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1.
Highly oriented solid-supported lipid membranes in stacks of controlled number N ≃ 16 (oligo-membranes) have been prepared by spin-coating using the uncharged lipid model system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The samples have been immersed in aqueous polymer solutions for control of osmotic pressure and have been studied by X-ray reflectivity. The bilayer structure and fluctuations have been determined by modelling the data over the full q-range. Thermal fluctuations are described using the continuous smectic Hamiltonian with the appropriate boundary conditions at the substrate and at the free surface of the stack. The resulting fluctuation amplitudes and the pressure-distance relation are discussed in view of the inter-bilayer potential.  相似文献   

2.
We present the experimental and theoretical background of a method to characterize the protein-protein attractive potential induced by one of the mostly used crystallizing agents in the protein-field, the poly(ethylene glycol) (PEG). This attractive interaction is commonly called, in colloid physics, the depletion interaction. Small-Angle X-ray Scattering experiments and numerical treatments based on liquid-state theories were performed on urate oxidase-PEG mixtures with two different PEGs (3350 Da and 8000 Da). A “two-component” approach was used in which the polymer-polymer, the protein-polymer and the protein-protein pair potentials were determined. The resulting effective protein-protein potential was characterized. This potential is the sum of the free-polymer protein-protein potential and of the PEG-induced depletion potential. The depletion potential was found to be hardly dependent upon the protein concentration but strongly function of the polymer size and concentration. Our results were also compared with two models, which give an analytic expression for the depletion potential. Received 29 April 2002 RID="a" ID="a"Present address: CRMC2-CNRS, Campus de Luminy, case 913, F-13288 Marseille Cedex 09, France; e-mail: vivares@crmc2.univ-mrs.fr RID="b" ID="b"e-mail: bonnete@crmc2.univ-mrs.fr RID="c" ID="c"Laboratory associated to Universities Aix-Marseille II and III.  相似文献   

3.
Growth kinetics of polymer crystals in bulk   总被引:1,自引:0,他引:1  
Temperature-dependent measurements of spherulite growth rates carried out for i-polystyrene, poly(ε -caprolactone) and linear polyethylene show that the controlling activation barrier diverges at a temperature which is 14K, 22K and 12K, respectively, below the equilibrium melting points. We discuss the existence of such a “zero growth temperature” T zg in the framework of a recently introduced thermodynamic multiphase scheme and identify T zg with the temperature of a (hidden) transition between the melt and a mesomorphic phase which mediates the crystal growth. The rate-determining step in our model of crystal growth is the attachment of chain sequences from the melt onto the lateral face of a mesomorphic layer at the growth front. The necessary straightening of the sequence prior to an attachment is the cause of the activation barrier. A theory based on this view describes correctly the observations. With a knowledge of T zg it is possible to fully establish the nanophase diagram describing the stability ranges of crystalline and mesomorphic layers in a melt. An evaluation of data from small-angle X-ray scattering, calorimetry and optical growth rate measurements yields heats of transition and surface free energies of crystals and mesophase layers, as well as the activation barrier per monomer associated with the chain stretching. According to the theory, the temperature dependence of the crystallization rate is determined by both the activation energy per monomer and the surface free energy of the preceding mesomorphic layer. Data indicate that the easiness of crystallization in polyethylene is first of all due to a particularly low surface free energy of the mesomorphic layer.  相似文献   

4.
We first review the results of an analysis of ππ interactions in S, P and D waves for the two-pion effective mass from threshold to about 1.4GeV. In particular, we show a recent improvement of this analysis above the Kˉ threshold using more data for phase shifts and including the S0-wave inelasticity from ππKˉ. In addition, we have improved the fit to the f 2(1270)-resonance and used a more flexible P-wave parametrization above the Kˉ threshold and included an estimation of the D2-wave inelasticity. The better accuracy thus achieved also required a refinement of the Regge analysis above 1.42GeV. Finally, in this work we check that the ππ scattering amplitudes obtained in this approach satisfy remarkably well forward dispersion relations and Roy's equations.  相似文献   

