Acidity and Lactonization of Xylonic Acid: A Nuclear Magnetic Resonance Study

In acidic aqueous solutions the xylonic acid/xylonate equilibrium is coupled with the formation of the γ- and δ-lactones. The γ-lactone is formed more readily, whereas the δ-lactone can only be observed in traces at very low pH values (<2.5). By means of ¹³C NMR, both the lactone hydrolization constant and the acid dissociation constant could be determined (K_L = 4.08, = 3.65 ± 0.34). Further, a second deprotonation of one of the hydroxyl groups could be observed at very high pH ( = 13.3 ± 0.76).     

Screening the Thermodynamics of Trimethylaluminum‐Hydrolysis Products and Their Co‐Catalytic Performance in Olefin‐Polymerization Catalysis

Herein, we introduce an approach for the computational screening of stoichiometric reactions between trimethylaluminum (TMA) and water. The thermodynamic products of these reactions are methylaluminoxanes (MAOs) with different compositions, which have the general formula (AlOMe)_n(AlMe₃)_m, in which n describes the degree of oligomerization and m is the number of associated TMA molecules. These reaction products were thoroughly explored up to n=4, thus demonstrating the thermodynamically preferable association of up to four AlMe₃ molecules, that is, TMA molecules in their monomeric form. The relative Lewis acidities of the Al sites in these MAOs were systematically explored and we found that the associated TMA molecules were a key ingredient for co‐catalytic activity in olefin‐polymerization catalysis. This conclusion was supported by computational studies on catalyst activation, which revealed an exergonic insertion of ethene into the metallocene/MAO complex.     

Aqueous Self‐Assembly and Cation Selectivity of Cobaltabisdicarbollide Dianionic Dumbbells

The anion [3,3′‐Co(C₂B₉H₁₁)₂]^? ([COSAN]^?) produces aggregates in water. These aggregates are interpreted to be the result of C?H???H?B interactions. It is possible to generate aggregates even after the incorporation of additional functional groups into the [COSAN]^? units. The approach is to join two [COSAN]^? anions by a linker that can adapt itself to act as a crown ether. The linker has been chosen to have six oxygen atoms, which is the ideal number for K⁺ selectivity in crown ethers. The linker binds the alkaline metal ions with different affinities; thus showing a distinct degree of selectivity. The highest affinity is shown towards K⁺ from a mixture containing Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺; this can be indicative of pseudo‐crown ether performance of the dumbbell. One interesting possibility is that the [COSAN]^? anions at the two ends of the linker can act as a hook‐and‐loop fastener to close the ring. This facet is intriguing and deserves further consideration for possible applications. The distinct affinity towards alkaline metal ions is corroborated by solubility studies and isothermal calorimetry thermograms. Furthermore, cryoTEM micrographs, along with light scattering results, reveal the existence of small self‐assemblies and compact nanostructures ranging from spheres to single‐/multi‐layer vesicles in aqueous solutions. The studies reported herein show that these dumbbells can have different appearances, either as molecules or aggregates, in water or lipophilic phases; this offers a distinct model as drug carriers.     

A posteriori estimates for the Stokes eigenvalue problem

We consider the Stokes eigenvalue problem. For the eigenvalues we derive both upper and lower a‐posteriori error bounds. The estimates are verified by numerical computations. © 2008 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2009     

Organizational emergence in networked collaboration

Research on complex adaptive systems has generated several conceptual parables to explain systems with emergent behaviour. One prominent use for terms such as self‐organization, evolutionary trajectories, co‐evolution and punctuated equilibrium has been in understanding human organizations. In such systems, emergent behaviour is demonstrated in novel structures, processes and spin‐offs that cannot be explained just by studying single components of the organization and the intelligence embedded in them. Instead of solely exploiting the qualitative explanatory power of the evolutionary concepts, this paper focuses also on quantitative methods to track emergent behaviour in a globally distributed, constantly fluctuating and highly networked project organization. The underlying case is that of CERN (CERN, the European Laboratory for Particle Physics, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the United Kingdom. Israel, the Russian Federation, Turkey, Yugoslavia (status suspended after the UN embargo, June 1992), the European Commission and UNESCO have observer status.) and its decade long accelerator project, which strongly relies on electronic communication and networking to achieve its major objectives due to be accomplished by the year 2006. By using time series and self‐organizing maps to analyse the global interaction among project groups and individuals the paper provides new insight to the understanding of emergent behaviour in human organizations. The key result of the study concerns the rigid deep structure of each case organization that seems to remain intact for the duration of the whole project. Copyright © 2002 John Wiley & Sons, Ltd.     

