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Spatial structures of a chymotryptic fragment C2 (residues 1-71) of bacterioopsin from HaEobacterium halobium, solubilized in a mixture of methanolkhloroform (1: 1, by vol.) and 0.1 M 'HCO,N&, or in perdeuterated sodium (2H)dodecyl sulfate (SDS) micelles in the presence of perdeuterated (2,2,2-2H)trifluoroethanol, were determined by two-dimensional and three-dimensional heteronuclear "N-lH NMR techniques. The influence of (2,2,2-'H)trifluoroethanol on the conformational dynamics of C2 in micelles and the effect of the salt (organic mixture) were studied. Under the best conditions, 'H and I5N resonances of 15N-uniformly enriched protein were assigned in both milieus by homonuclear two-dimensional NOE (NOESY) and two-dimensional total-correlated (TOCSY) spectra and heteronuclear three-dimensional NOESY-multiple-quantum-correlation (HMQC) and TOCSY-HMQC spectra. 651 (organic mixture) and 520 (micelles) interproton-distance constraints, derived from volumes of cross-peaks in two-dimensional NOESY and three-dimensional NOESY-HMQC spectra, along with deuterium exchange rates of amide groups measured in both milieus and 51 HN-CaH coupling constants obtained in the case of the organic mixture, were used in the construction of C2 spatial structures. Obtained structures are similar in both milieus and have two right-handed a-helical regions stretching from Pro8 to Met32 and Phe42 to Tyr64 (organic mixture), and from Pro8 to Met32 and Ala39 to Leu62 (micelles). In micelles, the second a helix is terminated by C-cap Gly63, adopting a conformation characteristic of a left-handed helix. Residues Gly65 to Thr67 form the turn of a right-handed helix. In the isotropic medium of the organic mixture, the C-terminal region of residues 65-71 lacks an ordered structure. Torsion angles x' were unequivocally determined for 18 a-helical residues in both milieus. In the isotropic organic mixture and anisotropic micellar system, C2 remains a compact structure with a characteristic size of 3.0-3.5 nm. C2 seems to be present in at least two conformational states, packed and unpacked. Using NMR data, along with the electron cryomicroscopy model of bacteriorhodopsin [Henderson, R., Baldwin, J. M., Ceska, T. A., Zemlin, F., Beckman, E. & Downing, K. H. (1990) J. Mol. Biol. 213, 899-9291, we suggested a model for the conformation of C2 in this putative close-packed state. However, no NOE contact between a helices was found in either milieu.Bacteriorhodopsin (BR) is a transmembrane protein acting as a light-dependent proton pump in the purple membrane of Halobacterium halobium (for a review see Ovchinnikov, 1982). Electron cryomicroscopy (ECM) data suggested a seven-a-helix structural motif for BR (Henderson et al., 1990). Arseniev et al. (1987) and Abdulaeva et al. (1991) used methanolkhloroform (1 : 1, by vol.), 0.1 M LiClO, for the I9F-NMR study of BR. The individual signals in the IT-NMR spectra of "F-Trp-labeled and IgF-Phe-labeled BR species were assigned and several long-range contacts between Correspondence to A. S. Arseniev, Shemyakin ...

Chemical shifts of the 23Na nucleus were measured for sodium tetraphenylborate, perchlorate, iodide,

The primary solvation sphere surrounding the thiocyanate or the selenocyanide anion in protic solvents, such as methanol, N-methylformamide and formamide, forms intimate hydrogen bonds with these anions. These interactions perturb the electron density and vibrational modes of these anions and can therefore be studied by NMR and infrared spectroscopies. In neat solutions, these solvents form hydrogen bonds to the nitrogen end along the molecular axis and nonaxially to the π cloud of the C−N bond, although a substantial proportion of the anions is not hydrogen bonded. In nitromethane solutions, the thiocyanate and selenocyanide anions form weak complexes with methanol and the amide solvents (K < 1 M-1), which is an order of magnitude smaller than that of OCN- with the same solvents, determined in our previous investigation of the cyanate anion. The remarkable differences in the solvation of these singly charged anions can be understood by theoretical calculations of their electrostatic potentials.

Publication costs assisted by Michigan State UniversityChemical shifts of lithium-7 nucleus were measured in 11 nonaqueous solvents against 4.0 M aqueous lithium perchlorate solution. Lithium perchlorate, chloride, bromide, iodide, triiodide, and tetraphenylborate were used. The shifts ranged from +2.80 ppm for acetonitrile, down to -2.54 ppm for pyridine. In dimethyl sulfoxide and dimethylformamide no evidence for contact ion pairing was observed. Formation of contact ion pairs was particularly evident in tetrahydrofuran, nitromethane, and tetramethylguanidine. The large broadening of the 35C1 resonance of the perchlorate ion in these solvents is in agreement with the above explanation. In contrast to sodium-23 nmr, no correlation was found between limiting chemical shifts in different solvents and the Gutmann donor numbers of these solvents.

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