Mass mapping of large globin complexes by scanning transmission electron microscopy.

Scanning transmission electron microscopy (STEM) of unstained, freeze-dried biological macromolecules in the dark-field mode provides an image based on the number of electrons elastically scattered by the constituent atoms of the macromolecule. The image of each isolated particle provides information about the projected structure of the latter, and its integrated intensity is directly related to the mass of the selected particle. Particle images can be sorted by shape, providing independent histograms of mass to study assembly/disassembly intermediates. STEM is optimized for low-dose imaging and is suitable for accurate measurement of particle masses over the range from about 30 kDa to 1,000 MDa. This article describes the details of the method developed at the Brookhaven National Laboratory STEM facility and illustrates its application to the mass mapping of large globin complexes.