The lipidic polymer, poly-3-hydroxybutyrate (PHB), is found in the plasma membranes of Escherichia col complexed to calcium polyphosphate (CaPPi). The composition, location, and putative structure of the polymer salt complexes led Reusch and Sadoff (1988) to propose that the complexes function as Ca2+ channels. Here we use bilayer patch-clamp techniques to demonstrate that voltage-activated Ca2+ channels composed of PHB and CaPPi are in the plasma membranes of E. coli. Single channel calcium currents were observed in vesicles of plasma membranes incorporated into planar bilayers of synthetic 1-palmitoyl, 2-oleoyl phosphatidylcholine. The channels were extracted from cells and incorporated into bilayers, where they displayed many of the signal characteristics of protein Ca2+ channels: voltage-activated selective for divalent over monovalent cations, permeant to Ca2+, manner by La3+, Co2+, Cd2+, and Mg2+, in that order. The channel-active extract, purified by size exclusion chromatography, was found to contain only PHB and CaPPi. This composition was confirmed by the observation of comparable single channel currents with complexes reconstituted from synthetic CaPPi and PHB, isolated from E. coli. This is the first report of a biological non-proteinaceous calcium channel. We suggest that poly-3-hydroxybutyrate/calcium polyphosphate complexes are evolutionary antecedents of protein Ca2+ channels.

Atomic-level understanding of structural characteristics and thermal behaviors of nanocatalysts is important for their syntheses and applications. In this article, we present a systematic study on structural and thermal stabilities of Pt−Pd bimetallic nanoparticles with core−shell and alloyed structures by using atomistic simulations. It was revealed that the Pd-core/Pt-shell structures are the least structurally stable, while the inverted Pt-core/Pd-shell nanoparticles are more stable than the alloyed ones when the Pt percentage exceeds 42% or so. The origin for this order was clarified through analysis of atomic energy distribution in these structures. Furthermore, the core−shell structures exhibit enhanced thermal stability as compared to the alloyed ones for Pt composition more than about 30%. The diverse melting behaviors of bimetallic nanoparticles, associating with their thermally driven structural evolutions under the heating process, were characterized by the measurement of the Lindemann index. In addition, the analyses of diffusion behavior and atomic distribution suggest that the minimization of surface energy tends to form Pd surface segregation. This study is of considerable importance not only to experimental preparation of Pt−Pd nanocatalysts but also to design of bimetallic (even multimetallic) nanostructures of high catalytic activity and excellent stability.

A complex of polyhydroxybutyrate (PHB), Ca2+, and inorganic polyphosphate (polyP) was proposed as the membrane component responsible for competence for DNA entry in Escherichia coli (Reusch, R. N., and Sadoff, H. L. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 4176-4180). While chemical and immunological assays and 1H NMR have unequivocally established the identity and content of PHB in the complex, comparable methods were not available for polyP. With specific enzyme assays developed for polyP, we have identified, in chloroform extracts of competent cell membranes, a novel form of polyP of about 60 to 70 residues in a stoichiometric ratio of PHB to polyP of 2:1. In E. coli mutants, incapable of synthesizing the predominant, thousand-long polyP chains, appearance of this short polyP and its inclusion in membranes can account for their capacity to develop competence and indicates an auxiliary pathway for polyP synthesis. A variety of fluorescent lipid probes demonstrate the appearance of extensive rigid domains in membranes of competent cells. We propose that the PHB.Ca2+.polyP complex perturbs the conformation of the lipid matrix, making it more permeable to charged molecules and thus allowing the entry of DNA.

