High-pressure polymorphism of BaFe2Se3.

BaFe2Se3 is a potential superconductor material exhibiting transition at 11 K and ambient pressure. Here we extended the structural and performed electrical resistivity measurements on this compound up to 51 GPa and 20 GPa, respectively, in order to distinguish if the superconductivity in this sample is intrinsic to the BaFe2Se3 phase or if it is originating from minor FeSe impurities that show a similar superconductive transition temperature. The electrical resistance measurements as a function of pressure show that at 5 GPa the superconducting transition is observed at around 10 K, similar to the one previously observed for this sample at ambient pressure. This indicates that the superconductivity in this sample is most likely intrinsic to the BaFe2Se3 phase and not to FeSe with T c > 20 K at these pressures. Further increase in pressure suppressed the superconductive signal and the sample remained in an insulating state up to the maximum achieved pressure of 20 GPa. Single-crystal and powder x-ray diffraction measurements revealed two structural transformations in BaFe2Se3: a second order transition above 3.5 GPa from Pnma (CsAg2I3-type structure) to Cmcm (CsCu2Cl3-type structure) and a first order transformation at 16.6 GPa. Here, γ-BaFe2Se3 transforms into δ-BaFe2Se3 (Cmcm, CsCu2Cl3-type average structure) via a first order phase transition mechanism. This transition is characterized by a significant shortening of the b lattice parameter of γ-BaFe2Se3 (17%) and accompanied by an anisotropic expansion in the orthogonal ac plane at the transition point.

Tomonaga-Luttinger liquid and Coulomb blockade in multiwall carbon nanotubes under pressure.

We report that the conductance of macroscopic multiwall nanotube (MWNT) bundles under pressure shows power laws in temperature and voltage, as corresponding to a network of bulk-bulk connected Tomonaga-Luttinger liquids (LLs). Contrary to individual MWNTs, where the observed power laws are attributed to Coulomb blockade, the measured ratio for the end and bulk obtained exponents, approximately 2.4, can be accounted for only by LL theory. At temperatures characteristic of interband separation, it increases due to thermal population of the conducting sheets unoccupied bands.

Pressure control of conducting channels in single-wall carbon nanotube networks.

We measure electrical transport on networks of single-wall nanotube ropes as a function of temperature T, voltage V, and pressure up to 22 GPa. We observe Luttinger liquid (LL) behavior, a conductance proportional to T(alpha), and a dynamic conductance proportional to V(alpha). With pressure, conductance increases while alpha decreases, enabling us to test the theoretical prediction for LL behavior on the alpha dependence of the T and V independent coefficient of the tunneling conductance, and to obtain the high frequency cutoff of LL modes. The possible transition to a Fermi liquid at alpha-->0 is unattainable, as nanotubes collapse to an insulating state at high pressures.

Pressure dependence of the superconducting transition temperature of magnesium diboride.

We studied the pressure and temperature dependence of the electrical resistivity of the superconducting compound magnesium diboride (MgB(2)). The superconducting transition temperature decreases monotonically with pressure, being parabolic or linear, depending on samples. The rate of decrease under pressure is higher than in conventional superconductors. We discuss our results in terms of the semimetallic character of the electronic band structure of MgB(2).

"Debye-Scherrer Ellipses" from 3D fullerene polymers: An anisotropic pressure memory signature

High-pressure studies on fullerenes have previously shown the existence of one- and two-dimensional (2D) polymerized C60 structures. Synchrotron radiation measurements, performed on C60 samples quenched from 13 gigapascals and 820 kelvin, yield unambiguous proof for the existence of a three-dimensional (3D) polymerized C60 derivative. Moreover, unusual ellipsoidal Debye-Scherrer diffraction patterns are observed, which shows that the giant anisotropic deformation induced by the nonhydrostatic compression is retained in the quenched samples. The multiple bonding possibilities of the highly symmetrical C60 allow the retention (down to ambient pressure) of the deformation, a phenomenon reported previously only under high pressure.

Identification of specific mRNAs affected by treatments producing long-term facilitation in Aplysia.

