Cholesterol monohydrate nucleation in ultrathin films on water.

The growth of a cholesterol crystalline phase, three molecular layers thick at the air-water interface, was monitored by grazing incidence x-ray diffraction and x-ray reflectivity. Upon compression, a cholesterol film transforms from a monolayer of trigonal symmetry and low crystallinity to a trilayer, composed of a highly crystalline bilayer in a rectangular lattice and a disordered top cholesterol layer. This system undergoes a phase transition into a crystalline trilayer incorporating ordered water between the hydroxyl groups of the top and middle sterol layers in an arrangement akin to the triclinic 3-D crystal structure of cholesterol x H(2)O. By comparison, the cholesterol derivative stigmasterol transforms, upon compression, directly into a crystalline trilayer in the rectangular lattice. These results may contribute to an understanding of the onset of cholesterol crystallization in pathological lipid deposits.

Crystalline films of interdigitated structures formed via amidinium-carboxylate interactions at the air-water interface.

Electrostatic interactions between amidinium and carboxylates were used for the construction of interdigitated architectures at the air-solution interface. Spreading the water-insoluble amphiphile p-pentadecylbenzoic acid (A) on an aqueous solution of p-methylbenzamidinium (B) ions results in an intercalation of the water-soluble base between the acidic headgroups of the water-insoluble amphiphile to form an amorphousA-B-A-B monolayer according to grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity measurements. Upon compression the monolayer transforms into a crystalline film composed of three bilayers with interdigitated hydrocarbon chains, and a top layer whose chains are disordered. Water-insoluble p-heptadecylbenzamidinium spread on an aqueous solution of benzoic acid displays a surface pressure-area isotherm similar to that obtained from the above system. A mechanism that accounts for the formation of these films is presented. Deposition of p-heptadecylbenzamidinium and p-pentadecylbenzoic acid amphiphiles in a 1:1 ratio on pure water led to the formation of a crystalline monolayer phase but which is partially disordered. Over an aqueous solution containing a 1:1 mixture of benzamidinium and benzoic acid no measurable binding of these solute molecules to the polar headgroups of the 1:1 mixed monolayer could be detected by X-ray reflectivity or GIXD.

Synchrotron X-ray study of lung surfactant-specific protein SP-B in lipid monolayers.

This work reports the first x-ray scattering measurements to determine the effects of SP-B(1-25), the N-terminus peptide of lung surfactant-specific protein SP-B, on the structure of palmitic acid (PA) monolayers. In-plane diffraction shows that the peptide fluidizes a portion of the monolayer but does not affect the packing of the residual ordered phase. This implies that the peptide resides in the disordered phase, and that the ordered phase is essentially pure lipid, in agreement with fluorescence microscopy studies. X-ray reflectivity shows that the peptide is oriented in the lipid monolayer at an angle of approximately 56 degrees relative to the interface normal, with one end protruding past the hydrophilic region into the fluid subphase and the other end embedded in the hydrophobic region of the monolayer. The quantitative insights afforded by this study lead to a better understanding of the lipid/protein interactions found in lung surfactant systems.

Langmuir and Langmuir-Blodgett films of amphiphilic hexa-peri-hexabenzocoronene: new phase transitions and electronic properties controlled by pressure.

We present the synthesis as well as the structural and electronic properties of an amphiphilic derivative of hexaalkylhexa-peri-hexabenzocoronene (HBC), which contains one alkyl substituent that is terminated with a carboxylic acid group. The molecules form well-defined Langmuir films when spread from a solution at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity studies of the Langmuir monolayer reveal two crystallographic phases at room temperature which depend on the surface pressure applied to the film. Scattering from very well-ordered (zeta = 200-400 A) pi-stacked lamellae of HBC molecules tilted approximately 45 degrees relative to the surface normal is observed in the low-pressure phase. In this phase, the HBC molecules pack in a rectangular two-dimensional unit cell with a = 22.95 A and b = 4.94 A. In the high-pressure phase, coherence from the pi stack is lost. This is a consequence of stress induced by the crystallization of the substituent alkyl chains into a hexagonal lattice, which has a trimerized superstructure in one direction: a = 3 x b = 15.78 A, b = 5.26 A, gamma = 120 degrees, A = 71.9 A2 = 3 x 23.9 A2. Thin monolayer films can be transferred to solid supports by the Langmuir-Blodgett (LB) technique. Atomic force microscopy (AFM) with atomic resolution reveals the crystalline packing of alkyl chains in the high-pressure phase. Kelvin force microscopy (KFM) shows a clear potential difference between the high- and low-pressure phases. This is discussed in terms of orbital delocalization (band formation) in the highly coherent low-pressure phase, which is in contrast to the localized molecular orbitals present in the high-pressure phase. The highly coherent pi stack is expected to sustain a very high charge-carrier mobility.

X-ray diffraction from a single layer of purple membrane at the air/water interface.

X-ray diffraction patterns have been recorded from a single layer of purple membrane ( approximately 50 A thickness) at the air/water interface in a Langmuir trough. Grazing-incidence X-ray diffraction is demonstrated to be a promising method for obtaining structural information on membrane proteins under physiological conditions. The method is so sensitive that diffraction can be measured from samples with only 10(13) protein molecules in the beam. Diffraction from hexagonal crystals of purple membrane with a lattice constant of 61. 3 A was observed up to the order {h,k}={4,3}, corresponding to a resolution of approximately 9 A. The work reported here is a first step towards a new way of protein crystallography using grazing-incidence X-ray diffraction at the air/water interface.

Bacterial S-layer protein coupling to lipids: x-ray reflectivity and grazing incidence diffraction studies.

The coupling of bacterial surface (S)-layer proteins to lipid membranes is studied in molecular detail for proteins from Bacillus sphaericus CCM2177 and B. coagulans E38-66 recrystallized at dipalmitoylphosphatidylethanolamine (DPPE) monolayers on aqueous buffer. A comparison of the monolayer structure before and after protein recrystallization shows minimal reorganization of the lipid chains. By contrast, the lipid headgroups show major rearrangements. For the B. sphaericus CCM2177 protein underneath DPPE monolayers, x-ray reflectivity data suggest that amino acid side chains intercalate the lipid headgroups at least to the phosphate moieties, and probably further beyond. The number of electrons in the headgroup region increases by more than four per lipid. Analysis of the changes of the deduced electron density profiles in terms of a molecular interpretation shows that the phosphatidylethanolamine headgroups must reorient toward the surface normal to accommodate such changes. In terms of the protein structure (which is as yet unknown in three dimensions), the electron density profile reveals a thickness lz approximately 90 A of the recrystallized S-layer and shows water-filled cavities near its center. The protein volume fraction reaches maxima of >60% in two horizontal sections of the S-layer, close to the lipid monolayer and close to the free subphase. In between it drops to approximately 20%. Four S-layer protein monomers are located within the unit cell of a square lattice with a spacing of approximately 131 A.