High-density Side Research Articles

Lasing properties of the J = 0-1 and the J = 2-1 lines of a neonlike germanium soft-x-ray laser

Lasing properties of a collisional-excitation Ne-like Ge soft-x-ray laser have been studied with exploding-foil, single-slab, and double-slab targets under identical pumping conditions. Experimental results for the angular intensity distributions and the temporal variations of the lasing intensities are examined with a hydrodynamic code and ray-trace calculations. The observed angular distributions are well reproduced by these analyses, and it is found that the effective gain regions are located on the high-density side of the expected gain regions. It is shown that the observed lasing intensity of the J = 0 to J = 1 line is strongly correlated with the temporal change of the calculated electron temperature for both the slab and the exploding-foil targets.

The metal-non-metal transition and the dynamic structure factor of expanded fluid alkali metals

This paper discusses the electrical and optical properties and, in particular the neutron scattering measurements of the dynamic structure factor in the metal-non-metal transition range. This transition occurs when the dense liquid evaporates to the dilute vapour or when the fluid is expanded by heating to its liquid-vapour critical point. The shape of S(Q, omega ) changes considerably on approaching the transition from the high-density liquid side, indicating a change in the interparticle interaction and the molecular structure. S(Q, omega ) of rubidium in the density range between the melting point density and three times the critical density is characterized by the existence of well defined acoustic-phonon-like collective density excitations at high momentum transfer, whereas S(Q, omega ) at a density of about twice the critical density is consistent with excitations of an optic-type mode in which two species tend to move in opposite directions.

Superconducting Phase Diagram of the Two-Band Model Investigated by the Exact-Diagonalization Method

By means of the exact-diagonalization method we have studied the finite-size two-band model with the interband pair-transfer interaction by calculating the superconducting (SC) correlation function P s etc. in the ground state. Occurrence of SC was clearly indicated by the increase of P s . The diagram of the observed SC region was constructed in the plane of the relative position of the second band versus on-site Coulomb energy U . In the low-density limit it proved close to the region where a negative- U situation is realized by the pair-transfer interaction. On the high-density side, the SC region was found to be more extended than the latter in accord with the previous interband-ladder-diagram results. Inclusion of the exchange interaction was found to reduce the SC region in a restricted region where two bands substantially share electrons. The two-band situations in cuprates are argued to be relevant to the high T c through the present mechanism.

Subcellular localization of squalene synthase in human hepatoma cell line Hep G2

Using the Hep G2 cell line as a model for the human hepatocyte the question was studied whether Hep G2-peroxisomes could be able to synthesize cholesterol. Hep G2 cell homogenates were applied to density gradient centrifugation on Nycodenz, resulting in good separation between the organelles. The different organelle fractions were characterized by assaying the following marker enzymes: catalase for peroxisomes, glutamate dehydrogenase for mitochondria and esterase for endoplasmic reticulum. Squalene synthase activity was not detectable in the peroxisomal fraction. Incubation of Hep G2 cells with U18666A, an inhibitor of the cholesterol synthesis at the site of oxidosqualene cyclase, together with heavy high density lipoprotein, which stimulates the efflux of cholesterol, led to a marked increase in the activity of squalene synthase as well as HMG-CoA reductase, whereas no significant effect on the marker enzymes was observed. Neither enzyme activity was detectable in the peroxisomal density gradient fraction, suggesting that in Hep G2-peroxisomes cholesterol synthesis from the water-soluble early intermediates of the pathway cannot take place. Both stimulated and non-stimulated cells gave rise to preparations where squalene synthase activity was comigrating with the reductase activity at the lower density side of the microsomal fraction; however, it was also present at the high density side of the microsomal peak, where reductase activity was not detected.

High-density properties of hard spheres within a modified Percus-Yevick theory: The role of thermodynamic consistency

Using an integral-equation approach based upon an approximation for the tail function, the equilibrium properties of a system of hard spheres are studied with special concern for the behavior in the region of close packing. The closure adopted is such that full, internal consistency is ensured in the thermodynamics of the model with respect to both the two zero-separation theorems as well as to the more standard virial and fluctuation routes to the equation of state. The scheme also makes use of the continuity properties of the tail function and of the cavity distribution function at contact. These properties are explictly tested in the low-density limit up to the fourth derivative. The theory generates an equilibrium branch bounded on the high-density side by a point corresponding to a packing fractionη≃0.78, a value which closely matches Rogers' least upper bound for the densest packing of spheres. The pair structure of the fluid in the state of random close packing is also compared to the type of local order predicted by the theory at similar densities.

Critical behavior of the Lennard-Jones fluid in the Percus-Yevick approximation.

