39 research outputs found
The low-density/high-density liquid phase transition for model globular proteins
The effect of molecule size (excluded volume) and the range of interaction on
the surface tension, phase diagram and nucleation properties of a model
globular protein is investigated using a combinations of Monte Carlo
simulations and finite temperature classical Density Functional Theory
calculations. We use a parametrized potential that can vary smoothly from the
standard Lennard-Jones interaction characteristic of simple fluids, to the ten
Wolde-Frenkel model for the effective interaction of globular proteins in
solution. We find that the large excluded volume characteristic of large
macromolecules such as proteins is the dominant effect in determining the
liquid-vapor surface tension and nucleation properties. The variation of the
range of the potential only appears important in the case of small excluded
volumes such as for simple fluids. The DFT calculations are then used to study
homogeneous nucleation of the high-density phase from the low-density phase
including the nucleation barriers, nucleation pathways and the rate. It is
found that the nucleation barriers are typically only a few and that
the nucleation rates substantially higher than would be predicted by Classical
Nucleation Theory.Comment: To appear in Langmui
Dynamics of Bacteriorhodopsin in Solid-Supported Purple Membranes Studied with Tapping-Mode Atomic Force Microscopy
Structural Changes in Bacteriorhodopsin Caused by Two-Photon-Induced Photobleaching
Bacteriorhodopsin (BR) is the key protein of the halobacterial photosynthetic system. BR assembles into two-dimensional crystalline patches, the so-called purple membranes (PM), and acts as a light-driven proton pump converting light energy into the chemical energy of a proton gradient over the cell membrane. The two-photon absorption (TPA) of BR is so far not fully understood. Astonishingly high TPA cross sections have been reported, but the molecular mechanisms have not been elucidated. In this work, we address structural changes in BR and PM upon TPA, investigating its TPA photochemistry by spectroscopy, small-angle X-ray scattering, as well as electron and atomic force microscopy. We observe that TPA of BR leads to formation of an UV-absorbing N-retinyl-bacterioopsin state, which is accompanied by the loss of crystalline order in PM. FTIR and CD spectroscopy confirm that BR trimers as well as the secondary structure of the BR molecules are preserved. We demonstrate that excitation by TPA results in the photochemical reduction of the retinal Schiff base, which in turn causes a permanent asymmetric shape change of BR, similar to the one transiently observed during the photocycle-related opening and closing of the cytoplasmic proton half channel. This shape change causes PM sheets to merely roll up toward the extracellular side and causes the loss of crystallinity of PM. We present a model for the TPA photoresponse of BR, which also explains the irreversibility of the process in terms of a photochemical reduction of the Schiff base
Crystallinity of Purple Membranes Comprising the Chloride-Pumping Bacteriorhodopsin Variant D85T and Its Modulation by pH and Salinity
Controlled growth of periodically aligned copper-silicide nanocrystal arrays on silicon directed by laser-induced periodic surface structures (LIPSS)
Assembly of poly(methacrylate)/purple membrane lamellar nanocomposite films by intercalation and in situ polymerisation
High-Temperature Spray-Dried Polymer/Bacteria Microparticles for Electrospinning of Composite Nonwovens
Living Micrococcus luteus (M. luteus) and Escherichia coli (E. coli) are encapsulated in poly(vinyl alcohol), poly(vinylpyrrolidone), hydroxypropyl cellulose, and gelatin by high-temperature spray drying. The challenge is the survival of the bacteria during the standard spray-drying process at temperatures of 150 °C (M. luteus) and 120 °C (E. coli). Raman imaging and transmission electron microscopy indicate encapsulated bacteria in hollow composite microparticles. The versatility of the spray-dried polymer bacteria microparticles is successfully proved by standard polymer solution-processing techniques such as electrospinning, even with harmful solvents, to water-insoluble polyacrylonitrile, polystyrene, poly(methyl methacrylate), and poly(vinyl butyrate) nanofiber nonwovens, which opens numerous new opportunities for novel applications
Interactions by the Fungal Flo11 Adhesin Depend on a Fibronectin Type III Like Adhesin Domain Girdled by Aromatic Bands
Towards an optimum design for electrostatic phase plates
Charging of physical phase plates is a problem that has prevented their routine use in transmission electron microscopy of weak-phase objects. In theory, electrostatic phase plates are superior to thin-film phase plates since they do not attenuate the scattered electron beam and allow freely adjustable phase shifts. Electrostatic phase plates consist of multiple layers of conductive and insulating materials, and are thus more prone to charging than thin-film phase plates, which typically consist of only one single layer of amorphous material. We have addressed the origins of charging of Boersch phase plates and show how it can be reduced. In particular, we have performed simulations and experiments to analyze the influence of the insulating Si3N4 layers and surface charges on electrostatic charging. To optimize the performance of electrostatic phase plates, it would be desirable to fabricate electrostatic phase plates, which (i) impart a homogeneous phase shift to the unscattered electrons, (ii) have a low cut-on frequency, (iii) expose as little material to the intense unscattered beam as possible, and (iv) can be additionally polished by a focused ion-beam instrument to eliminate carbon contamination accumulated during exposure to the unscattered electron beam (Walter et al., 2012, Ultramicroscopy, 116, 62–72). We propose a new type of electrostatic phase plate that meets the above requirements and would be superior to a Boersch phase plate. It consists of three free-standing coaxial rods converging in the center of an aperture (3-fold coaxial phase plate). Simulations and preliminary experiments with modified Boersch phase plates indicate that the fabrication of a 3-fold coaxial phase plate is feasible