Electron microscope study of the kinetics of the fiber-to-crystal transition of sickle cell hemoglobin.

Abstract

The intermediates and the rate-limiting step in crystallization of deoxygenated [human] sickle Hb were determined by a kinetic study with the use of EM. In slowly stirred solutions of deoxygenated Hb S, the sequential appearance of fibers having a diameter of .apprxeq. 210 .ANG., bundles of aligned fibers in well-ordered arrays, thick fibers of .apprxeq. 470 .ANG. diameter and microcrystals was observed. Only fibers having a diameter of .apprxeq. 210 .ANG. and bundles of aligned fibers were assigned as kinetically important intermediates of the fiber-to-crystal transition. Addition of microscopic seed crystals obtained from slowly stirred solutions of deoxyhemoglobin S to a solution composed of only fibers and Hb monomers resulted in more rapid crystallization than in control solutions. Addition of seed crystals after the formation of bundles of aligned fibers does not alter the overall kinetics of crystallization. Alignment of fibers is the rate-limiting step in the crystallization process and results in formation of nucleation sites for crystal growth.