We study the correlations and the isothermal compressibility of the Lennard-Jones fluid in the Percus-Yevick approximation in the critical region. The equation is solved numerically and the critical behavior is found similar to that for short-range forces: classical exponents but a nonclassical and nonuniversal scaling function that leads to an asymmetry between liquid and vapor with a true spinodal present only on the high-density side of the critical density.

Line emission from H II blister models

Numerical techniques to calculate the thermal and geometric properties of line emission from H II 'blister' regions are presented. It is assumed that the density distributions of the H II regions are a function of two dimensions, with rotational symmetry specifying the shape in three-dimensions. The thermal and ionization equilibrium equations of the problem are solved by spherical modeling, and a spherical sector approximation is used to simplify the three-dimensional treatment of diffuse ionizing radiation. The global properties of H II 'blister' regions near the edges of a molecular cloud are simulated by means of the geometry/density distribution, and the results are compared with observational data. It is shown that there is a monotonic increase of peak surface brightness from the i = 0 deg (pole-on) observational position to the i = 90 deg (edge-on) position. The enhancement of the line peak intensity from the edge-on to the pole-on positions is found to depend on the density, stratification, ionization, and electron temperature weighting. It is found that as i increases, the position of peak line brightness of the lower excitation species is displaced to the high-density side of the high excitation species.

Systematics of multilayer adsorption phenomena on attractive substrates

This paper presents a systematic classification of multilayer-adsorption phenomena on attractive substrates, with emphasis on the buildup of thick films. The approach is based on statistical mechanics and includes adsorption-desorption effects and the interrelation of bulk and surface behavior. The surface phase diagram depends qualitatively on the relative strengths and ranges of adatom-adatom and adatom-substrate attractions. When the adatom-substrate attraction dominates (strong substrate), the film builds up uniformly, as the bulk adatom density increases, and the excess surface density diverges at coexistence (complete wetting). The buildup proceeds via an infinite sequence of discrete layer transitions (layering) at low temperatures (below the roughening temperature ${T}_{R}$ and smoothly at higher temperatures, as originally noted by de Oliveira and Griffiths. Substrates of intermediate strength are characterized by a wetting temperature ${T}_{W}$ above which wetting at coexistence is approached. The relative values of ${T}_{W}$ and ${T}_{R}$ define three subregions: When ${T}_{W}<{T}_{R}$ layering occurs, with an infinite sequence of transitions between ${T}_{W}$ and ${T}_{R}$ when ${T}_{R}\ensuremath{\lesssim}{T}_{W}$, layer transitions have coalesced into a single thick-film---thin-film transition (prewetting); when ${T}_{R}\ensuremath{\ll}{T}_{W}$, prewetting may disappear, leaving only a critical-wetting transition on the coexistence axis. For still weaker substrates, wetting is incomplete at all temperatures; however, a variety of drying phenomena may occur on the high-density side of bulk coexistence. Specific calculations are given for a lattice-gas model at $T=0$ and in the mean-field approximation. Conclusions are informed, in addition, by certain exact results and symmetries. The last section includes a critical discussion of the relation of the lattice-gas model to the real world and a brief review of relevant experimental data.

Theory of cooperative phenomena in monolayers of hydroxyhexadecanoic acid isomers

Kellner and Cadenhead have obtained surface pressure/area isotherms for monolayers of several hydroxyhexadecanoic acid isomers. They find considerable differences in behaviour, depending on the position of the hydroxyl group on the carbon chain, and divide the isomers into “monopolar” and “bipolar” types. A lattice fluid model is introduced here with monomer states to represent erect conformations of the molecules, a dimer state to represent conformations parallel to the interface and vacant sites or “holes”. Using a Flory–Huggins type approximation the main behaviour types are reproduced. There are second-order (slope-discontinuity) transitions on some isotherms for all cases and first-order (liquid–gas) transitions in addition for the “monopolar” case with the monomer states energetically favoured. For “bipolar” cases with the dimer state energetically favoured the isotherms are more extended with nearly horizontal portions on the high-density side of the transition at some temperatures. With appropriate interaction parameters there is a region of negative thermal expansion on the low-density side of the transition similar to that found experimentally in 9-HHA monolayers.

Specific granules of the rat atrial muscle cell.

