At this year’s annual meeting of the American Crystallographic Association in New Orleans, Maryam Khoshouei of the Max Planck Institute of Biochemistry (MPIB) described how she and her collaborators used a novel technique to obtain a 3.2-Å-resolution structure of human hemoglobin (see accompanying figure). The feat is significant because, at 64 kDa, the protein complex is smaller than the 100 kDa lower limit that had previously prevailed when using cryoelectron microscopy (cryo-EM) to determine the structure of protein complexes.
In EM, contrast arises from differences in phase between scattered and unscattered electrons. (See the article by Bob Glaeser, Physics Today, January 2008, page 48.) To create that difference, microscopists either defocus their beams or insert an ultrathin phase plate with a micron-sized hole at the focus; scattered electrons that pass through the plate acquire a phase shift of π/2 with respect to unscattered electrons that fly...
