Structures of M2-TM. Left to right: ssNMR, solid state NMR structure (3). NMR, solution NMR structure (19). x-ray (D4), D4 model derived by replication of chain D of crystallographic structure (20). x-ray (A4), A4 model derived by replication of chain A of crystallographic structure (20). The HOLE profiles (40) (color coding scale in Å on the right), both N-terminal and C-terminal views are shown. Relevant pore lining residues are shown as VDW spheres: Val-27 (ice blue), His-37 (orange), and Trp-41 (green).