Size-Induced Depression of First-Order Transition Lines and Entropy Jump in Extremely Layered Nanocrystalline Vortex Matter.

We detect the persistence of the solidification and order-disorder first-order transition lines in the phase diagram of nanocrystalline Bi_{2}Sr_{2}CaCu_{2}O_{8} vortex matter down to a system size of less than one hundred vortices. The temperature location of the vortex solidification transition line is not altered by decreasing the sample size although there is a depletion of the entropy jump at the transition with respect to macroscopic vortex matter. The solid order-disorder phase transition field moves upward on decreasing the system size due to the increase of the surface-to-volume ratio of vortices entailing a decrease on the average vortex binding energy.

First-order transition in the magnetic vortex matter in superconducting MgB2 tuned by disorder.

The field-driven transition from an ordered Bragg glass to a disordered vortex phase in single-crystalline MgB2 is tuned by an increasing density of point defects, introduced by electron irradiation. The discontinuity observed in magnetization attests to the first-order nature of the transition. The temperature and defect density dependences of the transition field point to vortex pinning mediated by fluctuations in the quasiparticle mean free path, and reveal the mechanism of the transition in the absence of complicating factors such as layeredness or thermal fluctuations.

Pump-probe experiments on the single-molecule magnet Fe8: measurement of excited level lifetimes.

We present magnetization measurements on the single-molecule magnet Fe8 in the presence of pulsed microwave radiation. A pump-probe technique is used with two microwave pulses with frequencies of 107 and 118 GHz and pulse lengths of several nanoseconds to study the spin dynamics via time-resolved magnetization measurements using a Hall probe magnetometer. We find evidence for short spin-phonon relaxation times of the order of 1 micros. The temperature dependence of the spin-phonon relaxation time in our experiments is in good agreement with previously published theoretical results. We also established the presence of very short energy diffusion times that act on a time scale of about 70 ns.

Composite to tilted vortex lattice transition in Bi2Sr2CaCu2O8+delta in oblique fields.

Precision measurements of the vortex phase diagram in single crystals of the layered superconductor Bi2Sr2CaCu2O8+delta in oblique magnetic fields confirm the existence of a second phase transition, in addition to the usual first-order vortex-lattice melting line Hm(T). The transition has a strong first-order character, is accompanied by strong hysteresis, and intersects the melting line in a tricritical point (Hm perpendicular, Hcr parallel). Its field dependence and the changing character of the melting line at the tricritical point strongly suggest that the ground state for magnetic fields closely aligned with the superconducting layers is a lattice of uniformly tilted vortex lines.

PTEN does not modulate GLUT4 translocation in rat adipose cells under physiological conditions.

PTEN is a 3'-inositol lipid phosphatase that dephosphorylates products of PI 3-kinase. Since PI 3-kinase is required for many metabolic actions of insulin, we investigated the role of PTEN in insulin-stimulated translocation of GLUT4. In control rat adipose cells, we observed a approximately 2-fold increase in cell surface GLUT4 upon maximal insulin stimulation. Overexpression of wild-type PTEN abolished this response to insulin. Translocation of GLUT4 in cells overexpressing PTEN mutants without lipid phosphatase activity was similar to that observed in control cells. Overexpression of PTEN-CBR3 (mutant with disrupted membrane association domain) partially impaired translocation of GLUT4. In Cos-7 cells, overexpression of wild-type PTEN had no effect on ERK2 phosphorylation in response to acute insulin stimulation. However, Elk-1 phosphorylation in response to chronic insulin treatment was significantly decreased. Thus, when PTEN is overexpressed, both its lipid phosphatase activity and subcellular localization play a role in antagonizing metabolic actions of insulin that are dependent on PI 3-kinase but independent of MAP kinase. However, because translocation of GLUT4 in cells overexpressing a dominant inhibitory PTEN mutant (C124S) was similar to that of control cells, we conclude that endogenous PTEN may not modulate metabolic functions of insulin under normal physiological conditions.

Double mutant cycle analysis of aspartate 69, 97, and 103 to asparagine mutants in the m2 muscarinic acetylcholine receptor.