Signal processing challenges for applying software radio principles in future wireless terminals: an overview

The general idea of software radio is to develop highly integrated radio transceiver structures with high degree of flexibility and multimode capabilities, achieved through increased role of digital signal processing software in defining the functionalities which have traditionally been implemented with analog RF techniques. This paper explores the software radio concept from the receiver architecture and signal processing points of view, with mainly the wireless terminal application in mind. We first discuss the critical issues in alternative receiver architectures with simplified analog parts and increased configurability. We also introduce certain advanced digital signal processing techniques which could potentially relieve some of the essential problems and pave the way towards DSP‐based, highly integrated, and highly configurable terminals. Big emphasis is on efficient digital multirate signal processing methods and complex (I/Q) signal processing. Copyright © 2002 John Wiley & Sons, Ltd.     

Influence of ethanol on lipid membranes: from lateral pressure profiles to dynamics and partitioning

We have combined experiments with atomic-scale molecular dynamics simulations to consider the influence of ethanol on a variety of lipid membrane properties. We first employed isothermal titration calorimetry together with the solvent-null method to study the partitioning of ethanol molecules into saturated and unsaturated membrane systems. The results show that ethanol partitioning is considerably more favorable in unsaturated bilayers, which are characterized by their more disordered nature compared to their saturated counterparts. Simulation studies at varying ethanol concentrations propose that the partitioning of ethanol depends on its concentration, implying that the partitioning is a nonideal process. To gain further insight into the permeation of alcohols and their influence on lipid dynamics, we also employed molecular dynamics simulations to quantify kinetic events associated with the permeation of alcohols across a membrane, and to characterize the rotational and lateral diffusion of lipids and alcohols in these systems. The simulation results are in agreement with available experimental data and further show that alcohols have a small but non-vanishing effect on the dynamics of lipids in a membrane. The influence of ethanol on the lateral pressure profile of a lipid bilayer is found to be prominent: ethanol reduces the tension at the membrane-water interface and reduces the peaks in the lateral pressure profile close to the membrane-water interface. The changes in the lateral pressure profile are several hundred atmospheres. This supports the hypothesis that anesthetics may act by changing the lateral pressure profile exerted on proteins embedded in membranes.     

Quasistatic Dynamics with Intermittency

We study an intermittent quasistatic dynamical system composed of nonuniformly hyperbolic Pomeau–Manneville maps with time-dependent parameters. We prove an ergodic theorem which shows almost sure convergence of time averages in a certain parameter range, and identify the unique physical family of measures. The theorem also shows convergence in probability in a larger parameter range. In the process, we establish other results that will be useful for further analysis of the statistical properties of the model.     

Control of calcium oxalate crystal growth by face-specific adsorption of an osteopontin phosphopeptide

Mineral-associated proteins have been proposed to regulate many aspects of biomineralization, including the location, type, orientation, shape, and texture of crystals. To understand how proteins achieve this exquisite level of control, we are studying the interaction between the phosphoprotein osteopontin (OPN) and the biomineral calcium oxalate monohydrate (COM). In the present study, we have synthesized peptides corresponding to amino acids 220-235 of rat bone OPN (pSHEpSTEQSDAIDpSAEK), one of several highly phosphorylated, aspartic-, and glutamic acid-rich sequences found in the protein. To investigate the role of phosphorylation in interaction with crystals, peptides containing no (P0), one (P1), or all three (P3) phosphates were prepared. Using a novel combination of confocal microscopy and scanning electron microscopy, we show that these peptides adsorb preferentially to {100} faces of COM and inhibit growth of these faces in a phosphorylation-dependent manner. To characterize the mechanism of adsorption of OPN peptides to COM, we have performed the first atomic-scale molecular-dynamics simulation of a protein-crystal interaction. P3 adsorbs to the {100} face much more rapidly than P1, which in turn adsorbs more rapidly than P0. In all cases, aspartic and glutamic acid, not phosphoserine, are the amino acids in closest contact with the crystal surface. These studies have identified a COM face-specific adsorption motif in OPN and delineated separate roles for carboxylate and phosphate groups in inhibition of crystal growth by mineral-associated phosphoproteins. We propose that the formation of close-range, stable, and face-specific interactions is a key factor in the ability of phosphoproteins to regulate biomineralization processes.