Neural correlates of long-term sensitization of defensive withdrawal reflexes in Aplysia occur in sensory neurons in the pleural ganglia and can be mimicked by exposure of these neurons to serotonin (5-HT). Studies using inhibitors indicate that transcription is necessary for production of long-term facilitation by 5-HT. Several mRNAs that change in response to 5-HT have been identified, but the molecular events responsible for long-term facilitation have not yet been fully described. To detect additional changes in mRNAs, we investigated the effects of 5-HT (1.5 hr) on levels of mRNA in pleural-pedal ganglia using in vitro translation. Four mRNAs were affected by 5-HT, three of which were identified as calmodulin (CaM), phosphoglycerate kinase (PGK), and a novel gene product (protein 3). Using RNase protection assays, we found that 5-HT increased all three mRNAs in the pleural sensory neurons. CaM and protein 3 mRNAs were also increased in the sensory neurons by sensitization training. Furthermore, stimulation of peripheral nerves of pleural-pedal ganglia, an in vitro analog of sensitization training, increased the incorporation of labeled amino acids into CaM, PGK, and protein 3. These results indicate that increases in CaM, PGK, and protein 3 are part of the early response of sensory neurons to stimuli that produce long-term facilitation, and that CaM and protein 3 could have a role in the generation of long-term sensitization.

Identification of two phosphoproteins affected by serotonin in Aplysia sensory neurons.

Protein phosphorylation appears to play important roles in the mechanisms responsible for presynaptic facilitation in Aplysia. To screen for phosphoproteins that may be involved in facilitation, we previously examined protein phosphorylation in pleural sensory neurons as a function of different durations (2 min, 25 min and 1.5 h) of serotonin treatments. Different durations of serotonin had unique effects on the phosphorylation of different sets of proteins. To determine the functions of these phosphoproteins, we have begun to obtain their amino acid sequences using protein microsequencing techniques. We report here partial sequencing of 2 such proteins. One protein (S6), whose phosphorylation was affected by 2 min treatments with serotonin, appeared to be an intermediate filament protein. Another protein (L55), whose phosphorylation was affected by 1.5-h treatments with serotonin, appeared to be a calmodulin-like Ca(2+)-binding protein. Although the exact cellular functions for S6 and L55 are not known, obtaining partial sequences of these proteins sets the stage for future studies that will examine their regulation and their specific roles in facilitation.

Identification of three proteins in the eye of Aplysia, whose synthesis is altered by serotonin (5-HT). Possible involvement of these proteins in the ocular circadian system.

Previous results using translation inhibitors in the ocular circadian system of Aplysia suggest that protein synthesis may be involved in the light and serotonin (5-HT) entrainment pathways or perhaps in the circadian oscillator. Proteins have been previously identified whose synthesis was altered by treatments of light capable of perturbing the phase of the circadian rhythm in the eye of Aplysia. We extended these studies by investigating the effects of other treatments that perturb the ocular circadian rhythm on protein synthesis. 5-HT altered the synthesis of nine proteins. Interestingly, five of the proteins affected by treatments with 5-HT were previously shown to be affected by treatments with light. Four of the proteins affected by treatments with 5-HT were also affected by treatments with analogs of cAMP, a treatment which mimics the effects of 5-HT on the ocular circadian rhythm. To identify the cellular function of some of these proteins, we obtained their partial amino acid sequences. Based on these sequences and additional characterizations, a 78-kDa, pI 5.6 Aplysia protein appears to be glucose-regulated protein 78/binding protein, and a 36-kDa, pI 5.7 Aplysia protein appears to be porin/voltage-dependent anion channel. Heat shock experiments on Aplysia eyes revealed that yet another one of the Aplysia proteins (70 kDa) affected by 5-HT appears to be a heat-inducible member (heat shock protein 70) of the family of heat shock proteins. These findings suggest that these three identified proteins, together or individually, may be involved in some way in the regulation of the timing of the circadian oscillator in the eye of Aplysia.

Effects on protein synthesis produced by pairing depolarization with serotonin, an analogue of associative learning in Aplysia.

A form of associative plasticity in Aplysia, activity-dependent neuromodulation, involves the convergence of neuronal activity and the effects of a modulatory transmitter. To investigate the role of protein synthesis in associative plasticity, we examined the effects of a biochemical analogue of activity-dependent neuromodulation on the level of incorporation of labeled amino acid into proteins. To mimic associative training, abdominal ganglia were exposed to paired treatments of a depolarizing agent, elevated potassium, and a modulatory transmitter, serotonin. The effects of elevated potassium and serotonin applied alone were also examined. At least two proteins (nos. 9 and 17) were affected in a nonadditive way by the paired procedure. Incorporation of label into protein 9 was increased by the paired procedure but was not affected by either elevated potassium or serotonin. Incorporation of label into protein 17 was significantly affected by elevated potassium or serotonin, but the effect of the paired procedure was significantly less than the summed effects of elevated potassium and serotonin applied alone. These results indicate that changes in protein synthesis may be important in the induction of associative plasticities. Amino acid sequences of two peptides derived from protein 9 were obtained. Then, a partial cDNA clone for protein 9 was obtained by performing PCR with degenerate primers corresponding to portions of the sequences of the two peptides. The sequence of protein 9 is related to sequences previously reported for a family of genes comprising the stringent starvation protein of Escherichia coli, auxin-induced proteins of plants, and glutathione S-transferases of a number of organisms.

