bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic measurements of the velocity distribution of neutral particles in a plasma in front of a wall
Authors: Souw E. K. , Hackmann J. , Uhlenbusch J.
Published: March 2009

The influence of a metallic wall on the velocity distribution of the neutral particles in a plasma has been experimentally determined by spectroscopic measurements of a Doppler broadened line. Typical characteristic facts of the plasma –wall interaction can be explained quantitatively by means of a one-dimensional kinetic equation, which is solved in the plasma –wall interaction region, assuming reasonable boundary conditions on the wall surface.
Stability and eigenfrequency bounds for steady flow of a cylindrical electrostatic plasma
Authors: Lucas R. J.
Published: March 2009

The stability of steady flow of an N-component, warm or cold, cylindrical electrostatic plasma is considered. The plasma is immersed in a magnetic field B0(r), where r is the radial co-ordinate, which is allowed to have both axial and azimuthal components. The unperturbed quantities are allowed to be arbitrary functions of r consistent with the zero-order equations. A sufficient condition for the stability of a rotating flow to axisymmetric perturbations is obtained.
Effects of ambient plasma and background temperature on the beam–plasma instability and beam–plasma discharge at low pressures in bounded systems
Authors: Roth I. , Cuperman S.
Published: March 2009

The electrostatic instability of bounded beam –plasma systems as well as its consequences on the ignition of the beam –plasma discharge (BPD) at low pressures and magnetic field strengths are investigated. For this purpose, configurations consisting of a finite radius electron-beam –plasma system whose edge is far away from a confining coaxial metallic container are considered; an ambient, partially ionized gas filling the container is also present.
The interaction of high-frequency and low-frequency waves in magnetized plasmas
Authors: Relke I. V. , Rubenchik A. M.
Published: March 2009

The interaction of high-frequency and low-frequency waves in magnetized plasmas is considered. The narrowness of high-frequency wave packets makes possible a concise Hamiltonian description of the problem. Some concrete problems are studied with the help of the derived equations. The competitive role of scattering in self-consistent density and magnetic-field fluctuation are considered. The self-focusing and solitons of potential plasma waves and magnetohydrodynamic waves are studied.
Electron cyclotron absorption for oblique propagation in loss-cone plasmas
Authors: Ziebell L. F.
Published: March 2009

The components of the dielectric tensor for a plasma described by a relativistic loss-cone electron distribution are written in a simple way, which takes full account of relativistic effects, harmonics and Larmor radius, for perpendicular and oblique propagation. For sufficiently oblique propagation and temperatures in the thermonuclear range, a still simpler form of the dielectric tensor is derived. The role of the wave parameters in the absorption is discussed, and some comments are made about the weakly relativistic and non-relativistic approaches. A numerical example is given for both the extraordinary and ordinary modes.
Anomalous transport and anomalous heating due to lower-hybrid wave fields
Authors: Krämer M. , Sollich N. , Dietrich J.
Published: March 2009

The microscopic and macroscopic behaviours of a linear reflex discharge in the presence of low-frequency turbulence are investigated under the action of moderate lower-hybrid wave power. The frequency and wavenumber spectra of both the low-frequency fluctuations and the high-frequency waves are measured using a correlation-analysis technique with two probes. The low-frequency fluctuations may be attributed to drift-wave turbulence. The fluctuation level is raised when RF power is coupled to the plasma, thus leading to considerably enhanced radial transport. The coupling between low-frequency fluctuations and high-frequency waves can be seen clearly from the spectra. The high-frequency wavenumber spectra measured inside the antenna are in reasonable agreement with the lower-hybrid wave dispersion. However, the wavenumbers observed in the lower-hybrid resonance region outside the antenna are – in contrast with expectation – not larger than in the plasma edge region. From the electric-field energy-density spectra and from measurements of the density and the temperatures, a detailed energy balance can be performed. The calculated heating rates are anomalously large for both the electrons and the ions. The absorption processes, relevant for the present experiment, are discussed.
Analytical solutions for oblique wave growth from a ring-beam distribution
Authors: Thorne Richard M. , Summers Danny
Published: March 2009

Analytical solutions are presented for the linear growth rate of oblique plasma waves in a magnetized plasma due to resonant interactions with a model ringbeam distribution. Explicit closed-form solutions for the angular dependence are obtained in terms of modified Bessel functions of the first kind. In the limits of either quasi-longitudinal or quasi-transverse propagation the analytical solutions take the form of simple algebraic expansions, which allow an immediate comparison of the relative contributions from different harmonic resonances, and which also determine the conditions for marginal stability for any specific resonance. The results can be applied, for instance, to the growth of waves following ionization of neutrals originating from cometary, planetary, or interstellar material in the solar wind. In a weakly unstable plasma the analytical results also provide an important check on the complex numerical codes that hitherto constituted the only method available for evaluating the growth of oblique plasma waves.
The time-asymptotic stability of shocks in cosmic-ray hydrodynamics
Authors: Zank G. P.
Published: March 2009

