Flexural pivots for space applications

Design and fabrication of flexible pivots for aerospace structure

Lipid conformation in model membranes and biological membranes

Protein molecules in solution or in protein crystals are characterized by rather well-defined structures in which α-helical regions, β-pleated sheets, etc., are the key features. Likewise, the double helix of nucleic acids has almost become the trademark of molecular biology as such. By contrast, the structural analysis of lipids has progressed at a relatively slow pace. The early X-ray diffraction studies by V. Luzzati and others firmly established the fact that the lipids in biological membranes are predominantly organized in bilayer structures (Luzzati, 1968). V. Luzzati was also the first to emphasize the liquid-like conformation of the hydrocarbon chains, similar to that of a liquid paraffin, yet with the average orientation of the chains perpendicular to the lipid-water interface. This liquid-crystalline bilayer is generally observed in lipid-water systems at sufficiently high temperature and water content, as well as in intact biological membranes under physiological conditions (Luzzati & Husson, 1962; Luzzati, 1968; Tardieu, Luzzati & Reman, 1973; Engelman, 1971; Shipley, 1973). In combination with thermodynamic and other spectroscopic observations these investigations culminated in the formulation of the fluid mosaic model of biological membranes (cf. Singer, 1971). However, within the limits of this model the exact nature of lipid conformation and dynamics was immaterial, the lipids were simply pictured as circles with two squiggly lines representing the polar head group and the fatty acyl chains, respectively. No attempt was made to incorporate the well-established chemical structure into this picture. Similarly, membrane proteins were visualized as smooth rotational ellipsoids disregarding the possibility that protruding amino acid side-chains and irregularities of the backbone folding may create a rather rugged protein surfac

Chaos, Coherence and the Double-Slit Experiment

We investigate the influence that classical dynamics has on interference patterns in coherence experiments. We calculate the time-integrated probability current through an absorbing screen and the conductance through a doubly connected ballistic cavity, both in an Aharonov-Bohm geometry with forward scattering only. We show how interference fringes in the probability current generically disappear in the case of a chaotic system with small openings, and how they may persist in the case of an integrable cavity. Simultaneously, the typical, sample dependent amplitude of the flux-sensitive part $g(\phi)$ of the conductance survives in all cases, and becomes universal in the case of a chaotic cavity. In presence of dephasing by fluctuations of the electric potential in one arm of the Aharonov-Bohm loop, we find an exponential damping of the flux-dependent part of the conductance, $g(\phi) \propto \exp[-\tau_{\rm L}/\tau_\phi]$, in term of the traversal time $\tau_{\rm L}$ through the arm and the dephasing time $\tau_\phi$. This extends previous works on dephasing in ballistic systems to the case of many conducting channels.Comment: 8 pages, 4 figures in .eps format; Final version, to appear in Physical Review

Deuterium magnetic resonance: theory and application to lipid membranes

Proton and carbon-13 nmr spectra of unsonicated lipid bilayers and biological membranes are generally dominated by strong proton-proton and proton-carbon dipolar interactions. As a result the spectra contain a large number of overlapping resonances and are rather difficult to analyse. Nevertheless, important information on the structure and dynamic behaviour of lipid systems has been provided by these techniques (Wennerström & Lindblom, 1977

Planung von Biotopverbundsystemen im Tiefland

Für die Entwicklung eines landesweiten Biotopverbundes in Sachsen-Anhalt gibt die Landschaftsgliederung des Landes [71] den Rahmen vor. Entsprechend dieser Landschaftsgliederung umfasst der als Tiefland bezeichnete Landschaftsraum alle Landschaftseinheiten „am Südrand des Tieflandes“ sowie die Landschaftseinheiten „Werbener, Tangermünder und Dessauer Elbetal“, „Ohreniederung“, „Muldetal“, „Drömling“, „Rhin-Havel-Luch“ und „Fiener Bruch“, die zu den „Flusstälern und Niederungslandschaften“ gehören. Die Landschaftseinheiten des Tieflandes sind durch zusammenhängende Fließgewässersysteme und großflächige Lebensraumkomplexe geprägt, die meist von überregionaler Bedeutung sind. Fließgewässersysteme eignen sich in besonderer Weise zum Aufbau eines ÖVS, da sie als lineare Strukturen unterschiedliche Landschaften durchqueren und damit verschiedenartige Lebensräume verbinden und zugleich als weit verzweigtes Netz für die überregionale bis örtliche Planungsebene geeignete Grundbausteine liefern

Robustness and modularity properties of a non-covalent DNA catalytic reaction

The biophysics of nucleic acid hybridization and strand displacement have been used for the rational design of a number of nanoscale structures and functions. Recently, molecular amplification methods have been developed in the form of non-covalent DNA catalytic reactions, in which single-stranded DNA (ssDNA) molecules catalyze the release of ssDNA product molecules from multi-stranded complexes. Here, we characterize the robustness and specificity of one such strand displacement-based catalytic reaction. We show that the designed reaction is simultaneously sensitive to sequence mutations in the catalyst and robust to a variety of impurities and molecular noise. These properties facilitate the incorporation of strand displacement-based DNA components in synthetic chemical and biological reaction networks

