The electron microscope band pattern of human fibrin: various stains, lateral order, and carbohydrate localization

Human fibrin negatively contrasted with a variety of heavy metal compounds and examined by electron microscopy displays a distinctive, nonpolar band pattern with a repeat of 22.5 nm. These results together with a reversal of contrast observed in images of positively stained fibrin, indicate that the striations reflect the protein density along the fiber. All major features of the band pattern can be accounted for directly in terms of a model for the structure of fibrinogen. Optical and computed diffraction patterns of micrographs of fibrin show that most specimens are highly ordered along the fiber axis but have only diffuse equatorial reflections arising from the average spacing of the protofibrils, although occasional fibers have discrete reflections at about 19 nm. Finally, the resulting change in negative staining pattern upon binding of lectins to the carbohydrate moieties is distinctive and allows the carbohydrate-containing beta domain of the molecule to be localized.