Metal binding to bipyridine-modified PNA

Substitution of natural nucleobases in PNA oligomers with ligands is a strategy for directing metal ion incorporation to specific locations within a PNA duplex. In this study, we have synthesized PNA oligomers that contain up to three adjacent bipyridine ligands and examined the interaction with Ni2+ and Cu2+ of these oligomers and of duplexes formed from them. Variable-temperature UV spectroscopy showed that duplexes containing one terminal pair of bipyridine ligands are more stable upon metal binding than their nonmodified counterparts. While binding of one metal ion to duplexes that contain two adjacent bipyridine pairs makes the duplexes more stable, additional metal ions lower the duplex stability, with electrostatic repulsions being, most likely, an important contributor to the destabilization. UV titrations showed that the presence of several bipyridine ligands in close proximity of each other in PNA oligomers exerts a chelate effect. A supramolecular chelate effect occurs when several bipyridines are brought next to each other by hybridization of PNA duplexes. EPR spectroscopy studies indicate that even when two Cu2+ ions coordinate to a PNA duplex in which two bipyridine pairs are next to each other, the two metal-ligand complexes that form in the duplex are far enough from each other that the dipolar coupling is very weak. EXAFS and XANES show that the Ni2+-bipyridine bond lengths are typical for [Ni(bipy)2]2+ and [Ni(bipy)3]2+ complexes.     

Towards the simulation of grinding processes – a thermoplastic single grain approach

The current work aims at the modelling and simulation of metal grinding processes with focus on the thermal behaviour resulting from the interaction of tool and workpiece. Starting from a two-dimensional model of a single grain, representative numerical experiments are carried out based on the variation of grain geometry and its spatial position relative to the workpiece to predict the resulting load-displacement-behaviour. Here, a thermoelastic viscoplastic constitutive model is used to capture thermal softening of the material taken into account. Referring to previous research, an adaptive remeshing scheme, based on a combination of error estimation and refinement indication is used to overcome mesh dependence, allowing to resolve the complex deformation patterns and to predict a realistic thermomechanical state of the resulting workpiece surface. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The minimum rank problem over the finite field of order 2: Minimum rank 3

Our main result is a sharp bound for the number of vertices in a minimal forbidden subgraph for the graphs having minimum rank at most 3 over the finite field of order 2. We also list all 62 such minimal forbidden subgraphs. We conclude by exploring how some of these results over the finite field of order 2 extend to arbitrary fields and demonstrate that at least one third of the 62 are minimal forbidden subgraphs over an arbitrary field for the class of graphs having minimum rank at most 3 in that field.     

Zinc‐Mediated Formation of Trifluoromethyl Ethers from Alcohols and Hypervalent Iodine Trifluoromethylation Reagents

A (fluor)ry of activity : The transfer of an intact trifluoromethyl group from a hypervalent iodine reagent to an aliphatic alcohol occurs smoothly upon activation by zinc bis(triflimide). This constitutes a straightforward method for the preparation of trifluoromethoxy alkyl derivatives, compounds otherwise difficult to access.

     

Moderate Deviations for Random Field Curie-Weiss Models

The random field Curie-Weiss model is derived from the classical Curie-Weiss model by replacing the deterministic global magnetic field by random local magnetic fields. This opens up a new and interestingly rich phase structure. In this setting, we derive moderate deviations principles for the random total magnetization S _n , which is the partial sum of (dependent) spins. A typical result is that under appropriate assumptions on the distribution of the local external fields there exist a real number m, a positive real number ??, and a positive integer k such that (S _n ?nm)/n ^?? satisfies a moderate deviations principle with speed n ^1?2k(1???) and rate function ??x ^2k /(2k)!, where 1?1/(2(2k?1))<??<1.     

Compact diode-laser-pumped quantum light source based on four-wave mixing in hot rubidium vapor

Using a nondegenerate four-wave mixing process in hot rubidium vapor, we demonstrate a compact diode-laser-pumped system for the generation of intensity-difference squeezing down to 8?kHz with a maximum squeezing of -7?dB. To the best of our knowledge, this is the first demonstration of kilohertz-level intensity-difference squeezing using a semiconductor laser as the pump source. This scheme is of interest for experiments involving atomic ensembles, quantum communications, and precision measurements. The diode-laser-pumped system would extend the range of possible applications for squeezing due to its low cost, ease of operation, and ease of integration.     

Uniform diagonalisation of matrices over regular rings

The fundamental Separativity Problem for von Neumann regular rings is shown to be equivalent to a linear algebra problem: for a field F, is there a ``uniform formula' for diagonalising a matrix A over , independently of n? Here P and Q are required to be invertible matrices whose entries are fixed regular algebra expressions in the entries of A. Received July 10, 2000; accepted in final form September 26, 2000.     

Differential Complexing by Cyclodextrin Twin-cavity Hosts

The twin-cavity cyclodextrin (1) in which the link is a dithiotrehalosyl unit, and the flexibly-linked dimer (2) were shown to distinguish between the heterocyclic guests 3 and 4 (clofazimine drug) in spite of the guests' small structural difference. Both cyclodextrin dimer hosts form 1:1 complexes with methyl orange and with 3, as shown by double reciprocal plots of UV-absorbance change and host concentration. However with 4, both host molecules formed a 2 : 1 (host : guest) complex . Since both dimer cavities are used to create this effect, it is a new type of selectivity for macrocyclic hosts.