Spatial structures of proteolytic segment A (sA) of bacterioopsin of H. halobium (residues 1-36) solubilized in a mixture of methanol-chloroform (1:1), 0.1 M LiClO4 organic mixture, or in perdeuterated sodium dodecyl sulfate (SDS) micelles, were determined by 2D 1H-NMR techniques. 324 and 400 NOESY cross-peak volumes were measured in NOESY spectra of sA in organic mixture and SDS micelles, respectively. The sA spatial structures were determined by local structure analysis, distance geometry calculation with program DIANA and systematic search for energetically allowed side chain rotamers consistent with NOESY cross-peak volumes. The structures of sA are similar in both milieus and have the right-handed alpha-helical region from Pro8 to Met32 with root mean square deviation (RMSD) of 0.25 A between backbone heavy atoms and fit well with Pro8 to Met32 alpha-helical region in electron cryo-microscopy model of bacteriorhodopsin. The N-terminal region Ala2-Gly6 of sA in organic mixture has a fixed structure of two consecutive gamma-turns as 2 * 2(7)-helix (RMSD of 0.25 A) stabilized by the Thr5 NH...O = C Gln3 and Ile4 NH...O = C Ala2 hydrogen bonds while this region in SDS micelles has disordered structure with RMSD of 1.44 A for backbone heavy atoms. The C-terminal region Gly33-Asp36 of sA is disordered in both milieus. Torsion angles chi 1 of sA were unequivocally determined for 13 (SDS) and 11 (organic mixture) of alpha-helical residues and are identical in both milieus.