The specific granules found in the atrial cardiac muscle cells of the normal rat were studied. The ultrastructural appearance of these granules demonstrated a fixative-dependent lability. Fixation with cacodylate buffered aldehydes yields three types of granules. However, fixation with phosphate buffered aldehydes or primary fixation with OsO4 yields granules of uniform appearance. The granules are found predominantly in the perinuclear zone; 78% of the granules are within ten linear micrometers of the center of the nucleus. Two independent methods of measurement demonstrate spherical diameters of these granules of 0.30 micron and 0.37 micron respectively. The granules are found in greater concentration at one pole of the nucleus than at the other. On the high density side there are 4.07 granules/micrometers3 which occupy 5.8% of the cytoplasmic volume. On the low density side there are 2.15 granules/micrometers3 which occupy 3.0% of the cytoplasmic volume. The granules at both poles are the same size. Atrial walls were incubated in a modified Tyrode's solution. One hour of incubation caused no change in the atrial granules. Addition of norepinephrine or L-Dopa resulted in the appearance of more granules but the size of the granules remained the same. Incubation with reserpine had no effect upon the atrial granules. Apparently the atrial myocardial cell is stimulated by exogenous catecholamine to synthesize more atrial granules which themselves do not appear to contain catecholamines.

Phenomenological theory of first- and second-order metal-insulator transitions at absolute zero

A phenomenological theory of metal-insulator transitions at $T=0$ is set up in which the order parameter is the discontinuity $q$ in the single-particle occupation probability at the Fermi surface. Applied to the case of the second-order metal-insulator transition in a half-filled Hubbard band, $q$ is found to have a critical exponent of unity, and the relation to Gutzwiller's variational treatment is exposed. The enhancement of the spin susceptibility by the Hubbard interaction is also treated. The same phenomenology is then applied to jellium, where a first-order metal-insulator transition occurs when the conducting electron fluid crystallizes on to the Wigner lattice. The form of the spin susceptibility, chemical potential, and energy is given near to, but on the high-density side of, the transition.

Flows of 22Na in layer-type composite membranes

The permeability of 22Na across simple and layer-type composite membranes of parlodion containing different amounts of polystyrenesulfonic acid has been measured as a function of external NaC1 concentration and corrected for the effects of aqueous stationary layers present at the membrane-solution interfaces. It was found that the permeability of 22Na in the two opposite directions across the composite membrane was different whereas it was the same across simple membranes. In both cases, the permeability of 22Na increased with increase in the concentration of the external solution. In the presence of a concentration gradient, the composite membranes generated ±58 mV for a tenfold difference in the concentration and gave values for the permeability of 22Na which were different in the two directions across the membrane. Corrections applied to the two permeability values in the two opposite directions due to the presence of the electric potential difference across the membrane gave values which agreed qualitatively with those measured in the absence of the concentration gradient. The asymmetry or rectification in the permeability of 22Na was attributed to the presence of a permanent Na-solubility gradient in the composite membrane which assisted the flow of Na in one direction (high charge density side to low charge density side of the membrane) and opposed it in the opposite direction. Measurement of distribution of 22Na in the various layers of the composite membrane showed that there was accumulation and depletion of 22Na in the membrane phase, i.e., 22Na concentration profile asymmetric, when 22Na was flowing from the high charge density side to the low charge density side and vice versa, respectively. The need to recognize the existence of such a directional quantity (solubility gradient) and its contribution to fluxes measured across the biological membranes which have a layered structure is pointed out.

Studies with Composite Membranes: III. Measurement of Water Permeability

The permeability of tritiated water (THO) across simple and layer-type composite membranes of collodion containing different amounts of polystyrenesulfonic acid has been measured and corrected for the effects of aqueous stationary layers present at the membrane-solution interfaces. It was found that the water permeabilities in the two opposite directions across the composite membranes were different, whereas they were the same across simple membranes. The theoretical permeability value for the composite membrane (formed by putting one simple membrane on top of another simple membrane of increasing charge density and gently pressing them together), calculated from the values due to simple membranes, was found to be always greater than the two measured values. It was shown that the aqueous layers trapped between membranes were not responsible for the low measured values. The factor causing this was ascribed to the mechanism which produced rectification of water flow in the composite membranes. Establishment of the THO concentration profile in the layered membranes showed that accumulation and depletion of THO in the membrane phase when the THO was flowing from the high charge density side to the low charge density side and vice versa, respectively, were responsible for the unequal flows observed across the composite membrane in the two directions.

Dependence of the Normal Modes of Plasma Oscillation at a Bimetallic Interface on the Electron Density Profile

Using the high-frequency expansion of the random-phase approximation, we study the normal modes of plasma oscillations at a bimetallic interface. We find that the surface plasmons decay by damping into bulk oscillations of the low-density metal, whose wavelength becomes shorter as the low-density-side density decreases. If the low-density-side electron density is less than approximately a third that on the high-density side, the high-frequency expansion fails because the surface plasmons begin to decay into individual particle-hole states. Thus we are unable to use this expansion to study the modes at metal-vacuum interfaces, where the low-density-side electron density is zero. For bimetallic systems that allow the high-frequency expansion, we show how to determine the dependence of the plasmon dispersion relations on the density profile and stress tensor.

