Electron microscopy of fibers and discs of hemoglobin S having sixfold symmetry

Abstract

Aggregated forms of deoxyhemoglobin S were examined with a field emission transmission electron microscope. Images of isolated helical fibers were obtained from sickled cell lysates stained directly on the EM grid. Optical and digital analyses of the electron micrographs showed that the fibers are similar to those characterized by others in that they consist of stacked discs each composed of 6 Hb molecules. The fibers exhibit an outer diameter of 160-170 .ANG. and an inner diameter of about 60 .ANG. with an axial spacing of 58 .ANG. per disc. The fiber can be described as a helix consisting of 56 discs per helical turn. Discs of 6 Hb molecules, which may be stable substructural components of the fibers, were observed in preparations of Hb fibers and exhibited 6-fold symmetry by power spectrum analysis. A reconstructed image of a disc digitally filtered for 6-fold symmetry had a maximum external diameter of .apprx. 170 .ANG. and a central hole of 60 .ANG. diameter and was similar to the axial protection of a single disc from a low-resolution, 3 dimensional reconstructed model of a fiber.