The nonlinear behaviour of short-wavelength perturbations in the two-fluid cosmic-ray hydrodynamical model is examined. We show that such a perturbation leads to shock formation and derive the appropriate wave equation. We show that a discontinuous perturbation incident on a weak cosmic-ray shock destabilizes, in a time-asymptotic sense, the shock.
Cosmic-ray particle transport in weakly turbulent plasmas. Part 2. Mean free path of cosmic-ray protons
Authors: Schlickeiser Reinhard , Achat Ulrich
Published: March 2009

We use the general Fokker–Planck coefficients derived in the first paper of this series, which describe the interaction of energetic charged particles with weak plasma turbulence in a magnetized plasma, to calculate the mean free path λ of cosmic-ray particles along the uniform background magnetic field. This quantity is a key parameter for confining energetic charged particles in cosmic plasmas, and can be experimentally inferred from interplanetary in-situ observations of solar particle events
Reflection and absorption of ordinary waves in an inhomogeneous plasma
Authors: Croci R.
Published: March 2009

This study treats the system of Vlasov and Maxwell equations for the Fourier transform in space and time of a plasma referred to Cartesian co-ordinates with the z co-ordinate parallel to the uniform equilibrium magnetic field; the equilibrium plasma density depends on ηχ, where η is a parameter, and goes to zero as │χ│ → ∞. The component ky of the wave vector is taken to be zero, whereas kz is non-zero, thereby allowing for Landau damping. Coupling of the components of the electric field parallel and perpendicular to the equilibrium magnetic field (the ordinary and extraordinary waves in a homogeneous plasma) is neglected.
The neighbouring vibrating ‘multiple water-bag’ plasma potential and related aspects
Authors: Chee-seng Lim
Published: March 2009

The spatial factor Φ(x) of the vibratory-state MWB (‘multiple water-bag’) plasma potential Φ = Φ(x) exp (- iwt) in the vicinity of its generating electric charge rgr;(x) exp (- iwt) is proved to be the Poisson potential U(x) of ρ plus an error function ξ(x) whose magnitude can be rendered as small as desired, in relation to │Φ(x)│, by locating the recording point sufficiently close to the charge. How close this should be is independent of ρ, but depends quantitatively on the MWB model and the frequency ω, and qualitatively on whether the frequency of the vibrations is greater or less than the plasma frequency. A high vibration frequency reinforces accuracy in the Poissonian approximation. The question as to whether Φ is weaker or stronger than U(x) is tacked. Shielding efficiency is explored in the vibratory state.
Millimetre-wave second-harmonic generation in an underdense magnetoplasma in the presence of a magnetic wiggler
Authors: Parashar Jetendra , Pandey H. D. , Ramachandran K. , Singh R. K.
Published: March 2009

A high-power millimetre wave (w1, k1) propagating through a magnetized plasma in the presence of a wiggler magnetic field (0, k0) produces density perturbations at (w1, K1 +K0). The density perturbations couple with the oscillatory velocity at (wl, K1) to produce a nonlinear current at (2w1, 2K1 +K0) driving second-harmonic electromagnetic radiation. The amplitude of the generated wave is sensitive to cyclotron resonance.
Bifurcations in an advanced two-chamber model for the edge plasma in a divertor tokamak
Authors: Punjabi Alkesh , Soe† Min
Published: March 2009

An advanced two-chamber model for the main plasma scrape-off and the divertor chamber of a tokamak is constructed. It is studied in the framework of catastrophe theory. The model shows bifurcations in the topology of the equilibrium surfaces. The results are in qualitative agreement with some of the experimentally observed features of the H-mode transition.
Helicon waves in a cylindrical plasma column
Authors: Shoucri M.
Published: March 2009

Helicon waves have been used for efficiently coupling radio-frequency power to plasmas, and are studied for their potential application for low-frequency current drive in tokamks. In this paper the electromagnetic field components and the dispersion relation for azimuthally independent helicon-wave oscillations in a cylindrical plasma column are derived. The coupling of transverse electric TE and transverse magnetic TM modes associated with these oscillations is discussed. The effect of the collisional damping on determining the nature of the TM mode (either surface wave or body wave) is analysed.
Transverse electromagnetic modes in a hot cylindrical plasma column
Authors: Shoucri M.
Published: March 2009

We derive the dielectric tensor of a hot plasma by including the smallgyroradius corrections up to the first harmonic (ω ≈ 2ωcα). We show that these corrections introduce radial wavenumbers associated with transverse electromagnetic modes (TEM). The special case of a cylindrical plasma column is discussed for a hydrogen plasma at the ion-cyclotron frequency and for an electron plasma at the electron-cyclotron frequency and its first harmonic.
The peripheral auditory apparatus
Authors: Johnstone B. M. , Sellick P. M.
Published: March 2009

This review of the peripheral auditory apparatus represents an attempt to analyse critically recent developments in the field. The coverage is not exhaustive, the emphasis is on functional aspects and no attempt is made to review the anatomy of the ear. Particular emphasis is placed on three broad sections: the physiology of the middle ear, basilar membrane mechanics and the electrophysiology of the cochlea. It is in these areas that recent technical advances have led to experiments which throw doubt on traditionally held concepts. Such an advance is the application of the Mössbauer technique to the problem of middle- and inner-ear mechanics. Because of its novelty, this technique is discussed in detail. Other new methods such as laser holography are just starting to be used, as are the ion selective microelectrodes for measuring dynamic changes in endolymph concentration. After many years of slow progress there is a sudden spark of enthusiasm for peripheral auditory research, and the coming decade promises to be most exciting indeed.
Molecular quantum mechanics in biology
Authors: Fischer-Hjalmars Inga
Published: March 2009

Among the different fields of natural sciences, physics is concerned with the most basic properties of matter. It is possible to define the necessary physical concepts in a rather unique way and carry out experiments under well-controlled conditions. The interpretation of experiments is therefore comparatively easy. In chemistry the situation is more complicated and the interpretation becomes more ambiguous. Biology finally is characterized by its high degree of complexity, making the detailed analysis difficult. Since the problems of biology are so essential, it is very important to use every tool available to clarify the situation, state the problems as carefully as possible and approximate the solution from as many sides as are feasible. Therefore, not only the methods developed within biology but also those pertinent to chemistry and physics should be applied. For this reason also quantum mechanics may be able to contribute to the elucidation of biological problems.
Biophysical contributions to membrance structure
Authors: Finean J. B.
Published: March 2009

Applications of physical techniques to studies of membrane structure have increased greatly in the past few years and they have begun to provide more precise structural parameters for membranes and also some indication of the physical states of the molecular constituents in the membranes. Direct measurements of membrane features have been made by electron microscopy and by X-ray diffraction methods. Spectroscopic techniques especially infrared absorption, nuclear magnetic resonance absorption and optical rotatory dispersion and circular dichroism measurements have provided additional data relating to the physical states of the molecular components in the membranes.
Control of muscle contraction
Authors: Ebashi Setsuro , Endo Makoto , Ohtsuki Iwao
Published: March 2009

As is well known, the memorable discovery of Galvani (1791) was followed by the development of two new fields of science, electrochemistry and electrophysiology. During the course of this development, the most remarkable feature of the original finding, i.e. ‘contraction of muscle induced by a piece of metal’, gradually came to be ignored. As a consequence, the simple question as to how electrical stimulation might induce muscle contraction was left unanswered until the middle of this century, when several physiologists became aware of the crucial nature of the problem and tried to attack it from various directions. This resulted in a marked progress of physiological and morphological studies which were intentionally or unintentionally concerned with the mechanism of the link between excitation, that is the electrical phenomenon at the surface membrane, and the contractile process.
Differential gene activity and segregation of cell lines: an attempt at a molecular interpretation of the primary events of embryonic development
Authors: Rossi Mosé , Augusti-Tocco Gabriella , Monroy Alberto
Published: March 2009

The study of differentiation is concerned with the analysis of the processes responsible for ‘the cellular changes in macromolecular synthesis and composition, patterned in time and space and resulting in specialized functions, forms and organization’ (Moscona, 1973).
Acetylcholine receptors
Authors: Rang H. P.
Published: March 2009

The idea that certain drugs and neurotransmitters produce their effects by combining with specific receptors was first clearly expressed by Langley (1905) on the basis of the selective and localized effect of nicotine on striated muscle fibres. In 1914, Langley published a paper in which the antagonism between ‘curari’ and nicotine was analysed and measured as the ratio by which the nicotine concentration had to be increased in order to produce a standard response in the presence of tubocurarine. It was clear that Langley had in mind the idea of competition between nicotine and curare for the receptor sites and it was surprising that he did not formulatethe theory quantitatively, particularly since Hill, working in Langley's laboratory in 1909, published a mathematical analysis of the action of nicotine on frog muscle giving kinetic and equilibrium equations based on the law of mass action, which could easily have been extended to give an account of competitive antagonism. Barger & Dale (1910) did not favour the idea of specific receptors for sympathomimetic amines.
Nucleosomes structure and its dynamic transitions
Authors: Mirzabekov Andrei D.
Published: March 2009

The discovery of nucleosomes as a basic repeating unit of the chromatin structure organizing the major part of eukaryotic DNA greatly catalyzes the expansion of our knowledge on chromatin. Several lines of experimental evidence have led to the formulation of the nucleosome conception: the observation of chains of globular particles in electron micrographs of chromatin (Olins & Olins, 1974); the demonstration that DNA is released as a set of discrete sizes upon digestion of chromatin with endogenous nucleases (Hewish & Burgoyne, 1973); the isolation of discrete nucleoprotein particles upon digestion of chromatin with micrococcal nuclease (Rill & Van Holde, 1973).
The use of X-ray synchrotron radiation for structural research in biology
Authors: Stuhrmann H. B.
Published: March 2009

Synchrotron radiation is intense electromagnetic radiation with a continuous spectral distribution emitted by high-energy electron (or positron) synchrotrons or storage rings in the visible, vacuum ultraviolet and X-ray regions. For a long time only an unwanted but inevitable by-product of ring accelerators in elementary particle research, synchrotron radiation is now becoming a most widely used tool in atomic, molecular and solid-state spectroscopy, surface physics, structural research on solids, soft X-ray microscopy, lithography. It is not yet clear how large the field of applications really is nor which line of research will prove to be the most fruitful one in the long run.
Lineages, quantal cell cycles, and the generation of cell diversity
Authors: Holtzer H. , Rubinstein N. , Fellini S. , Yeoh G. , Chi J. , Birnbaum J. , Okayama M.
Published: March 2009

Most theories of determination or differentiation assume that embryonic cells differ from mature cells. Embryonic cells are thought to have metastable control mechanisms. These labile controls are believed to become progressively more stabilized as the cells differentiate. Zygote, blastula, neural plate, limb bud, somite, or ‘stem’ cells are conceived of as undifferentiated, totipotent, or multipotential cells. As such, these cells supposedly have available for activation a larger repertoire of phenotypic programmes than their progeny. A necessary corollary to this view is that the activation of one particular phenotypic programme out of the many available is a function of instructive exogenous inducing molecules.
Functional reassembly of membrane proteins in planar lipid bilayers
Authors: Montal M. , Darszon A. , Schindler H.
Published: March 2009

Recent progress in membrane biology has brought us to a stage where it is possible to associate complex biological processes to identifiable membrane proteins. Technical advances in the biochemical characterization and purification of membrane proteins have contributed a wealth of structural information. The reconstitution approach has proved to be valuable in our efforts to understand the molecular mechanisms of membrane transport and energy transduction.
Photon correlation spectroscopy of molecular processes in solution
Authors: De Maeyer L. , Gnädig K. , Hendrix J. , Saleh B.
Published: March 2009

In the initial period of application of photon correlation spectroscopy to molecular phenomena, attention has mainly been focused on the improvement of experimental approaches and instrumental techniques, on feasibility studies of certain cases where theoretical considerations seemed to indicate the prospective usefulness and on various applications that are relatively straightforward, such as the measurement of translational or rotational diffusion coefficients, or the determination of flow velocity by Doppler anemometry.
Relaxation of rotational and internal modes of macromolecules determined by dynamic scattering
Authors: Schurr J. Michael
Published: March 2009

About 11 years ago Ford & Benedek (1964) and Cummins, Knable & Yeh (1964) developed some fascinating new light-scattering techniques for the study of macromolecular motions in solution. Enthusiastic researchers have over the years endowed these new methods with a variety of colourful names, including optical mixing or light-beating spectroscopy, heterodyne or homodyne beating, intensity fluctuation spectroscopy, Rayleigh spectroscopy, and quasielastic or dynamic light scattering.
Haemocyanins
Authors: Holde K. E. van , Miller Karen I.
Published: March 2009

About ten years ago, one of the authors participated in a review of haemocyanin structure and function (van Holde & van Bruggen, 1971). At that time, it was possible to describe the field in terms of a limited amount of exciting new structural information, and a long list of unanswered questions. While the stoichiometry of oxygen binding was understood, virtually nothing was known about the active site. Even the oxidation state of the copper was a matter of conjecture. The size of the haemocyanin polypeptide chains was the subject of intense debate, with very little substantive knowledge available. While the haemocyanins were known to be allosteric proteins, there were virtually no experimental studies of oxygen binding on a level that could be meaningfully interpreted in terms of extant theories.
Peptide–protein interactions: an overview
Authors: Zvelebil Markéta J. J. M. , Thornton Janet M.
Published: March 2009

Most biological processes involve recognition and binding between molecules. Protein-peptide interactions are particularly common as, for example, in the action of hormones, such as oxytocin and vasopressin, or neurotransmitters including morphine-like peptides.
Protein dynamics: comparison of simulations with inelastic neutron scattering experiments
Authors: Smith J. C.
Published: March 2009

To deepen our understanding of the principles determining the folding and functioning of globular proteins the determination of their three-dimensional structures must be supplemented with the characterization of their internal motions. Although dynamical events in proteins occur on time-scale ranging from femtoseconds to at least seconds, the physical properties of globular proteins are such that picosecond (ps) time-scale motions make a particularly important contribution to the internal fluctuations of the atoms from their mean positions.
Proton-detected heteronuclear edited and correlated nuclear magnetic resonance and nuclear Overhauser effect in solution
Authors: Griffey Richard H. , Redfield Alfred G.
Published: March 2009

The proton has been the nucleus of choice for NMR studies of macromolecules because it is ubiquitous; it provides the highest sensitivity; its resonances can be identified with types of amino and nucleic acids by means of experiments utilizing proton spin-spin interaction and chemical shift; and, most important, proton NMR yields distance information via the nuclear Overhauser effect (NOE). Many of these advantages are lost for larger biopolymers (molecular weight more than 15 kDa) for which the line width is considerably greater than the proton-proton spin-spin interaction. The spin-spin interaction is then useless or difficult to use for assignment; and furthermore the proton line width and the number of proton resonances both increase in proportion to the molecular weight, thereby increasing the problem of resonance overlap to an intolerable degree.
Medical applications of nuclear magnetic resonance imaging
Authors: Margulis A. R. , Hricak H. , Crooks L.
Published: March 2009

In the very short time since magnetic resonance imaging (MRI) was born it has gained surprisingly rapid and enthusiastic acceptance and has speedily proliferated, particularly in the United States and Western Europe. Magnetic resonance imaging (MRI) has successfully challenged computed tomography (CT) in all areas of the body where respiratory motion does not degrade the image (Steinberg, 1986). Newer techniques using a multiplicity of approaches are starting to close the gap between CT and MRI, even in the upper abdomen where the effects of respiratory motion are most pronounced. Although MR is already widely clinically applied and is an accepted everyday diagnostic modality in most large medical centres in the United States, it is not a mature modality. It is rapidly evolving, with whole new areas opening to investigation which will vastly broaden its applications.
Microscopic imaging of cells
Authors: Kam Zvi
Published: March 2009

The microworld was revealed to investigators through a glass bead or a hanging water droplet long before optics was understood. The cellular structure of plants was well resolved by such simple magnifying glasses, van Leeuwenhoek, the Dutch merchant and amateur microscopist, was the first to report to the English Royal Society his observations of bacteria with his single-lens microscope in 1665.
Phenomena at the advancing ice–liquid interface: solutes, particles and biological cells
Authors: Körber Christoph
Published: March 2009

Ice formation in aqueous solutions and suspensions involves a number of significant changes and processes in the residual liquid. The resulting effects were described concerning the redistribution of dissolved salts, the behaviour of gaseous solutes and bubble formation, the rejection and entrapment of secondphase particles. This set of conditions is also experienced by biological cells subjected to freezing. The influences of ice formation in that respect and their relevance for cryopreservation were considered as well.

A model of transient heat conduction and solute diffusion with a planar ice front, propagating through a system of finite length was found to be in good agreement with measured salt concentration profiles. The spacing of the subsequently developing columnar solidification pattern was of the same order of magnitude as the pertubation wavelengths predicted from the stability criterion. Non-planar solidification of binary salt solutions was described by a pure heat transfer model under the assumption of local thermodynamic equilibrium.

The rejection of gaseous solutes and the resulting gas concentration profile ahead of a planar ice front has been estimated by means of a test bubble method, yielding a distribution coefficient of 0·05 for oxygen. The nucleation of gas bubbles has been observed to occur at slightly less than 20-fold supersaturation. The subsequent radial growth of the bubbles obeys a square-root time dependence as expected from a diffusion controlled model until the still expanding bubbles become engulfed by the advancing ice-liquid interface. The maximum bubble radii decrease for increasing ice front velocities.

The transition between repulsion and entrapment of spherical latex particles by an advancing planar ice-front has been characterized by a critical value of the velocity of the solidification interface. The critical velocity is inversely proportional to the particle radius as suggested by models assuming an undisturbed ice front. The increase of the critical velocity for increasing thermal gradients shows good agreement with a theoretically predicted square-root type of dependence. Critical velocities have also been measured for yeast and red blood cells.

The effect of freezing on biological cells has been analyzed for human lymphocytes and erythrocytes. The reduction of cell volume observed during non-planar freezing agrees reasonably well with shrinkage curves calculated from a water transport model. The probability of intracellular ice formation has been characterized by threshold cooling rates above which the amount of water remaining within the cell is sufficient for crystallization. The cooling rate dependence of viability exhibits largely different maxima for lymphocytes and erythrocytes at about 30 and 4700 K/min, respectively. The decrease of viability above these values has been attributed to the damaging effect of intracellular ice formation.
Structure and function of inhibitory glycine receptors
Authors: Betz Heinrich
Published: March 2009

Ion transport by peptide channels has been the major theme in the work of the late P. Läuger. His theoretical and experimental approaches provided the basis for a deeper understanding of pore-mediated ion permeation through biological membranes. This review on a ligand–gated ion channel protein from the mammalian brain is dedicated to the memory of this outstanding scientist.
The functional architecture of the acetylcholine nicotinic receptor explored by affinity labelling and site-directed mutagenesis
Authors: Changeux Jean-Pierre , Galzi Jean-Luc , Devillers-Thiéry Anne , Bertrand Daniel
Published: March 2009

The scientific community will remember Peter Läuger as an exceptional man combining a generous personality and a sharp and skilful mind. He was able to attract by his views the interest of a large spectrum of biologists concerned by the mechanism of ion translocation through membranes. Yet, he was not a man with a single technique or theory. Using an authentically multidisciplinary approach, his ambition was to ‘understand transmembrane transport at the microscopic level, to capture its dynamics in the course of defined physiological processes’ (1987). According to him, ‘new concepts in the molecular physics of proteins’ had to be imagined, and ‘the traditional static picture of proteins has been replaced by the notions that proteins represent dynamic structures, subjected to conformational fluctuations covering a very wide time-range’ (1987).
The inorganic biochemistry of molybdoenzymes
Authors: Bray Robert C.
Published: March 2009