Seventh Virial Coefficients for Hard Spheres and Hard Disks

The seventh virial coefficient B7 is expressed as a sum of modified star integrals instead of the usual Mayer star integrals. The new graphs contain both Mayer f functions and f̃(≡1+f) functions. This simplifies the calculation of B7. That is, instead of the 468 star integrals that appear in Mayer's formulation, only 171 modified star integrals now appear in the evaluation of B7. Furthermore, for D-dimensional particles with a hard core, these 171 integrals are strongly dependent on the number of dimensions D. For hard rods, hard disks, and hard spheres, respectively, at most one, 78, and 164 of these integrals give a nonvanishing contribution to B7. When Monte Carlo integration is used to evaluate these integrals using hard-sphere and hard-disk potentials we obtain the following values of B7: hard spheres, B7/(B2)6=0.0138±0.0004; hard disks, B7/(B2)6=0.1141±0.0005. For hard spheres, the truncated seven-term virial series for the pressure agrees within 10% with the results of the molecular dynamics data throughout the fluid phase. The Padé approximants to the pressure obtained from Bn(n<8) data agree even better (≲5%) with the molecular-dynamics data on the fluid side. Estimated values of B8/(B2)7 obtained from the Padé approximants are 0.005 and 0.063 for hard spheres and hard disks, respectively. Furthermore, for hard disks, the Padé approximants obtained from the seven known values of Bn(n<8) indicate that the virial series shows an unstable phase at ρ=ρ0. If this is in fact true, then the low-density study of the equation of state for hard disks would yield some information on the high-density side. We have also calculated the first seven coefficients Cn of the density expansion of the isothermal compressibility for hard spheres, hard disks, hard parallel cubes, and hard parallel squares. We find that these Cn alternate in sign. For two-dimensional continuum particles with a hard core, the corresponding compressibility series appear to converge faster than the usual virial series of the pressure. We also compare the present values of B7 with the various values of B7 calculated from approximate theories.

Interaction between denatured DNA and RNA from Bacillus stearothermophilus involving only one-half of total DNA

A new type of interaction between denatured DNA and RNA is described, which takes place on mixing solutions of DNA and RNA at room temperature. The formation of a hybrid molecule containing RNA fragments bound to a DNA strand is revealed by the appearance of a new band in the analytical ultracentri-fuge patterns in a CsCl density-gradient on the high-density side of the band of denatured DNA. The action of nucleases, the effect of the mean molecular weight of RNA on the characteristics of the band and the importance of the ratio RNA/DNA have been studied quantitatively by analysing the ultracentrifuge patterns into their Gaussian components. The results show that when the ratio RNA/DNA is higher than 2, the ratio between the areas of the two bands is always very near unity. The denatured DNA not involved in the formation of the complex has been isolated by chromatography on a methylated albumin column and has been shown incapable of returning to the native configuration after annealing. This kind of interaction does not show species-specifity, since it also takes place in mixtures containing DNA from Bacillus stearothermophilus and RNA from Escherichia coli . So far no such interaction has been observed for DNA from E. coli .

