Micelle Formation in Aqueous Solutions of the Cholesterol-Based Detergent Chobimalt Studied by Small-Angle Scattering

The structure and interaction parameters of the water-soluble cholesterol-based surfactant, Chobimalt, are investigated by small-angle neutron and X-ray scattering techniques. The obtained data are analyzed by a model-independent approach applying the inverse Fourier transformation procedure as well as considering a model fitting procedure, using a core-shell form factor and hard-sphere structure factor. The analysis reveals the formation of the polydisperse spherical or moderately elongated ellipsoidal shapes of the Chobimalt micelles with the hard sphere interaction in the studied concentration range 0.17-6.88 mM. The aggregation numbers are estimated from the micelle geometry observed by small-angle scattering and are found to be in the range of 200-300. The low pH of the solution does not have a noticeable effect on the structure of the Chobimalt micelles. The critical micelle concentrations of the synthetic surfactant Chobimalt in water and in H2O-HCl solutions were obtained according to fluorescence measurements as ~3 μM and ~2.5 μM, respectively. In-depth knowledge of the basic structural properties of the detergent micelles is necessary for further applications in bioscience and biotechnology

Low Density Lipoproteins as Circulating Fast Temperature Sensors

Background: The potential physiological significance of the nanophase transition of neutral lipids in the core of low density lipoprotein (LDL) particles is dependent on whether the rate is fast enough to integrate small (62uC) temperature changes in the blood circulation. Methodology/Principal Findings: Using sub-second, time-resolved small-angle X-ray scattering technology with synchrotron radiation, we have monitored the dynamics of structural changes within LDL, which were triggered by temperature-jumps and-drops, respectively. Our findings reveal that the melting transition is complete within less than 10 milliseconds. The freezing transition proceeds slowly with a half-time of approximately two seconds. Thus, the time period over which LDL particles reside in cooler regions of the body readily facilitates structural reorientation of the apolar core lipids. Conclusions/Significance: Low density lipoproteins, the biological nanoparticles responsible for the transport of cholesterol in blood, are shown to act as intrinsic nano-thermometers, which can follow the periodic temperature changes during blood circulation. Our results demonstrate that the lipid core in LDL changes from a liquid crystalline to an oily state within fractions of seconds. This may, through the coupling to the protein structure of LDL, have important repercussions o

Fast time-resolved X-ray diffraction for studying laser T-jump-induced phase transitions

Phase transitions of phospholipid/water systems, triggered by a short 1–2 ms infrared laser heat pulse (Er-laser, wavelength: 1.5 μm, energy 1–2 J), were studied by recording their real time X-ray powder diffraction patterns at a time resolution down to 0.5 ms using synchrotron radiation. Theoretical calculations of the temperature (T) profile show that the thermal gradient within the heated region of the aqueous sample is less than 2°C and that the influence of temporal heat diffusion can be neglected on the time scale of these experiments, i.e., ⩽ 5 s. With this technique, combining the fast heating source with a fast X-ray detection system, it is possible to study the molecular spatial rearrangements and to evaluate the kinetics of thermotropic phase transitions of phospholipids in the millisecond time rang

In situ small angle X-ray scattering reveals solution phase discharge of Li-O2 batteries with weakly solvating electrolytes

Electrodepositing insulating and insoluble Li2O2 is the key process during discharge of aprotic Li-O2 batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved LiO2 governs whether Li2O2 grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, governing factors for Li2O2 packing density and capacity need better understanding, requiring in situ metrologies with structural sensitivity from the atomic to sub-micron scale. Here, we establish in situ small and wide angle X-ray scattering as a suitable method to record the Li2O2 phase evolution with atomic to sub-micrometer resolution during cycling a custom-built in situ Li-O2 cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multi-phase systems. Surprisingly, we find that features are absent that would point at a Li2O2 surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin Li2O2 platelets eventually forming large toroidal particles. Higher discharge overpotentials (high currents) lead to smaller Li2O2 particles, but there is no transition to an electronically passivating, conformal Li2O2 coating. This implies that mass transport of reactive species rather than electronic transport through a Li2O2 film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in poorly solvating electrolytes. The currently accepted Li-O2 reaction mechanism ought to be reconsidered.<br /

Millisecond time-resolved x-ray diffraction on liquid-crystalline phase transitions using infrared laser T-jump technique and synchrotron radiation

An erbium laser (λ=1.5 μm) with a pulse energy of 2 J delivered in 2 ms was used for rapid heating to investigate the kinetics and structural mechanism of the thermotropic phase transitions of a phospholipid/water system by fast x‐ray small‐angle powder diffraction. The phase transitions were induced by a single laser pulse, leading to a T jump of about 10 °C and the structural relaxations following the T jump were detected in 1‐ms resolution. The results show that the transition at 29 °C (Lβ‐Lα) is at least as fast as the heating time (2 ms) and that the lamellar/hexagonal transition around 57 °C (Lα‐HII) is considerably slower with a half‐time of the order of 100 ms