Scanning Transmission Electron Microscopy (STEM) of Annelid Extracellular Hemoglobins
Scanning transmission electron microscopy (STEM) was used to study the structure of the extracellular hemoglobin of the annelid Lumbricus terrestris. These are giant molecules possessing a molecular weight of approximately 3.9 x 106 and appear in electron micrographs as a hexagonal bilayer, The samples for this study were diluted to 100 μg/ml with distilled water and applied to a thin ( ∽15 Å) carbon substrate supported on a microgrid. One percent (w/v) uranyl acetate solution was used for negative staining for 2 minutes and dried in air. STEM images were recorded using both a high resolution CRT with 1024 x 1024 picture elements and an on-line computer recording with 512 x 512 pixels. The images were rotated in on-line fashion with the computer to enhance contrast.The hemoglobin of Lumbricus assumes three common positions on the substrate (Fig. 1). The top view (Fig. la) demonstrates that the 1/12th subunits possess an approximately pentagonal shape with a central region of low protein density.