THE ORIGIN, METABOLISM, AND STRUCTURE OF NORMAL HUMAN SERUM LIPOPROTEINS

The main fractions obtained by ultracentrifugal separation of normal human serum lipoproteins form distinct classes. On the high-density side (d > 1.063) HDL1, HDL2, and HDL3; on the low-density side: LDL1, LDL2, and chylomicra are represented by particles of increasing density, lipid content, and diameter, the latter ranging from about 100 Å (HDL3) to about 15.000 Å (largest chylomicron).Distinctions between classes are evidenced by the action of solvents, the analysis of the N-terminal amino acids of peptides and immunochemical and metabolic properties.High-density lipoproteins (HDL) and low-density lipoproteins (LDL) form distinct groups displaying dissimilarities on the basis of many criteria. There can be little doubt that the origin, structure, and functions of the two groups are different. Among the LDL, LDL2, and LDL1are both very similar and complex, LDL2appearing to be a precursor of LDL1. HDL on the other hand may be partially derived from the chylomicra as a result of intravascular lipoprotein action upon some of these transient, very low-density lipoproteins with which they share characteristic peptides.Since no satisfactory structural model for the lipoproteins has yet been published, it was the purpose of this paper to re-examine all pertinent data and to present new speculations regarding these structures.It could be demonstrated that the lipoproteins on the low-density side are particles characterized by a protein coverage which is uniform over their extensive dimensional range and which corresponds to 36 A2per amino acid residue. LDL1, the smallest of the group and the most abundant of the normal human serum lipoproteins, appears to consist of a single monolayer of lipids bearing adsorbed protein and surrounding a central core of occluded water. The general configuration of the adsorbed protein is still unknown. It is demonstrated that our present knowledge of the configuration of protein and synthetic polypeptide monolayers, most of which is derived from experiments with the film balance at air–water and petroleum ether – water (so-called oil–water) interface, cannot apply to these lipoproteins since there exists a considerable difference in physical and ionic topography between the experimental "oil–water" interface and the interface involved in lipoproteins.Part of the demonstration consists of the structural analysis of the lipid molecules involved. Diagrams representing these molecules, which are exact as to interatomic distances and bond angles are presented and used to explain the important phenomenon of sterol–phospholipid association. The latter is expected to play a considerable role in LDL1(and presumably in the external lipid layer of LDL2and chylomicra) where phospholipids and cholesterol (free and bound) represent 90% of the total molecular species with cholesterol (free and bound) accounting for two-thirds of them. By virtue of their associative properties, such molecules must exist in organized arrays conferring a directive influence to the lipid film on the configuration of the adsorbed protein. This should be quite different from the dispersive action of the disorganized petroleum ether layer in the so-called "oil–water" interface which induces partial random uncoiling of the protein chains.A demonstration of the method used in studying lipoprotein films by the use of diagrams is given. Its application to the cholesterol monolayer yielded for the limiting area a value in excellent agreement with that obtained with the film balance.

THE ORIGIN, METABOLISM, AND STRUCTURE OF NORMAL HUMAN SERUM LIPOPROTEINS

The main fractions obtained by ultracentrifugal separation of normal human serum lipoproteins form distinct classes. On the high-density side (d > 1.063) HDL1, HDL2, and HDL3; on the low-density side: LDL1, LDL2, and chylomicra are represented by particles of increasing density, lipid content, and diameter, the latter ranging from about 100 Å (HDL3) to about 15.000 Å (largest chylomicron).Distinctions between classes are evidenced by the action of solvents, the analysis of the N-terminal amino acids of peptides and immunochemical and metabolic properties.High-density lipoproteins (HDL) and low-density lipoproteins (LDL) form distinct groups displaying dissimilarities on the basis of many criteria. There can be little doubt that the origin, structure, and functions of the two groups are different. Among the LDL, LDL2, and LDL1are both very similar and complex, LDL2appearing to be a precursor of LDL1. HDL on the other hand may be partially derived from the chylomicra as a result of intravascular lipoprotein action upon some of these transient, very low-density lipoproteins with which they share characteristic peptides.Since no satisfactory structural model for the lipoproteins has yet been published, it was the purpose of this paper to re-examine all pertinent data and to present new speculations regarding these structures.It could be demonstrated that the lipoproteins on the low-density side are particles characterized by a protein coverage which is uniform over their extensive dimensional range and which corresponds to 36 A2per amino acid residue. LDL1, the smallest of the group and the most abundant of the normal human serum lipoproteins, appears to consist of a single monolayer of lipids bearing adsorbed protein and surrounding a central core of occluded water. The general configuration of the adsorbed protein is still unknown. It is demonstrated that our present knowledge of the configuration of protein and synthetic polypeptide monolayers, most of which is derived from experiments with the film balance at air–water and petroleum ether – water (so-called oil–water) interface, cannot apply to these lipoproteins since there exists a considerable difference in physical and ionic topography between the experimental "oil–water" interface and the interface involved in lipoproteins.Part of the demonstration consists of the structural analysis of the lipid molecules involved. Diagrams representing these molecules, which are exact as to interatomic distances and bond angles are presented and used to explain the important phenomenon of sterol–phospholipid association. The latter is expected to play a considerable role in LDL1(and presumably in the external lipid layer of LDL2and chylomicra) where phospholipids and cholesterol (free and bound) represent 90% of the total molecular species with cholesterol (free and bound) accounting for two-thirds of them. By virtue of their associative properties, such molecules must exist in organized arrays conferring a directive influence to the lipid film on the configuration of the adsorbed protein. This should be quite different from the dispersive action of the disorganized petroleum ether layer in the so-called "oil–water" interface which induces partial random uncoiling of the protein chains.A demonstration of the method used in studying lipoprotein films by the use of diagrams is given. Its application to the cholesterol monolayer yielded for the limiting area a value in excellent agreement with that obtained with the film balance.