[en] The authors describes the use of x-ray diffraction for lipid mesomorphic phase identification and phase transition temperature determination. Some sample diffraction patterns, characteristic of the more commonly encountered lipid phases, such as the lamellar, ripple, hexagonal, cubic, and fluid isotropic phases, are included for reference purposes

[en] A study of the dynamics and mechanism of the various thermotropic phase transitions undergone by the hydrated monoacylglycerides monoolein and monoelaidin, in the temperature range of 20-120⁰C and from 0 to 5 M NaCl, has been undertaken. Measurements were made by using time-resolved X-ray diffraction at the Cornell High-Energy Synchrotron Source. The lamellar chain order/disorder, lamellar/cubic (body centered, space group No.8), cubic (body centered, No.8)/cubic (primitive No.4), cubic (body centered, No.12)/cubic (primitive, No.4), cubic (primitive, No.4)/fluid isotropic, cubic (body centered, No.12)/inverted hexagonal, cubic (primitive, No.4)/inverted hexagonal, and hexagonal/fluid isotropic transitions were examined under active heating and passive cooling by using a jump in temperature to effect phase transformation. All of the transitions with the exception of the cubic (body centered, No.8)/cubic (primitive, No.4) and the cubic (body centered, No.12)/cubic (primitive, No.4) cooling transitions were found (1) to be repeatable, (2) to be reversible, and (3) to have an upper bound on the transit time (time required to complete the transition) of ≤ 3s. In addition to the time-resolved measurements, data were obtained on the stability of the various phases in the temperature range of 20-120⁰C and from 0 to 5 M NaCl. In the case of fully hydrated monoolein, high salt strongly favors the hexagonal over the cubic (body centered, No.8) phase and slightly elevates the hexagonal/fluid isotropic transition temperature. With fully hydrated monoelaidin, the hexagonal phase which is not observed in the absence of salt becomes the dominant phase at high salt concentration

[en] A method is described for observing and recording in real-time x-ray diffraction from an unoriented hydrated membrane lipid, dipalmitoylphosphatidylcholine (DPPC), through its thermotropic gel/liquid crystal phase transition. Synchrotron radiation from the Cornell High Energy Synchrotron Source (Ithaca, New York) was used as an x-ray source of extremely high brilliance and the dynamic display of the diffraction image was effected using a three-stage image intensifier tube coupled to an external fluorescent screen. The image on the output phosphor was sufficiently intense to be recorded cinematographically and to be displayed on a television monitor using a vidicon camera at 30 frames x s^-1. These measurements set an upper limit of 2 s on the DPPC gel → liquid crystal phase transition and indicate that the transition is a two-state process. The real-time method couples the power of x-ray diffraction as a structural probe with the ability to follow kinetics of structural changes. The method does not require an exogenous probe, is relatively nonperturbing, and can be used with membranes in a variety of physical states and with unstable samples. The method has the additional advantage over its static measurement counterpart in that it is more likely to detect transiently stable intermediates if present

[en] A method that enables temperature-composition phase diagram construction at unprecedented rates is described and evaluated. The method involves establishing a known temperature gradient along the length of a metal rod. Samples of different compositions contained in long, thin-walled capillaries are positioned lengthwise on the rod and equilibrated such that the temperature gradient is communicated into the sample. The sample is then moved through a focused, monochromatic synchrotron-derived x-ray beam and the image-intensified diffraction pattern from the sample is recorded on videotape continuously in live-time as a function of position and, thus, temperature. The temperature at which the diffraction pattern changes corresponds to a phase boundary, and the phase(s) existing (coexisting) on either side of the boundary can be identified on the basis of the diffraction pattern. Repeating the measurement on samples covering the entire composition range completes the phase diagram. These additional samples can be conveniently placed at different locations around the perimeter of the cylindrical rod and rotated into position for diffraction measurement. Temperature-composition phase diagrams for the fully hydrated binary mixtures, dimyristoylphosphatidylcholine (DMPC)/dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE)/DPPC, have been constructed using the new temperature gradient method. They agree well with and extend the results obtained by other techniques. In the DPPE/DPPC system structural parameters as a function of temperature in the various phases including the subgel phase are reported. The potential limitations of this steady-state method are discussed