Double mutant cycles provide a method for analyzing the effects of a mutation at a defined position in the protein structure on the properties of an amino acid at a second site. This approach was used to map potential interactions between aspartates 69, 97, and 103 in the m2 muscarinic acetylcholine receptor transmembrane helices 2 and 3. Receptors containing single and double aspartate to asparagine mutants were expressed in Chinese hamster ovary cells and their effects on ligand binding, signal transduction, and thermal stability determined. Analysis of the double mutant cycles showed that the mutations had approximately additive effects on ligand binding, signal transduction, and thermal stability. Ligand binding and thermal inactivation results support the conclusion that aspartate-103 is the ligand amine counterion. Effector coupling properties of the mutant receptors showed that aspartate-103 was also required for signal transduction activity. The mutation of aspartate-69 to asparagine completely eliminated signal transduction by the agonists acetylcholine, carbachol, and pilocarpine but not oxotremorine M, which caused reduced but significant inhibition of adenylyl cyclase and stimulation of phospholipase C. In contrast, adenylyl cyclase stimulation by the asparagine-69 mutant was elicited only by acetylcholine and carbachol but not by oxotremorine M. The variation in agonist-dependent effector coupling properties provides evidence that the asparagine-69 mutant can exist in activated receptor states that are different from the wild-type m2 muscarinic receptor.

Porcine m2 muscarinic acetylcholine receptor-effector coupling in Chinese hamster ovary cells.

The relationship between porcine m2 muscarinic receptor coupling to inhibition of cAMP formation and stimulation of phosphatidylinositol metabolism in Chinese hamster ovary cells was examined. Reduction of the number of receptors per cell with the slowly dissociating antagonist (-)-quinuclidinyl benzilate caused a decrease in maximal response with no effect on EC50 for coupling to phosphatidylinositol metabolism. Inhibition of cAMP formation showed the opposite dependence with no effect on maximal response but an increase in EC50 value as receptor density decreased. Pilocarpine appeared to be a partial agonist at low cell receptor density but displayed full agonism at higher receptor density. These results are compatible with a two-state model describing m2 muscarinic receptor acting via two different G proteins. This model is compatible with observations of negative antagonism where antagonists stimulated cAMP formation in adenylyl cyclase inhibition assays, and can also be used to estimate receptor affinities for G proteins in systems which display negative antagonism.

Kinetic and biophysical analysis of the m2 muscarinic receptor.

The recombinant Pm2 muscarinic receptor expressed in Chinese hamster ovary (CHO) cells was used as a model system to examine receptor-effector coupling and ligand binding. In CHO cells, equilibrium binding studies and the dependence on receptor number per cell of the maximum response and EC50 values for agonist stimulation of phosphatidylinositol metabolism and inhibition of cAMP formation were consistent with a modified ternary complex model of signal transduction that included a physiologically noncompetent receptor state. Detailed kinetic studies of oxotremorine M (Oxo-M) binding to CHO cell membranes suggested that agonist interactions at the high affinity class of binding sites are complicated and depend on receptor expression levels. At low levels of expression, kinetic data were consistent with a special case of a mechanism in which Oxo-M shifts the equilibrium between two receptor conformations while at high levels of expression, it was necessary to evoke receptor-receptor interactions to explain the kinetic data. Far ultraviolet circular dichroism studies of the purified recombinant receptor showed a high content of alpha-helical secondary structure and small changes in secondary structure upon antagonist, but not agonist, binding.

Preliminary crystallographic analysis of a plant pathogenic factor: pectate lyase.

Pectate lyase is a saccharide-binding enzyme that lyitically depolymerizes polypectate in higher plant cell walls, thus causing soft-rot diseases in food crops. A pectate lyase from Erwinia chrysanthemi, EC16 (PLe), crystallizes in the orthorhombic space group P2(1)2(1)2(1) with unit cell dimension of a = 39.0 A, b = 91.0 A and c = 103.4 A. The asymmetric unit consists of one molecule with a molecular mass of 38,118 daltons and the X-ray diffraction extends to a resolution of 1.8 A. The crystals reproducibly grow to large dimensions and are suitable for a high-resolution X-ray diffraction analysis.