Polyelectrolyte multilayers with a tunable Young's modulus: influence of film stiffness on cell adhesion

Mechanical properties of model and natural gels have recently been demonstrated to play an important role in various cellular processes such as adhesion, proliferation, and differentiation, besides events triggered by chemical ligands. Understanding the biomaterial/cell interface is particularly important in many tissue engineering applications and in implant surgery. One of the final goals would be to control cellular processes precisely at the biomaterial surface and to guide tissue regeneration. In this work, we investigate the substrate mechanical effect on cell adhesion for thin polyelectrolyte multilayer (PEM) films, which can be easily deposited on any type of material. The films were cross linked by means of a water-soluble carbodiimide (EDC), and the film elastic modulus was determined using the AFM nanoindentation technique with a colloidal probe. The Young's modulus could be varied over 2 orders of magnitude (from 3 to 400 kPa) for wet poly(L-lysine)/hyaluronan (PLL/HA) films by changing the EDC concentration. The chemical changes upon cross linking were characterized by means of Fourier transform infrared spectroscopy (FTIR). We demonstrated that the adhesion and spreading of human chondrosarcoma cells directly depend on the Young's modulus. These data indicate that, besides the chemical properties of the polyelectrolytes, the substrate mechanics of PEM films is an important parameter influencing cell adhesion and that PEM offer a new way to prepare thin films of tunable mechanical properties with large potential biomedical applications including drug release.     

A distorted cubic tetranuclear copper(II) phosphonate cage with a double-four-ring-type core

The reaction of Cu2(O2CMe)(4).2H2O with tert-butylphosphonic acid and 3,5-di-tert-butylpyrazole in the presence of triethylamine leads to a high-yield synthesis of the tetranuclear compound [Cu2(3,5-t-Bu2PzH)2(t-BuPO3)2]2 (1). The latter has a distorted cubic cage structure and its core resembles the D4R (double-four-ring) motif found in zeolites. The phosphonate, [t-BuPO3]2-, functions as a dianionic tridentate ligand, while the pyrazole ligands are neutral and are monodentate. The coordination geometry at each copper atom is distorted square planar with a 3O,1N coordination environment. Magnetic measurements on 1 reveal that the chiT product continuously decreases to reach a value very close to zero at 1.8 K, indicating dominant antiferromagnetic interactions between Cu(II) ions that leads to an S=0 ground state. The tetranuclear cage 1 functions as a very effective artificial nuclease in the presence of an external oxidant, magnesium monoperoxyphthalate.     

Radical Anions of Oxidized vs. Reduced Oxytocin: Influence of Disulfide Bridges on CID and Vacuum UV Photo-Fragmentation

The nonapeptide oxytocin (OT) is used as a model sulfur-containing peptide to study the damage induced by vacuum UV (VUV) radiations. In particular, the effect of the presence (or absence in reduced OT) of oxytocin’s internal disulfide bridge is evaluated in terms of photo-fragmentation yield and nature of the photo-fragments. Intact, as well as reduced, OT is studied as dianions and radical anions. Radical anions are prepared and photo-fragmented in two-color experiments (UV?+?VUV) in a linear ion trap. VUV photo-fragmentation patterns are analyzed and compared, and radical-induced mechanisms are proposed. The effect of VUV is principally to ionize but secondary fragmentation is also observed. This secondary fragmentation seems to be considerably enabled by the initial position of the radical on the molecule. In particular, the possibility to form a radical on free cysteines seems to increase the susceptibility to VUV fragmentation. Interestingly, disulfide bridges, which are fundamental for protein structure, could also be responsible for an increased resistance to ionizing radiations.

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Graphical Abstract

     

Entropic stabilization of isolated beta-sheets

Temperature-dependent electric deflection measurements have been performed for a series of unsolvated alanine-based peptides (Ac-WA(n)-NH(2), where Ac = acetyl, W = tryptophan, A = alanine, and n = 3, 5, 10, 13, and 15). The measurements are interpreted using Monte Carlo simulations performed with a parallel tempering algorithm. Despite alanine's high helix propensity in solution, the results suggest that unsolvated Ac-WA(n)-NH(2) peptides with n > 10 adopt beta-sheet conformations at room temperature. Previous studies have shown that protonated alanine-based peptides adopt helical or globular conformations in the gas phase, depending on the location of the charge. Thus, the charge more than anything else controls the structure.     

Quantitative SIMS-analysis of erbium profiles in LiNbO3

Modern fiberoptic communication technology uses light of 1.5 m wavelength and Er³⁺ is the laser active ion for this wavelength. Doping of crystalline LiNbO₃ (an electrooptical material) with erbium ions permits the fabrication of signal-amplifying electrooptic devices. Novel results of three different approaches have been presented to dope the near surface area of LiNbO₃ for its application in planar optoelectronics: erbium indiffusion from the surface, implantation of erbium into LiNbO₃ and subsequent annealing schemes, and the homoepitaxial growth of Er-doped LiNbO₃ on LiNbO₃ single crystalline material by a laser deposition method. These experiments are not only useful for creating integrated optical devices with active amplifying functions, but they are also important examples for fabricating and studying novel thin ferroelectric films. Secondary ion mass spectrometry (SIMS) has been employed as the main analytical tool for quantitative determination of the erbium concentration profiles.     

An S = 6 cyanide-bridged octanuclear FeIII4NiII4 complex that exhibits slow relaxation of the magnetization

The synthesis and structural and magnetic characterization of an S = 6 cyanide-bridged octanuclear FeIII4NiII4 (1) complex is described. Ac susceptibility and mu-SQUID measurements suggest that fast magnetization relaxation is present in zero-field due to quantum tunneling of the ground spin state (QTM) while application of small magnetic fields induces slow relaxation of the magnetization.     

Fixed-rank matrix factorizations and Riemannian low-rank optimization

Motivated by the problem of learning a linear regression model whose parameter is a large fixed-rank non-symmetric matrix, we consider the optimization of a smooth cost function defined on the set of fixed-rank matrices. We adopt the geometric framework of optimization on Riemannian quotient manifolds. We study the underlying geometries of several well-known fixed-rank matrix factorizations and then exploit the Riemannian quotient geometry of the search space in the design of a class of gradient descent and trust-region algorithms. The proposed algorithms generalize our previous results on fixed-rank symmetric positive semidefinite matrices, apply to a broad range of applications, scale to high-dimensional problems, and confer a geometric basis to recent contributions on the learning of fixed-rank non-symmetric matrices. We make connections with existing algorithms in the context of low-rank matrix completion and discuss the usefulness of the proposed framework. Numerical experiments suggest that the proposed algorithms compete with state-of-the-art algorithms and that manifold optimization offers an effective and versatile framework for the design of machine learning algorithms that learn a fixed-rank matrix.     

Simulation of High‐Frequency Atmospheric Gas Discharges

The generation of atmospheric gas discharges is an important concern in many industrial applications. We are investigating here a special type of high‐frequency atmospheric gas discharge. The results of modelling the breakdown mechanism and of the development phase of the discharge will be presented. Furthermore we will show first numerical results. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)