[en] A prototype system implemented in LabVIEW for the intelligent monitoring of the movement of radioactive' material within a nuclear facility is presented. The system collects and analyzes radiation sensor and video data to identify suspicious movement of material within the facility. The facility system also transmits wavelet- compressed data to a remote system for concurrent monitoring. 2 refs., 2 figs

[en] Short communication

[en] Radiation damage of hydrated lecithin membranes brought about by exposure to wiggler-derived synchrotron radiation at 8.3 keV (1.5 A) is reported. Considerable damage was observed with exposures under 1 h at an incident flux density of 3 x 10¹⁰ photons s^-1mm^-2, corresponding to a cumulative radiation dose of <= 10 MRad. Damage was so dramatic as to be initially observed while making real-time X-ray diffraction measurements on the sample. The damaging effects of 8.3 keV X-rays on dispersions of dipalmitoyllecithin and lecithin derived from hen egg yolk are as follows: (1) marked changes were noted in the X-ray diffraction behaviour, indicating disruption of membrane stacking. (2) Chemical breakdown of lecithin was observed. (3) The X-ray beam visibly damaged the sample and changed the appearance of the lipid dispersion, when viewed under the light microscope. Considering the importance of X-ray diffraction as a structural probe and the anticipated use of synchrotron radiation in studies involving membranes, the problem of radiation damage must be duly recognized. Furthermore, since dipalmitoyllecithin, the major lipid used in the present study, is a relatively stable compound, it is not unreasonable to expect that X-ray damage may be a problem with other less stable biological and non-biological materials. These results serve to emphasize that whenever a high intensity X-ray source is used, radiation damage can be a problem and that the sensitivity of the sample must always be evaluated under the conditions of measurement. (orig.)

[en] Short communication

[en] Phenomena associated with x-rays glancing off a flat surface or mirror have become the focus of attention in many research fields and have been applied to surface and thin film related structure analysis. Recently, the authors have shown that x-ray standing waves generated during total external reflection at a gold mirror surface are well defined at up to 1,000 A above the surface [Wang et. al., Nature 354: 277, 1991]. The theoretical calculations described in the latter study indicated the existence of a potentially useful resonance effect which markedly enhances the electric field (E-field) of the penetrated x-rays in the organic thin film at low angles of incidence under certain interference conditions. In excellent agreement with theory, the authors have demonstrated experimentally that the primary resonant x-ray E-field confined in the organic thin film is 20 times more intense than that of the incident beam when measured at a position close to the center of the film [Wang et al., Science In press]. The sizable E-field intensity enhancement demonstrated here means that the significance of this new resonance effect goes beyond the realm of x-ray physics and x-ray devices and extends into the domain of thin film characterization

[en] Time resolved X-ray diffraction can provide information about temporal aspects of structural changes as they occur in a sample. The advent of powerful synchrotron radiation sources and the development of various fast detection systems make it possible to explore this developing area. The dynamics of wide and small angle X-ray diffraction have been recorded in real-time with a two-dimensional electro-optical detector. Performance characteristics of the detection system [3-stage image intensifier with ZnS (Ag) fluorescent screen] and read-out devices (video camera, cassette recorder/player, and monitor) are presented. Quantitative intensity information was obtained by interfacing the video recorder with a Grinnell digital image processing system. We have demonstrated the applicability of this real-time diffraction method for investigating the dynamics and mechanism of structural changes induced in a variety of materials. Samples were selected to (1) cover a range of material types representative of the various applied and basic sciences, (2) provide examples of materials that scatter well enough to investigate their wide or small angle diffraction patterns in live time, and (3) illustrate the type of perturbations that can be applied and the range of processes that can be examined. (orig.)