Evidence for pinning by (Sr,Ca)2CuOy particles in partial-melting processed bulk Bi2Sr2CaCu2O8+δ ceramics

Flux distributions of partial-melting processed Bi₂Sr₂CaCu₂O_8+δ ceramics are obtained using magneto-optic imaging. In remanent states (μ₀H_a=0 T), large amounts of trapped flux are observed along (Sr,Ca)₂CuO_y particles embedded in the Bi₂Sr₂CaCu₂O_8+δ matrix. Despite the relatively large size of these particles (up to 30 μm), the pinning effect is similar to that of Y₂BaCuO₅ particles in melt-processed YBa₂Cu₃O_7−δ. Furthermore, we discuss how the pinning capability of non-superconducting particles of different sizes and densities will show up in magneto-optic images.     

Crystal structure of a 1,4-dihydropyridine with enantiomers showing opposite effects on calcium channels: structural features of calcium channel agonists and antagonists

The structure of the calcium channel modulator isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-nitro- 3-pyridinecarboxylate has been determined by X-ray analysis. Structural and stereochemical features are discussed in relation to previously determined structures of calcium agonists and antagonists of the 1,4-dihydropyridine type, and in relation to a newly proposed model for the dihydropyridine binding site.     

pH-sensitive paramagnetic liposomes for MRI: assessment of stability in blood

The pH-dependent stability of dipalmitoyl phosphatidyl ethanolamine/palmitic acid (DPPE/PA) liposomal GdDTPA-BMA was investigated in human blood and after exposure to selected blood components. Relaxometry, visual observations and cryo-transmission electron microscopy (cryo-TEM) were employed for the assessment of stability. The liposomes were stable in buffer at physiological pH and the T(1)-relaxivity (r(1)) of the system was significantly lowered compared to that of non-liposomal GdDTPA-BMA, which could be explained by an exchange limited relaxation process. Lowering the pH, however, gave a marked increase in r(1), due to liposome aggregation and subsequent leakage of GdDTPA-BMA. After a few minutes incubation in human blood the liposomes were destabilised and leaky at both high and low pH, and blood components likely to cause the instability were studied. Physiological level of Na(+) (150 mM) did not affect the relaxometric behavior of the liposomes at pH 7.4, but shifted the pH-r(1) profile laterally to higher pH-values compared to a level of 50 mM Na(+). Increased screening of the surface charges and, concomitantly, a lowering of the energy-barrier against aggregation is a plausible explanation for this phenomenon. In contrast, both Ca(2+) and Mg(2+) (physiological level, both 2 mM) caused massive aggregation of the liposomes and leakage of contents, and were therefore much more detrimental to the stability of the liposomes than a physiological level of Na(+). This could be due to the higher screening ability of divalent cations, but aggregation could also be induced through an inter-liposomal "bridging" effect. Physiological level of both Na(+) and Ca(2+) caused less leakage than for lower Na(+) concentration (50 mM Na(+) and 2 mM Ca(2+)), probably due to competition for the negative surface charges. Albumin also destabilised the liposomes, and it was shown to be due to an interaction between albumin and PA in the liposomal membrane.     

RF field dependence of memory echoes in LiNbO3 and GaAs powder

Measurements of the rf electric field dependence of the amplitude of the “memory” echo in LiNbO₃ and GaAs powders are reported. Consistent with the torque-rotation model, the dependence on each of the three applied rf pulses is linear in the low power regime. At high power, more complex behavior is found including an irreversible decrease in the echo amplitude when the third applied pulse is larger than the preceeding two pulses.     

Ultrasonic investigation of the phase transition in flux-grown SrTiO3

Ultrasonic attenuation and velocity have been measured close to and above the 105 K transition in flux-grown SrTiO₃. The data are compared with data from Verneuil-grown SrTiO₃. Larger exponents than expected are observed. The difference between relaxation times in the two samples is interpreted as being due to different impurity contents.     

Compressibility study of quaternary phospholipid blend monolayers

The mechanical properties of liposome membranes are strongly dependent on type and ratio of lipid compounds, which can have important role in drug targeting and release processes when liposome is used as drug carrier. In this work we have used Brewster's angle microscopy to monitor the lateral compression process of lipid monolayers containing as helper lipids either distearoyl phosphatidylethanolamine (DSPE) or dioleoyl phophatidylethanolamine (DOPE) molecules on the Langmuir trough. The compressibility coefficient was determined for lipid blend monolayers containing the helper lipids above, cholesterol, distearoyl phosphatidylcholine (DSPC) and pegylated-DSPE at room temperature. Two variables, the cholesterol fraction and the ratio ρ between the helper lipid (either DSPE or DOPE) and the reference lipid DSPC, were studied by multivariate analysis to evaluate their impact on the compressibility coefficient of the monolayers. The cholesterol level was found to be the most significant variable for DSPE blends while the ratio ρ was the most significant one for DOPE blend monolayers. It was also found that these two variables can exhibit positive interaction and the same compressibility value can be obtained with different blend compositions.     

Experimental studies of the influence of defects and impurities on structural phase transitions

A review of experiments pertaining to the influence of defects and impurities on structural phase transitions is given. The systems covered are the KH₂P0₄-family, the TGS-family, the SrTiO₃-family and, finally, the K_1-x(Li, Na) _x TaO₃-system. Emphasis is placed on the influences of impurities and defects on static and dynamic critical and precritical effects, as well as their ability to induce glass-like precursor effects.     

Imaging cytoplasmic cAMP in mouse brainstem neurons

Background  

cAMP is an ubiquitous second messenger mediating various neuronal functions, often as a consequence of increased intracellular Ca²⁺ levels. While imaging of calcium is commonly used in neuroscience applications, probing for cAMP levels has not yet been performed in living vertebrate neuronal tissue before.     

Paramagnetic Liposomes as Thermosensitive Probes for MRI-Guided Thermal Treatment: In Vitro Feasibility Studies

In this work the potential of thermosensitive paramagnetic liposomes for in vitro temperature monitoring during radiofrequency heating has been assessed. Two thermosensitive liposome formulations with different phase-transition properties were investigated. Temperature-dependent spin–lattice (T ₁) relaxivity measurements were performed at 0.24 T. Magnetic resonance imaging was performed at 2 T in liposome-containing phantom models and T ₁ relaxation rates (R ₁) were quantified as a function of temperature. Independent temperature measurements were performed using both thermocouple and magnetic-resonance-based methods (proton resonance frequency and diffusion-based thermometry). The relaxometric measurements showed that the T ₁ relaxivity increased from low values (about 0.3 s⁻¹mM⁻¹ at 35 °C) to about 4 s⁻¹mM⁻¹ when the temperature approached and exceeded the phase-transition temperature (T _c) of the liposome preparations. These data correlated well to the imaging data where an increased signal intensity was observed on T ₁-weighted images at temperatures above T _c. The derived R ₁ maps reflected the measured liposomal temperature sensitivity and temperature quantification was possible on the basis of the measured linear temperature versus R ₁ correlation in the transition range of the liposomes. The studies have therefore shown that thermosensitive paramagnetic liposomes exhibit the required temperature sensitivity to allow for an accurate mapping of the temperature changes in an in vitro imaging model. Authors' address: Kamil A. Il'yasov, Physics Department, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation