Horospherical transform on real symmetric varieties: Kernel and cokernel

In this paper, we define a horospherical transform for a semisimple symmetric space Y. A natural double fibration is used to assign a more geometrical space Ξ of horospheres to Y. The horospherical transform relates certain integrable analytic functions on Y to analytic functions on Ξ by fiber integration. We determine the kernel of the horospherical transform and establish that the transform is injective on functions belonging to the most continuous spectrum of Y.     

Perturbation theorems for holomorphic semigroups

The concept of the gap function is used to give new perturbation results for generators of holomorphic semigroups. In particular, we show that if A is the generator of a holomorphic semigroup on a Banach space and , then every closed linear operator C such that for some and

Structure and Bonding in First‐Row Transition‐Metal Dicarbides: Are They Related to the Stability of Met‐cars?

First-row transition-metal dicarbides MC(2) (M=Sc-Zn) have been investigated by using quantum-mechanical techniques. The competition between cyclic and linear isomers in these systems has been studied and the bonding scheme for these compounds is discussed through topological analysis of electron density. All of the systems have been found to prefer a C(2v)-symmetric arrangement, although for ZnC(2) the energy difference between this and the linear isomer is rather small. In most cases the C(2v)-symmetric structure corresponds to a T-shaped structure, with the exceptions of TiC(2), CoC(2), and NiC(2) which have been shown to be true rings. A detailed analysis of the variation of the energy of the system with geometry has been carried out. An analysis of the bonding, taking into account the main interactions between the valence orbitals of both fragments, the M atom and the C(2) molecule, has allowed the main features of these compounds to be interpreted. A clear correlation between the dissociation energies of the first-row transition-metal dicarbides and the bonding energies of the corresponding met-cars was observed.     

Surface shear rheological studies of marine phytoplankton cultures-Nitzschia closterium, Thalassiosira rotula, Thalassiosira punctigera and Phaeocystis sp

The study of interfacial properties in the marine environment is important for the understanding of air-sea gas exchange processes, especially with respect to the behaviour of entrained air bubbles. Seawater contains surfactant material, much of which is thought to origin from the exudation of dissolved organic material (DOM) by phytoplankton. This study aims at investigating the influence of different phytoplankton species on the surface shear viscosity of an air-water interface. Measurements of surface shear viscosity were carried out with the ISR1 interfacial shear rheometer. Surface shear viscosities of stock cultures of Phaeocystis sp., Thalassiosira rotula, Thalassiosira punctigera and Nitzschia closterium as well as of F/2 nutrient medium and seawater were measured. The surface shear viscosity of N. closterium was investigated during different stages of its growth as well as for an unfiltered stock culture sample and its filtrate. Results reveal that the influence of phytoplankton on the surface shear viscosity is species specific. An increase in surface shear viscosity occurred for the N. closterium stock culture only. The remaining cultures showed similar behaviour to F/2 nutrient medium. The increase of surface shear viscosity during the growth of N. closterium occurred mainly during the exponential growth phase. The increases in surface shear viscosity depend on the presence of phytoplankton cells in the sample. The formation of compact mechanical structures at the air-water interface originating from the aggregation of DOM released by N. closterium as a cause for the observed increases in surface shear viscosity is discussed.     

Molecular assembly of metallacarboranes in water: light scattering and microscopy study

Polyhedral metallacarboranes are used mainly as ion-pairing agents and recently have been recognized as potent inhibitors of HIV protease. They are characterized by exceptional hydrophobicity, rigid geometry, delocalized negative charge, ion-pairing behavior, and strong acidity of their conjugated acids. The completely novel phenomenon, association of these promising pharmaceutical tectons in aqueous solutions, is described here. The behavior of two structural types of metallacarboranes, [bis(1,2-dicarbollide)cobaltate(1-)] and bis[(3)-1,2-dicarbollylcobalt]-(3,6)-1,2-dicarbacanastide(2-)], in aqueous solution was studied by a combination of static and dynamic light scattering and microscopy methods. Spherical aggregates with radii of ca. 100 nm and fairly monodisperse nanostructures were found in aqueous solutions. The behavior of nanoaggregates is fairly complex and depends on the concentration and aging of the solutions. The particles are stabilized in the solution by counterions. The formation of larger clusters upon dilution of bis(1,2-dicarbollide)cobaltate(1-) solutions was observed. The secondary aggregation can be suppressed by addition of NaCl. Gel permeation chromatography measurements of sodium bis(1,2-dicarbollide)cobaltate(1-) show that the majority of matallacarborane molecules form nanoaggregates and only a small amount of the metallacarborane remains molecularly soluble or forms small oligomers.     

Arraying of intact liposomes into chemically functionalized microwells

Here, we describe a protocol to bind individual, intact phospholipid bilayer liposomes, which are on the order of 1 microm in diameter, in microwells etched in a regular array on a silicon oxide substrate. The diameter of the wells is on the order of the liposome diameter, so only one liposome is located in each well. The background of the silicon oxide surface is functionalized with a PEG oligomer using the contact printing of a PEG silane to present a surface that resists the adsorption of proteins, lipid material, and liposomes. The interiors of the wells are functionalized with an aminosilane to facilitate the conjugation of biotin, which is then bound to Neutravidin. The avidin-coated well interiors bind the liposomes whose surfaces contain biotinylated lipids. The specific binding of the liposomes to the surface using the biotin-avidin linkage, together with the resistant nature of the background and the physical confinement of the wells, allows the liposomes to remain intact and to not unravel, rupture, and fuse onto the surface. We demonstrate this intact arraying using confocal laser scanning microscopy of fluorophores specifically tagging the microwells, the lipid bilayer, and the aqueous interior of the liposome.     

Hydroxyapatite coating on stainless steel pre-coated with bovine serum albumin at ambient conditions

A biomimetic process for coating of nanosized hydroxyapatite on stainless steel, which capitalises the dual nature of the protein bovine serum albumin in both metal binding and a strong affinity for calcium ions, has been developed. The novelty of the process lies in pre-conditioning the metallic surface using the above protein prior to its mineralization with hydroxyapatite at ambient conditions. The microporous morphology of these coatings may provide favourable solubility and resorbability as desired by many orthopaedic and orthodontic applications.     

Modification of fibrous silicates surfaces with organic derivatives: an infrared spectroscopic study

This contribution explores the interaction of the fibrous silicates, palygorskite, sepiolite and chrysotile with a wide range of organic agents. Infrared spectroscopy (IR) methods are essential for the characterization of solid surfaces and for the investigation of the kind of bonds formed between the surface of these silicates and the organic moieties. Thus, when sepiolite or palygorskite are treated, e.g., with polyurethanes, alcohols, isocyanates, amines or pyridines, specific SiNHC or SiOC bonds are derived from the linkage of the differently located OH groups in these fibrous silicates with the organic moieties. On the other hand, more stable, covalent SiOSiC bondings are formed when the fibrous silicates, especially chrysotile, are reacted with heterofunctional silylating agents like chlorosilanes or ethoxysilanes carrying, alkyl, alkenyl or aryl groups. Such reactions may occur in the presence or absence of HCl. An absorption band at 960 cm(-1)--which we assigned to SiOH groups--is detected only in the presence of HCl. The evolution of this band is related to the degree of grafting of the organic radicals with the silanol groups of the silicates. HCl-generated silanol groups are the main bridges for the coupling of organosilyl groups on chrysotile and other silicates by covalent bonding, leading the way to the preparation of interesting new materials, including fibrous sheet polymers.     

NaCu(II)4] cluster from alkali template assembly of two asymmetric end-on azido-bridged [Cu(II)2] units

The cluster [NaCu4L2(N3)2](ClO4) [1; H3L is 2-(2'-hydroxyphenyl)-1,3-bis[3'-aza-4'-(2'-hydroxyphenyl)prop-4'-en-1'-yl]-1,3-imidazolidine] has been synthesized and structurally characterized. Complex 1 is formed by the template assembly of two [Cu2L(N3)] neutral fragments through their weak oxophilic interactions with a central Na+ cation as observed in metallacrowns. The cluster exhibits a combination of ferro- and antiferromagnetic interactions. End-on N3- bridging of copper ions within the [Cu(II)2] units facilitates stabilization of S = 1 magnetic subunits that mutually cancel via antiferromagnetic coupling as mediated by the O...Na+...O bridges.     

Is Poisson-Boltzmann theory insufficient for protein folding simulations?

The Poisson-Boltzmann theory has been widely used in the studies of energetics and conformations of biological macromolecules. Recently, introduction of the efficient generalized Born approximation has greatly extended its applicability to areas such as protein folding simulations where highly efficient computation is crucial. However, limitations have been found in the folding simulations of a well-studied beta hairpin with several generalized Born implementations and different force fields. These studies have raised the question whether the underlining Poisson-Boltzmann theory, on which the generalized Born model is calibrated, is adequate in the treatment of polar interactions for the challenging protein folding simulations. To address the question whether the Poisson-Boltzmann theory in the current formalism might be insufficient, we directly tested our efficient numerical Poisson-Boltzmann implementation in the beta-hairpin folding simulation. Good agreement between simulation and experiment was found for the beta-hairpin equilibrium structures when the numerical Poisson-Boltzmann solvent and a recently improved generalized Born solvent were used. In addition simulated thermodynamic properties also agree well with experiment in both solvents. Finally, an overall agreement on the beta-hairpin folding mechanism was found between the current and previous studies. Thus, our simulations indicate that previously observed limitations are most likely due to imperfect calibration in previous generalized Born models but not due to the limitation of the Poisson-Boltzmann theory.