Deoxyhemoglobin S crystal. Chromatographically purified HbS (in 5 mmol/L KCl, 10 mmol/L Tris, pH 6.5) was deoxygenated, air-dried onto a graphite chip under a stream of nitrogen, and examined by scanning tunneling electron microscopy (STEM). Images at two levels of resolution are shown for a portion of a single crystalline bundle of HbS fibers that was 1,450 nm long and 65 nm in diameter. The size of the individual constituent subunits (6 × 5 nm) is consistent with their identity as hemoglobin tetramers. The vivid detail at the interface between tetramers (asterisk) suggests that STEM could be used to help define the fine structure of HbS polymers. (Courtesy of Mary M. Christopher, Yi Lin, Roy Matthew, D. Fennell Evans, and Robert P. Hebbel, Departments of Medicine and Chemical Engineering, University of Minnesota, Minneapolis, MN 55455.)