Sterols sense swelling in lipid bilayers

In the mimetic membrane system of phosphatidylcholine bilayers, thickening (pre-critical behavior, anomalous swelling) of the bilayers is observed, in the vicinity of the main transition, which is non-linear with temperature. The sterols cholesterol and androsten are used as sensors in a time-resolved simultaneous small- and wide angle x-ray diffraction study to investigate the cause of the thickening. We observe precritical behavior in the pure lipid system, as well as with sterol concentrations less than 15%. To describe the precritical behavior we introduce a theory of precritical phenomena.The good temperature resolution of the data shows that a theory of the influence of fluctuations needs modification. The main cause of the critical behavior appears to be a changing hydration of the bilayer.Comment: 11 pages, 7 ps figures included, to appear in Phys.Rev.

Nickel on Lead, Magnetically Dead or Alive?

Two atomic layers of Ni condensed onto Pb films behave, according to anomalous Hall effect measurements, as magnetic dead layers. However, the Ni lowers the superconducting T_{c} of the Pb film. This has lead to the conclusion that the Ni layers are still very weakly magnetic. In the present paper the electron dephasing due to the Ni has been measured by weak localization. The dephasing is smaller by a factor 100 than the pair-breaking. This proves that the T_{c}-reduction in the PbNi films is not due magnetic Ni moments

Psychology and aggression

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68264/2/10.1177_002200275900300301.pd

Simulation study of lateral diffusion in lipid-sterol bilayer mixtures

We employ off-lattice Monte Carlo simulations to study lateral diffusion in lipid-sterol bilayers using a two-dimensional model system which has been designed to simulate the experimental phase diagrams of both lipid-cholesterol and lipid-lanosterol systems. We focus on the effects of varying sterol concentration and temperature on the tracer diffusion coefficient, $D$, which characterizes the lateral motion of single tagged lipids in a bilayer. Generally, we find that increasing the cholesterol concentration suppresses $D$ due to an increased conformational ordering of lipid chains. We argue that this effect competes with an increase in the average free area per lipid, which favours an increase in $D$. At temperatures close to the main transition temperature, the competition between the two effects leads to intriguing behavior of $D$. Overall, the model results are in excellent qualitative agreement with available experimental results for lipid-cholesterol mixtures. Additional studies of a model lipid-lanosterol system, for which experimental diffusion results are not available, predict that the presence of lanosterol has a smaller effect than cholesterol on reducing $D$ relative to the pure lipid system. We conclude that the molecular model employed contains the essential features required to describe many of the qualitative features of the lateral diffusion behavior in lipid-sterol systems

AMORPHOUS AND SPIN GLASSES.Random magnetic anisotropy in amorphous alloys containing rare earth atoms : some recent developments

Le modèle HPZ relatif à l'anisotropie magnétique aléatoire dans les alliages amorphes est analysé en détail. Quelques calculs d'aimantation sont présentés et discutés en utilisant le modèle HPZ dans I'approximation du champ moléculaire et l'approximation des paires corrélées d'Oguchi, pour des interactions d'échange ferro- et antimagnétiques. L'analyse de Callen et al., qui traite des effets d'hystérésis dans les alliages amorphes, sera comparée aux résultats des calculs du type Monte Carlo pour les deux interactions.The HPZ model for random magnetic anisotropy in amorphous alloys containing non S-state rare earth ions is analysed in detail. Magnetisation calculations using this model in conjunction with the molecular field approximation (MFA) and the Oguchi pair approximation (OPA) are presented and discussed for both ferro- and antiferromagnetic exchange coupling. The analysis of Callen et al. for hysteresis effects in amorphous alloys using the HPZ model is related to the results of Monte Carlo calculations for both ferro- and antiferromagnetic coupling