Poly(3-hydroxybutyrate) (PHB) is well-known as a high molecular weight homopolymer of R-3-hydroxybutyrate which accumulates in storage granules within the cytosols of certain bacteria. Escherichia coli does not amass these granules; however, small amounts of low molecular weight PHB (<0.02% of dry weight) have been found in the plasma membranes in complexes with calcium polyphosphate; the complexes serve as voltage-activated calcium channels. Here we report that polyphosphate-complexed PHB is only a minor fraction of the polyester in E. coli. PHB comprises 0.36 to 0. 55% of the dry weight of log-phase cells, depending on culture medium, and this amount increases by 15 to 20% when the cells are made genetically competent. The PHB is widely distributed throughout the cell, wherein it is primarily associated with proteins. The identity of protein-associated PHB was established by antibody reaction, chemical assay, and 1H NMR spectroscopy. As expected, the physical and chemical properties of protein-associated PHB were found to be considerably different from those of the bulk polymer or granule PHB, e.g. protein-PHB complexes are normally insoluble in chloroform, soluble in water and alkaline hypochlorite, and are converted to crotonic acid more slowly on heating in concentrated sulfuric acid. Our studies indicate that the majority of cellular PHB (over 80%) is located in cytoplasmic proteins, especially proteins of the ribosomal fraction. Western immunoblots, probed with polyclonal anti-PHB IgG, revealed a number of PHB-polypeptides having a wide range of molecular weights in all cell fractions. These results suggest that PHB is a fundamental constituent of cells that may have physiological functions in addition to facilitating ion transmembrane transport or serving as a carbon reserve.

High index surfaces are introduced into Pt nanocrystals because they are expected to exhibit higher catalytic activity than low index planes such as {111}, {100}, and even {110}. This article presents a systematic investigation on the structure and stability of polyhedral Pt nanocrystals with both low-index and high-index facets by means of atomistic simulations. It has been found that the stability of Pt nanocrystals depends strongly on the particle shape and surface structures. Those nanocrystals, enclosed by high-index facets of {310}, {311}, and {331}, possess better stability and higher dangling bond density of surface compared with those ones with low-index facets, such as {100} and {110}, suggesting that they should become preferential candidates for nanocatalysts. The octahedral nanocrystals with {111} facets, though they have excellent structural and thermal stabilities, present the lowest dangling bond density of surface.National Natural Science Foundation of China[20833005, 21021002]; Fujian Provincial Department of Science and Technology[2008I0025

Microscopic understanding of thermodynamic behaviors of metallic nanoparticles is of significance for their applications in nanoscale catalysis and thermal energy storage. In this article, molecular dynamics simulations are used to investigate the thermal stabilities of Pt−Pd core−shell nanoparticles with different core sizes and shell thicknesses.Our study shows that a distinct two-stage melting occurs during the continuous heating of bimetallic nanoparticles. It has experienced a much broader temperature range compared with the melting of monometallic nanoparticles, although they have both developed from surface into interior. The temperature width for the two-stage melting is dependent not only on the bulk melting points of two component metals but also on the ratio of the shell thickness and core size. Furthermore, due to the melting of the Pd shell beforehand, the melting point of the Pt core is lower than that of the same size Pt nanoparticle not encapsulated by the Pd shell. This study provides a fundamental perspective on the melting behavior of bimetallic (even multimetallic) nanoparticles at the atomic level.

Comprehensive understanding of thermodynamic properties of metallic nanoparticles is of significance for their utility in catalysis. In this article, we have employed molecular dynamics simulations with quantum Sutton−Chen many-body potentials to examine the thermal stability of Au−Pt core−shell nanoparticles with different sizes during continuous heating. Our study shows that, for fixed particle size, the melting temperature is independent of core size for a small core while it is linearly decreased with a rising core radius for a large core. Diverse melting mechanisms have been discovered for different-core-sized nanoparticles. For a small core, the melting is progressively developed from the surface into the core, similar to that of monometallic nanoparticles. For a moderate or large core, an inhomogeneous melting has been found in these nanoparticles. The nucleation and activity of Shockley partial dislocations have initialized the local structural instability of the core−shell interface, leading to the inhomogeneous premelting of the Au core and the Pt shell for the moderate core. Nevertheless, when the core is large enough (resulting superthin shell), the diffusion of Au atoms from the core into the shell plays a dominant role in the destruction of the core−shell interface. This study provides a fundamental perspective on the melting behaviors of bimetallic (even multimetallic) nanoparticles at the atomistic level.

We investigated spin dependent transport through Fe/GaAs/Fe tunnel junctions.The tunneling magnetoresistance effect (TMR) was probed for different types of Fe/GaAs interfaces. For interfaces cleaned by hydrogen plasma the TMR effect is increased and observable at room temperature. If an epitaxial Fe/GaAs(001) interface is involved, the tunnel junction exhibits a bias dependent inversion of the TMR effect. This is a first experimental signature for band structure effects at a Fe/GaAs interface and relevant for spin injection experiments.