Molybednum-containing enzymes (Coughlan, 1980; Spiro, 1985) occupy a significant place in the development of the field now termed inorganic biochemistry. The importance of the metal as a biological trace element depends on its involvement in the known, and perhaps other as yet unknown, molybdoenzymes. That it plays a role in biological nitrogen fixation, the process whereby the enzyme nitrogenase in the root nodules of plants converts atmospheric nitrogen into ammonia, was recognized in the 1930s. The metal is also a constituent of a variety of other enzymes, having first been found in a mammalian enzyme, xanthine oxidase, in the 1950s.
Piezoelectric properties of biological polymers
Authors: Fukada Eiichi
Published: March 2009

Piezoelectricity and pyroelectricity in wool and hair were observed by Martin early in 1941. The piezoelectric effect in wood was investigated in detail by Bazhenov (1961). Both converse and direct effect of shear piezoelectricity in wood was demonstrated by Fukada (1955). Bending piezoelectricity in bone was first discovered by Yasuda (1953). Shear and longitudinal piezoelectricity in bone and tendon was quantitatively investigatged by Fukada & Yasuda (1957, 1964).
Analysis of the domain structure of membranes by fragmentation and separation in aqueous polymer two-phase systems
Authors: Albertsson Per-Åke
Published: March 2009

This paper consists of three parts. The first describes theoretically a general strategy for fragmentation and separation of membranes which can be used in the elucidation of their structure and function. The second part describes a practical separation method, partition in liquid aqueous polymer two-phase systems, which can be used for separation of macromolecules and membrane particles of biological origin. The third part gives examples of the application of this method to membrane vesicles, and how this separation in combination with the strategy described in the first part can be used for analysis of the structure of biological membranes.
The reaction of the psoralens with deoxyribonucleic acid
Authors: Hearst John E. , Isaacs Stephen T. , Kanne David , Rapoport Henry , Straub Kenneth
Published: March 2009

Psoralen photochemistry is specific for nucleic acids and is better understood at the molecular level than are all other methods of chemical modification of nucleic acids. These compounds are used both forin vivostructure analysis and for photochemotherapy since they easily penetrate both cells and virus particles. Apparently, natural selection has selected for membrane and virus penetrability during the evolution of these natural products. Most cells are unaffected by relatively high concentrations of psoralens in the absence of ultraviolet light, and the metabolites of the psoralens have thus far not created a problem. Finally, psoralens form both monoadduct and cross-links in nucleic acid helices, the yield of each being easily controlled by the conditions used during the photochemistry.
Biosynthetic Incorporation of15N and13C for Assignment and Interpretation of Nuclear Magnetic Resonance Spectra of Proteins
Authors: McIntosh Lawrence P. , Dahlquist Frederick W.
Published: March 2009

The use of isotopic substitution is a time-honoured method for simplifying the nuclear magnetic resonance spectra of biological macromolecules. For example, the biosynthetic incorporation of a heteronucleus such as15N or13C into a specific amino acid residue in a protein followed by direct observation of the15N or13C NMR spectrum could provide a means to specifically observe a given amino acid type in that protein. By observation of the chemical shift or relaxation properties as a function of pH, ligand concentration, etc. a number of important conclusions concerning the pKavalues of specific residues, the affinity of the protein for various ligands, or dynamic properties of the protein can be deduced. (See Henryet al.1986a,b; 1987 for an elegant modern example). In such situations, direct observation of the heteronucleus is a powerful means to observe environmental changes (Niuet al.1979) but often these measurements are not readily interpretable in terms of alterations of protein structure. Although proton-proton dipolar interactions (NOEs) typically provide the richest source of such structural information, these interactions are not monitored in most experiments which directly observe the heteronucleus.
Classification of macromolecular assemblies studied as ‘single particles’
Authors: Frank Joachim
Published: March 2009

In order to study proteins that do not occur in two- or three-dimensionally ordered form, one may take two different approaches: either search for conditions that induce the formation of crystals, and proceed with the established methods of X-ray or electron crystallography, or attempt to study the molecules in the form of single particles with the EM. Although many proteins have been successfully crystallized, and some general recipes for inducing ordered arrangement have been found (Mannella, 1984; Uzgiris & Kornberg, 1983), there exists a large number of proteins and protein assemblies that have resisted such attempts for a long time. Furthermore, there are macromolecular assemblies, associated with membranes and engaged in switching or gating, whose function is tied to their occurrence in isolated form, and hence are best studied without extraction from the membrane. For these reasons, the single-particle approach to the study of macromolecular structure (Franket al.1978, 1981; Radermacheret al.1987a, b; for recent reviews, see Franket al.1985, 1988e; Frank, 1989) has found numerous applications after initial technical and conceptual hurdles were overcome. Although atomic resolution cannot be achieved with this approach for a variety of reasons, a quantitative description of architecture on the quaternary level is nevertheless possible, as exemplified in the 3D studies of ribosomal particles (overviews, see Franket al.1988a) and, most recently, the junctional channel complex (Wagenknechtet al.1989a).
Biophysics of the structure and function of porins
Authors: Jap Bing K. , Walian Peter J.
Published: March 2009

