval of 64 A form double filaments similar to those found in the monoclinic crystals and in the fiber. The existence of the triclinic crystals which contain only one double filament per unit cell removes a postulated requirement that antipolar double filament pairs be the sole unit of the fiber architecture.