Pressure-induced enhancement of tc above 150 k in hg-1223.

The recently discovered homologous series HgBa(2)Can-1 Cun O2n+2+delta possesses remarkable properties. A superconducting transition temperature, T(c), as high as 133 kelvin has been measured in a multiphase Hg-Ba-Ca-Cu-O sample and found to be attributable to the Hg-1223 compound. Temperature-dependent electrical resistivity measurements under pressure on a (> 95%) pure Hg-1223 phase are reported. These data show that T(c) increases steadily with pressure at a rate of about 1 kelvin per gigapascal up to 15 gigapascals, then more slowly and reaches a T(c) = 150 kelvin, with the onset of the transition at 157 kelvin, for 23.5 gigapascals. This large pressure variation (as compared to the small effects observed in similar compounds with the optimal T(c)) strongly suggests that higher critical temperatures could be obtained at atmospheric pressure.

Identification of an annexin-like protein and its possible role in the Aplysia eye circadian system.

Light and serotonin regulate the phase of the circadian rhythm of the isolated eye of Aplysia. To screen for possible protein components of the eye circadian oscillator, we identified a number of proteins whose synthesis was altered in opposite ways by light and serotonin. The cellular function of one of these proteins was investigated by obtaining a partial amino acid sequence of it and by examining its immunoreactivity. A 38-amino acid sequence was obtained from a 40-kDa (isoelectric point 5.6) protein. A greater than 60% amino acid identity existed between this sequence and sequences of a family of calcium/phospholipid-binding proteins called annexins. Furthermore, the 40-kDa protein reacted with antibodies generated against a conserved amino acid sequence of annexins and with antibodies raised against human annexin I. The identification of the 40-kDa, light- and serotonin-regulated protein as an annexin led us to hypothesize that arachidonic acid metabolism plays a role in the Aplysia eye circadian system. To test this hypothesis, we examined the ability of an inhibitor of the arachidonic acid metabolic pathway to perturb the eye rhythm. Pulse treatments of isolated eyes with a lipoxygenase inhibitor, nordihydroguaiaretic acid, phase shifted the rhythm. The phase-shifting ability of nordihydroguaiaretic acid suggests that arachidonic acid and some of its metabolites may play a role in the eye circadian system. The results of our studies raise the possibility that links may exist between the 40-kDa annexin-like protein, arachidonic acid metabolism, and the circadian oscillator.

Long-term changes in synthesis of intermediate filament protein, actin and other proteins in pleural sensory neurons of Aplysia produced by an in vitro analogue of sensitization training.

Electrical stimulation of peripheral nerves of isolated pleural-pedal ganglia, an in vitro analogue of long-term behavioral training in Aplysia, produced changes in the synthesis of specific proteins in pleural sensory neurons. The changes in incorporation of [35S]methionine into proteins occurring 24 h after electrical stimulation (late) were determined and compared with changes occurring immediately after stimulation (early). Eight proteins were affected 24 h after electrical stimulation. Three of these proteins were also affected immediately after electrical stimulation. Two of the proteins affected late are components of the cytoskeleton. One protein was identified as actin. The other protein was purified from preparative 2D-gels and partial amino acid sequences of 3 peptides derived from this protein were determined. The peptide sequences were found to be identical to those of an Aplysia intermediate filament protein.

Alteration of the phase and period of a circadian oscillator by a reversible transcription inhibitor.

A function for transcription in the mechanism of a circadian oscillator was investigated with the reversible transcription inhibitor 5,6-dichloro-1-beta-D- ribobenzimidazole (DRB). Two-hour treatments with DRB shifted the phase of the circadian rhythm of the isolated eye of Aplysia, and continuous treatments of DRB lengthened the free running period of this rhythm. Camptothecin, an inhibitor of transcription that is structurally unrelated to DRB, had similar effects on the circadian rhythm. These results suggest that transcription may be part of the circadian oscillating mechanism.