Gram-negative bacteria such asEscherichia coli(E. coli) andSalmonella typhimurium(S. typhimurium) have two layers of membranes in the cellular envelope – the cytoplasmic membrane and the outer membrane (Fig. I). Between these membranes is a periplasmic space in which there is a peptidoglycan layer that provides the cells with mechanical rigidity. In this periplasmic space, there are also a variety of hydrolases and binding proteins. The composition of the outer membrane is somewhat unusual. This membrane bilayer is asymmetric, having an inner (periplasmic) leaflet composed of phospholipids and an outer (extracellular) leaflet formed by lipopolysaccharide (LPS). Unlike phospholipids having two acyl chains, LPS has six or seven saturated fatty acid chains (see reviews, Lugtenberg & Van Alphen, 1983; Nikaido & Vaara, 1985; Nakae, 1986). The head groups of LPS have a strong affinity for divalent cations such as Ca2+, and given a sufficient concentration of these ions the outer membrane can form quite a formidable permeability barrier through this head group/salt bridge network (Nikaido & Vaara, 1985). The function of the outer membrane is to serve as a protective envelope against hostile environments such as those in the intestinal tract of animals where harmful and toxic substances - for example, bile salts and various enzymes - are often found. The outer membrane itself would be impermeable to most hydrophilic solutes were it not for the presence of membrane channels. The presence of a large number of pore-forming proteins provides both specific and nonspecific diffusion pathways across the outer membrane for solutes such as nutrients and waste products to diffuse into or out of the cell.
Crystal faces and cleavage planes in quartz as templates in biological processes
Authors: Langer Arthur M.
Published: March 2009

Some species of minerals, when comminuted and aerosolized in the form of respirable-size particulate dusts, are recognized as capable of producing disease in man after their inhalation. Comprehensive listings of such agents are in the current literature (e.g. in Aponte, 1970; Langer & Mackler, 1972; Ehrenreich et al. 1973a, b) and many of these and their effects are well known and receive much attention. Asbestos fibres of all varieties, ampbibole and quartz contaminated talcum powders (see Rohl et al. 1976), fibrous amphiboles and their cleavage fragments which contaminate the ambient environment (e.g., the contamination of Lake Superior with cummingtonite-grunerite, as discussed in Bowes, Langer & Rohl, 1977), have been the subject of recent intensive investigations mostly due to their protential impact to large segments of the general population.
Optical evanescent wave methods for the study of biomolecular interactions
Authors: Garland Peter B.
Published: March 2009

Implementation and regulation of the molecular mechanisms underlying biological processes is dependent on direct interactions between biological molecules. These interactions are characterised by specific binding between at least one molecule and another, and for binding to occur the molecules must be able to come close enough to each other to make contact. One of the partners in the interaction is invariably a macromolecule (e.g. protein or DNA) or an assembly of large size, such as a lipid bilayer. The interaction may take place with the partners in solution, or with at least one attached to a biological surface (e.g. a membrane) or a very large structure such as a chromosome. Where the partners are part of a membrane or other large structure then there must be a mechanism, such as lateral diffusion in the plane of the membrane, that permits them to come together close enough for interaction.
Cruciform structures and functions
Authors: Timsit Youri , Moras Dino
Published: March 2009

In this paper, a structure-function analysis of B-DNA self-fitting is reviewed in the light of recent oligonucleotide crystal structures. Their crystal packings provided a high-resolution view of B-DNA helices closely and specifically fitted by groove-backbone interaction, a natural and biologically relevant manner to assemble B-DNA helices. In revealing that new properties of the DNA molecule emerge during condensation, these crystallographic studies have pointed to the biological importance of DNA—DNA interactions.
On characterizations of sphere-preserving maps
Authors: LI BAOKUI , YAO GUOWU
Published: March 2009

Recently, the first author and Y. Wang proved that $f\colon \rn\to\rn$ (n ≥ 2) is a Möbius transformation if and only if f is a non-degenerate circle-preserving map. In this paper, we will further the result to show that f is a Möbius transformation if and only if f is a non-degenerate r–dimensional sphere-preserving map. The versions for the Euclidean and hyperbolic cases are also obtained. These results make no surjectivity or injectivity or even continuity assumptions on f. Moreover, certain degenerate sphere-preserving maps are given, which completes the characterizations of sphere-preserving maps.
Sepsis und Rhabdomyolyse bei einem Tropenrückkehrer
Authors: Schläpfer  , Imhof
Published: March 2009

Wir beschreiben einen Fall von schwerer Hypophosphatämie mit neurologischen, muskulären und hämatologischen Symptomen. Die symptomatisch relevante akute Hypophosphatämie kommt praktisch nur bei vorgängig phosphatdepletierten schwerkranken Patienten vor und kann schwere Folgen bis hin zum Tod haben. Die Diagnose ergibt sich aus Klinik und Laborwert und sollte eine unverzüglich Substitution zur Folge haben.
Modelling naive T‐cell homeostasis: consequences of heritable cellular lifespan during ageing
Authors: Dowling Mark R , Hodgkin Philip D
Published: March 2009