Catalyzed relaxation of a metastable DNA fuel

Practically all of life's molecular processes, from chemical synthesis to replication, involve enzymes that carry out their functions through the catalytic transformation of metastable fuels into waste products. Catalytic control of reaction rates will prove to be as useful and ubiquitous in nucleic-acid-based engineering as it is in biology. Here we report a metastable DNA "fuel" and a corresponding DNA "catalyst" that improve upon the original hybridization-based catalyst system (Turberfield et al. Phys. Rev. Lett. 90, 118102-1118102-4) by more than 2 orders of magnitude. This is achieved by identifying and purifying a fuel with a kinetically trapped metastable configuration consisting of a "kissing loop" stabilized by flanking helical domains; the catalyst strand acts by opening a helical domain and allowing the complex to relax to its ground state by a multistep pathway. The improved fuel/catalyst system shows a roughly 5000-fold acceleration of the uncatalyzed reaction, with each catalyst molecule capable of turning over in excess of 40 substrates. With k_(cat)/K_M ≈ 10^7/M/min, comparable to many protein enzymes and ribozymes, this fuel system becomes a viable component enabling future DNA-based synthetic molecular machines and logic circuits. As an example, we designed and characterized a signal amplifier based on the fuel-catalyst system. The amplifier uses a single strand of DNA as input and releases a second strand with unrelated sequence as output. A single input strand can catalytically trigger the release of more than 10 output strands

Decoherence in ballistic mesoscopic interferometers

We provide a theoretical explanation for two recent experiments on decoherence of Aharonov-Bohm oscillations in two- and multi-terminal ballistic rings. We consider decoherence due to charge fluctuations and emphasize the role of charge exchange between the system and the reservoir or nearby gates. A time-dependent scattering matrix approach is shown to be a convenient tool for the discussion of decoherence in ballistic conductors.Comment: 11 pages, 3 figures. To appear in a special issue on "Quantum Computation at the Atomic Scale" in the Turkish Journal of Physic

The Role of Size and Charge for Blood-Brain Barrier Permeation of Drugs and Fatty Acids

The lipid bilayer is the diffusion barrier of biological membranes. Highly protective membranes such as the blood-brain barrier (BBB) are reinforced by ABC transporters such as P-glycoprotein (MDR1, ABCB1) and multidrug resistance associated proteins (MRPs, ABCCs). The transporters bind their substrates in the cytosolic lipid bilayer leaflet before they reach the cytosol and flip them to the outer leaflet. The large majority of drugs targeted to the central nervous system (CNS) are intrinsic substrates of these transporters. Whether an intrinsic substrate can cross the BBB depends on whether passive influx is higher than active efflux. In this paper, we show that passive influx can be estimated quantitatively on the basis of Stokesian diffusion, taking into account the ionization constant and the cross-sectional area of the molecule in its membrane bond conformation, as well as the lateral packing density of the membrane. Active efflux by ABC transporters was measured. The calculated net flux is in excellent agreement with experimental results. The approach is exemplified with several drugs and fatty acid analogs. It shows that compounds with small cross-sectional areas (A D  70Å2) or highly charged compounds show higher efflux than influx. They cannot cross the BBB and are, thus, apparent substrates for ABC transporters. The strict size and charge limitation for BBB permeation results from the synergistic interaction between passive influx and active efflu

Origins of Protein Functions in Cells

In modern organisms proteins perform a majority of cellular functions, such as chemical catalysis, energy transduction and transport of material across cell walls. Although great strides have been made towards understanding protein evolution, a meaningful extrapolation from contemporary proteins to their earliest ancestors is virtually impossible. In an alternative approach, the origin of water-soluble proteins was probed through the synthesis and in vitro evolution of very large libraries of random amino acid sequences. In combination with computer modeling and simulations, these experiments allow us to address a number of fundamental questions about the origins of proteins. Can functionality emerge from random sequences of proteins? How did the initial repertoire of functional proteins diversify to facilitate new functions? Did this diversification proceed primarily through drawing novel functionalities from random sequences or through evolution of already existing proto-enzymes? Did protein evolution start from a pool of proteins defined by a frozen accident and other collections of proteins could start a different evolutionary pathway? Although we do not have definitive answers to these questions yet, important clues have been uncovered. In one example (Keefe and Szostak, 2001), novel ATP binding proteins were identified that appear to be unrelated in both sequence and structure to any known ATP binding proteins. One of these proteins was subsequently redesigned computationally to bind GTP through introducing several mutations that introduce targeted structural changes to the protein, improve its binding to guanine and prevent water from accessing the active center. This study facilitates further investigations of individual evolutionary steps that lead to a change of function in primordial proteins. In a second study (Seelig and Szostak, 2007), novel enzymes were generated that can join two pieces of RNA in a reaction for which no natural enzymes are known. Recently it was found that, as in the previous case, the proteins have a structure unknown among modern enzymes. In this case, in vitro evolution started from a small, non-enzymatic protein. A similar selection process initiated from a library of random polypeptides is in progress. These results not only allow for estimating the occurrence of function in random protein assemblies but also provide evidence for the possibility of alternative protein worlds. Extant proteins might simply represent a frozen accident in the world of possible proteins. Alternative collections of proteins, even with similar functions, could originate alternative evolutionary paths