Xenon and krypton binding to Ngb. (A) Stereo view of Ngb, showing the location of the four Xe atoms and the two Kr atoms represented as spheres. α-Helices are labeled according to the globin fold nomenclature. The shape and position of the main tunnel, connected with the bulk, and its distal branch were calculated using CAVER (46). (B) The XeIII and XeIV atoms in the main cavity, limited by the heme and neighboring side chains, are shown. The electron density for XeIII and XeIV atoms is depicted (2Fo-Fc map, contoured at 1.8σ).