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Neutron reflectivity experiments on the interface of pure D2O against thin films of perdeuterated polystyrene (d-PS) spin-coated onto silicon blocks were performed to study the intrinsic structure of the interface of water against hydrophobic substrates. The experiments reveal nonvanishing scattering contrast at the polymer/water interface, although the two materials (d-PS and D2O) have closely similar scattering length densities. Organic (nondeuterated) contaminants or macroscopic air bubbles trapped at the polymer/water interface can be ruled out as the origin of this observation. From a systematic study of this system, it is concluded that the source of the nonvanishing contrast is a depletion of water in the boundary layer against the hydrophobic surface. It is conjectured that this depletion layer represents a precursor layer of submicroscopic gas bubbles recently observed by Tyrrell and Attard. The existence of such gas nanobubbles in the present system is confirmed by atomic force microscopy (AFM) of the surface of d-PS coatings in contact with bulk water. The thickness of the precursor gas layer as determined by neutron reflectometry is 2−5 nm, depending on the level of air saturation of the water sample and on the time elapsed after contacting it with the hydrophobic surface.

Abstract:: ErCl3 crystallizes in the AlCl3-type layer structure. The crystal structure was refined in the paramagnetic state by powder neutron diffraction. The monoclinic lattice parameters at 1.5 K are a = 6.8040(3)Å, b = 11.7456(5)Å, c = 6.3187(3)Å and . The space group is C2/m. Short-range, predominantly in-plane, magnetic ordering occurs above 350 mK up to several Kelvin. Below mK a three-dimensional antiferromagnetic order with a propagation vector of sets in. The magnetic structure of ErCl3 was determined by powder and single-crystal neutron diffraction at temperatures down to 45 mK. The Er3+ ions are located on two-dimensional honeycomb layers in the a–b plane. There are two antiferromagnetically coupled triangular sublattices which form right- and left-handed helices along the c-axis. The magnetic moments are oriented in the a–b plane and amount to 3.3(1) at saturation. From the temperature dependence of the integrated neutron magnetic peak intensity a critical exponent (2) was derived for the magnetic phase transition.

The lamellar spacing dlof purple membrane (PM) multilayer systems was investigated with neutron diffraction as a function of temperature and of the level of hydration. The observed large T-dependent variations of dlindicate that PM is partially dehydrated when cooled below a “hydration water freezing point”. This phenomenon is reversible, but a hysteresis is observed when PM is rehydrated upon reheating. The hydration water remaining bound to the membrane below about 240 K is non-freezing. Its amount was found to be hnf= 0.24(±0.02) g 2H2O/g BR for all samples equilibrated at room temperature in the presence of 2H2O vapour at ⩾84% r.h. It is evident, that the dehydration/rehydration behaviour of PM is strongly correlated with the temperature-dependent behaviour of the dynamical structure factor. Above the well-known “dynamical transition” announcing the onset of localized diffusive molecular motions between 190 K and 230 K, a second dynamical transition is caused by the temperature-induced rehydration of the PM starting near 255 K. This is also correlated with the deviation from a pure Arrhenius law of the rate-limiting process in the photocycle, known to occur upon cooling beyond the ice point into the same temperature region. Our results suggest that the phenomenon of dehydration and rehydration induced by cooling and reheating, respectively, is a general property of biological membranes.

Neutron diffraction from oriented purple membrane fragments at various hydration levels, coupled with H2O/2H2O exchange, was used to compare the structure and hydration of the light-adapted initial state (B-state) and the M photointermediate of bacteriorhodopsin mutant D96N. Diffraction patterns were recorded at 86%, 75% and 57% relative humidity (r.h.).

Bacteriorhodopsin (BR) was regenerated with two selectively deuterated retinals, one with 11 deuterons in the beta-ionone ring (D11) and the other with 5 deuterons (D5) at the end of the polyene chain closest to the Schiff base at carbon atoms C-14, C-15, and C-20. Both label positions (centers of deuteration) were obtained from difference Fourier maps of projections onto the plane of the membrane by neutron diffraction at 90 K, both in the light-adapted ground-state BR568 and in the photocycle intermediate M412. To retard the decay of M412, purple membrane films were soaked in 0.1 M or 1 M guanidine hydrochloride at pH 9.6. M412 was produced by illuminating oriented membrane films at physiological temperature (278 K), followed by rapid cooling to 90 K in the absence of light. The results show that in the projected structure the ring position is unaltered during the transition from BR568 to M412, whereas the position of the D5 label shifts by 1.4 +/- 0.9 A toward the ring. The shortened interlabel distance in the projected structure for the M412 state implies that as a result of the all-trans/13-cis isomerization, the C-5 to C-13 part of the polyene chain tilts out of the plane of the membrane toward the cytoplasm by about 11 degrees +/- 6 degrees. Pairwise comparison of data sets with the same retinal for the two photocycle states M412 and BR568 leads to four difference-density maps for the protein, which are in agreement with previous work. They show changes in the protein density near helices G and F.

Bacteriorhodopsin (bR) was expressed in Halobacterium halobium by using a multicopy plasmid containing the bop gene. The plasmid contains pGRB1, a 1.8-kilobase-pair plasmid; a 70-base-pair fragment from ISH11, a recently characterized insertion sequence; and a 1.6-kilobase-pair fragment carrying the bop gene from H. halobium S9. When transformed with this plasmid, a bop- insertion mutant of H. halobium yielded purple (Pum+) colonies. The insertion at the chromosomal bop locus remained intact in transformed cells, indicating that the plasmid bop gene was responsible for the Pum+ phenotype. bR was induced in early stationary phase in both wild-type and transformed cells. The final level of bR in transformed cells was 25-40% of that in wild type. The lower level of expression was presumably due to plasmid instability. Purple membrane purified from transformed strains had absorption and visible CD properties similar to wild type and contained bR in a hexagonal lattice with the same unit-cell dimension as wild type. The structure of bR from wild-type and transformed strains was identical at a resolution of 7.2 A. When reconstituted into vesicles, the purple membrane from wild-type and transformed strains showed similar light-dependent proton-pumping activity.