5.
We studied biomembrane adhesion using the micropipet aspiration technique. Adhesion was caused by contact site A, a laterally mobile and highly specific cell adhesion molecule from Dictyostelium discoideum, reconstituted in lipid vesicles of DOPC (L-α-dioleoylphosphatidylcholine) with an addition of 5 mol % DOPE-PEG2000 (1,2-diacyl-sn-glycero-3-phosphatidylethanolamine-N-[poly(ethyleneglycol) 2000]). The “fuzzy” membrane mimics the cellular plasma membrane including the glycocalyx. We found adhesion and subsequent receptor migration into the contact zone. Using membrane tension jumps to probe the equation of state of the two-dimensional “gas” of bound receptor pairs within the contact zone, we found strong, attractive lateral interactions. Received 16 February 2001  相似文献   

6.
We have investigated the structure of solid-supported, multilamellar membranes by X-ray reflectivity. The density profile is obtained by fitting the full q-range to a model using the bilayer Fourier coefficients as fitting parameters. The effect of hydration and the substrate boundary condition are discussed in view of the well-known Landau-Peierls effect and its implications for structure determination. The resulting bilayer density profile agrees remarkably well with previously published data of a molecular dynamics (MD) simulation for 1,2-oleoyl-palmitoyl-sn-glycero-3-phosphocholine (OPPC). Received 1 October 2001 and Received in final form 21 December 2001  相似文献   

7.
Hard exclusive production of mesons in deep-inelastic scattering allows one to probe the so-far unknown Generalized Parton Distributions (GPDs) of the nucleon. The HERMES experiment has measured several different observables in exclusive meson production by scattering the 27.6GeV HERA lepton beam off an internal fixed gaseous target. Recent results on exclusive π +, ρ 0 and pion pair production will be presented.  相似文献   

8.
9.
The dynamic structure factor of Li with [110] and 0.88 a.u. <q< 1.03 a.u., as measured with 1 eV resolution by means of synchrotron radiation based inelastic X-ray scattering spectroscopy (IXSS), exhibits, in the energy loss range between 3 and 12 eV, a fine structure, which appears as a resonance around 4 eV and an antiresonance around 8 eV, when the difference between the experimental -spectra with [110] and [111] is considered. In order to find out the origin of this fine structure we have interpreted recent TLDA (time dependent local density approximation) calculations of the Li- [#!ref12!#], which were based on the inversion of the full dielectric matrix, by utilizing a simple two-plasmon-band model. In this way the fine structure can be traced back to a Fano-like coupling of the discrete collective excitations (both the regular plasmon and the so-called zone-boundary collective states (ZBCS's)) and the particle-hole excitation continuum, mediated by the off-diagonal elements of the dielectric matrix, , where (1,1,0). Received: 29 December 1998 / Accepted: 16 March 1998  相似文献   

10.
A combined experimental-theoretical approach to modeling of building blocks of recognition complexes formed by nucleic acid bases and the amino-acids side-chain amino group is reviewed. The approach includes the temperature dependent field-ionization mass spectrometry and ab initio quantum chemical calculations. The mass spectrometric technique allows determination of interaction enthalpies of biomolecules in the gas phase, and the results it produces are directly comparable to the results obtained through theoretical modeling. In our works we have analyzed both thermodynamic and structural aspects of the recognition complexes of four canonical nucleic acid bases and acrylamide, which models the side chain of asparagine and glutamine. It has been shown that all bases can interact with amide group of the amino acids via their Watson-Crick sites when being incorporated into a single strand DNA or RNA. Stability of the complexes studied, expressed as - ΔH (kJ mole-1) decreases as: m9Gua (- 59.5) > m 1Cyt (- 57.0) > m 9Ade (- 52.0) ≫m 1Ura (- 40.6). We have determined that in the double stranded DNA only purine bases can be recognized. Received 5 February 2002 and Received in final form 14 March 2002 Published online 13 September 2002  相似文献   

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