Within an individual, the population of mature naive T cells is maintained throughout life by both input from the thymus and homeostatic proliferation in the periphery. Here, we develop a mathematical model of this process of naive T‐cell homeostasis, and use it to explore questions of lifespan, inheritance and receptor repertoire during ageing. By assuming lifespan is largely determined by a heritable trait reset on mitosis, we show that homeostatic proliferation leads naturally to a longer lived population with age. A plausible candidate for the heritable trait influencing lifespan is T‐cell receptor affinity for major histocompatibility molecules loaded with self‐peptides. Concurrently with increasing lifespan, receptor diversity decreases with age, thus quantitatively linking these two phenomena. These results depend on the thymus involuting with age so that homeostatic proliferation becomes the dominant mode of replacement of the naive T‐cell repertoire.
Allogeneic stromal cell implantation in brain tissue leads to robust microglial activation
Authors: Tambuyzer Bart R , Bergwerf Irene , De Vocht Nathalie , Reekmans Kristien , Daans Jasmijn , Jorens Philippe G , Goossens Herman , Ysebaert Dirk K , Chatterjee Shyama , Van Marck Eric , Berneman Zwi N , Ponsaerts Peter
Published: March 2009

Although adult and embryonic stem cell‐based therapy for central nervous system (CNS) injury is being developed worldwide, less attention is given to the immunological aspects of allogeneic cell implantation in the CNS. The latter is of major importance because, from a practical point of view, future stem cell‐based therapy for CNS injury will likely be performed using well‐characterised allogeneic stem cell populations. In this study, we aimed to further describe the immunological mechanism leading to rejection of allogeneic bone marrow‐derived stromal cells (BM‐SC) after implantation in murine CNS. For this, we first investigated the impact of autologous and allogeneic BM‐SC on microglia activation in vitro. Although the results indicate that both autologous and allogeneic BM‐SC do not activate microglia themselves in vitro, they also do not inhibit activation of microglia after exogenous stimuli in vitro. Next, we investigated the impact of allogeneic BM‐SC on microglia activation in vivo. In contrast to the in vitro observations, microglia become highly activated in vivo after implantation of allogeneic BM‐SC in the CNS of immune‐competent mice. Moreover, our results suggest that microglia, rather than T‐cells, are the major contributors to allograft rejection in the CNS.
Proton transfer in the mechanism of polyadenylate polymerase
Authors: Balbo Paul B. , Bohm Andrew
Published: March 2009

PAP (polyadenylate polymerase) is the template-independent RNA polymerase responsible for synthesis of the 3′ poly(A) tails of mRNA. To investigate the role of proton transfer in the catalytic mechanism of PAP, the pH dependence of the steady-state kinetic parameters of yeast PAP were determined for the forward (adenyl transfer) and reverse (pyrophosphorolysis) reactions. The results indicate that productive formation of an enzyme–RNA–MgATP complex is pH independent over a broad pH range, but that formation of an active enzyme–RNA–MgPPi complex is strongly pH dependent, consistent with the production of a proton on the enzyme in the forward reaction. The pH dependence of the maximum velocity of the forward reaction suggests two protonic species are involved in enzyme catalysis. Optimal enzyme activity requires one species to be protonated and the other deprotonated. The deuterium solvent isotope effect on Vmax is also consistent with proton transfer involved in catalysis of a rate-determining step. Finally, pKa calculations of PAP were performed by the MCCE (multiconformational continuum electrostatic) method. Together, the data support that the protonation of residues Lys215 and Tyr224 exhibit co-operativity that is important for MgATP2− and MgPPi2− binding/dissociation, and suggest these residues function in electrostatic, but not in general acid, catalysis.
GlcNAcstatins are nanomolar inhibitors of human O-GlcNAcase inducing cellular hyper-O-GlcNAcylation
Authors: Dorfmueller Helge C. , Borodkin Vladimir S. , Schimpl Marianne , van Aalten Daan M. F.
Published: March 2009

O-GlcNAcylation is an essential, dynamic and inducible post-translational glycosylation of cytosolic proteins in metazoa and can show interplay with protein phosphorylation. Inhibition of OGA (O-GlcNAcase), the enzyme that removes O-GlcNAc from O-GlcNAcylated proteins, is a useful strategy to probe the role of this modification in a range of cellular processes. In the present study, we report the rational design and evaluation of GlcNAcstatins, a family of potent, competitive and selective inhibitors of human OGA. Kinetic experiments with recombinant human OGA reveal that the GlcNAcstatins are the most potent human OGA inhibitors reported to date, inhibiting the enzyme in the sub-nanomolar to nanomolar range. Modification of the GlcNAcstatin N-acetyl group leads to up to 160-fold selectivity against the human lysosomal hexosaminidases which employ a similar substrate-assisted catalytic mechanism. Mutagenesis studies in a bacterial OGA, guided by the structure of a GlcNAcstatin complex, provides insight into the role of conserved residues in the human OGA active site. GlcNAcstatins are cell-permeant and, at low nanomolar concentrations, effectively modulate intracellular O-GlcNAc levels through inhibition of OGA, in a range of human cell lines. Thus these compounds are potent selective tools to study the cell biology of O-GlcNAc.