Three‐dimensional structure of proteolytic fragment 163–231 of bacterioopsin determined from nuclear magnetic resonance data in solution

546 NOESY cross‐peak volumes were measured in the two‐dimensional NOESY spectrum of proteolytic fragment 163–231 of bacterioopsin in organic solution. These data and 42 detected hydrogen bonds were applied for determining the peptide spatial structure. The fold of the polypeptide chain was determined by local structure analysis, a distance geometry approach and systematic search for energetically allowed side‐chain rotamers which are consistent with experimental NOESY cross‐peak volumes. The effective rotational correlation time of 6 ns for the molecule was evaluated from optimization of the local structure to meet NOE data and from the dependence on mixing time of the N i H/C i α H cross‐peak volumes of the residues in α‐helical conformation. The resulting structure has two well defined α‐helical regions, 168–191 and 198–227, with root‐mean‐square deviation 44 pm and 69 pm, respectively, between the backbone atoms in 14 final energy refined conformations. The α‐helices correspond to transmembrane segments F and G of bacteriorhodopsin. The segment F contains proline 186, which introduces a kink of about 25° with a disruption of the hydrogen bond with the NH group of the following residue. The segments are connected by a flexible loop region 192–197. Torsion angles X 1 are unequivocally defined for 62% of side chains in the α‐helices but half of them differ from electron cryo‐microscopy (ECM) model of bacteriorhodopsin, apparently because of the low resolution of ECM. Nevertheless, the F and G segments can be packed as in the ECM model and with side‐chain conformations consistent with all NMR data in solution.