1,6:9,14-bismethano[16]annulene - a new bridged [4n]annulene

The molecular structure and dynamic behaviour of the bridged [4n] annulene 1,6:9,14-bismethano-[16]annulene is unravelled by NMR and X-ray investigations, combined with force field calculations.     

Dynamic combinatorial carbohydrate libraries: probing the binding site of the concanavalin A lectin

Dynamic combinatorial chemistry (DCC) has emerged as an efficient approach to receptor/ligand identification based on the generation of combinatorial libraries by reversible interconversion of the library constituents. In this study, the implementation of such libraries on carbohydrate-lectin interactions was examined with the plant lectin Concanavalin A as a target species. Dynamic carbohydrate libraries were generated from a pool of carbohydrate aldehydes and hydrazide linker/scaffold components through reversible acylhydrazone exchange, resulting in libraries containing up to 474 constituents. Dynamic deconvolution allowed the efficient identification of the structural features required for binding to Concanavalin A and the selection of a strong binder, a tritopic mannoside, showing an IC(50)-value of 22 microM.     

The biotin-streptavidin interaction can be reversibly broken using water at elevated temperatures

The biotin-streptavidin system is the strongest noncovalent biological interaction known, having a dissociation constant, K(d), in the order of 4x10(-14) M. The strength and specificity of the interaction has led it to be one of the most widely used affinity pairs in molecular, immunological, and cellular assays. However, it has previously been impossible to re-use any streptavidin solid support, since the conditions needed to break the interaction with biotin has led to the denaturation of the streptavidin. Here, we show that a short incubation in nonionic aqueous solutions at temperatures above 70 degrees C can efficiently break the interaction without denaturing the streptavidin tetramer. Both biotin and the streptavidin remain active after dissociation and both molecules can therefore be re-used. The efficiency of the regeneration allowed solid supports with streptavidin to be used many times, here exemplified with the multiple re-use of streptavidin beads used for sample preparation prior to automated DNA sequencing. The results suggest that streptavidin regeneration can be introduced as an improvement in existing methods and assays based on the streptavidin system as well as emerging solid phase applications in fields, such as microfluidics and nanotechnology.     

Letter to the Editor: Friction between Surfaces—Polyacrylic Acid Brush and Silica—Mediated by Calcium Ions

With this letter, we report how friction can be controlled by inducing physical bonds solely within a polyelectrolyte brush layer, while keeping repulsive interactions between the brush layer and the bare surface that slides above. Our results imply that the nature of the bare surface is of minor importance as long as the repulsive surface interaction is maintained.     

Acid-Assisted Direct Olefin Metathesis of Unprotected Carbohydrates in Water

The ability to use unprotected carbohydrates in olefin metathesis reactions in aqueous media is demonstrated. By using water-soluble, amine-functionalized Hoveyda–Grubbs catalysts under mildly acidic aqueous conditions, the self-metathesis of unprotected alkene-functionalized α-d -manno- and α-d -galactopyranosides could be achieved through minimization of nonproductive chelation and isomerization. Cross-metathesis with allyl alcohol could also be achieved with reasonable selectivity. The presence of small quantities (2.5 vol %) of acetic acid increased the formation of the self-metathesis product while significantly reducing the alkene isomerization process. The catalytic activity was furthermore retained in the presence of large amounts (0.01 mm ) of protein, underlining the potential of this carbon–carbon bond-forming reaction under biological conditions. These results demonstrate the potential of directly using unprotected carbohydrate structures in olefin metathesis reactions under mild conditions compatible with biological systems, and thereby enabling their use in, for example, drug discovery and protein derivatization.     

Maximum modulus principle for “holomorphic functions” on the quantum matrix ball

We describe the Shilov boundary ideal for a q-analog of the algebra of holomorphic functions on the unit ball in the space of $n\times n$ matrices and show that its $C^{?}$-envelope is isomorphic to the $C^{?}$-algebra of continuous functions on the quantum unitary group $U_q(n)$.     

Tandem reversible addition-intramolecular lactonization for the synthesis of 3-functionalized phthalides

A new tandem process based on reversible nucleophilic addition and intramolecular lactonization of methyl 2-formylbenzoate leads to the efficient synthesis of 3-functionalized phthalides, which are important precursors for the synthesis of quinone skeletons via Hauser-Kraus annulation. The reactions are successfully carried out under mild conditions in single operations.     

Expanded bite angles in tridentate ligands. Improving the photophysical properties in bistridentate RuII polypyridine complexes

Bistridentate metal complexes as photosensitizers are ideal building blocks in the construction of rod-like isomer-free assemblies for intramolecular photoinduced charge separation. Approaches to obtain long-lived luminescent metal-to-ligand charge transfer excited states in bistridentate Ru^II polypyridine complexes via the manipulation of metal-centered state energies are discussed. Following an introduction to general strategies to prolong the excited state lifetimes, more recent work is explored in detail where tridentate ligands with expanded 2,2′:6′,2″-terpyridine cores are utilized. The synthesis of these tridentate ligands and their corresponding Ru^II complexes is covered. Bistridentate Ru^II complexes with microsecond metal-to-ligand charge transfer excited state lifetimes are described, and are used in electron donor–photosensitizer–electron acceptor assemblies for efficient vectorial photoinduced charge separation.     

Balancing Neumann-Neumann preconditioners for mixed approximations of heterogeneous problems in linear elasticity

Summary. Balancing Neumann-Neumann methods are extented to mixed formulations of the linear elasticity system with discontinuous coefficients, discretized with mixed finite or spectral elements with discontinuous pressures. These domain decomposition methods implicitly eliminate the degrees of freedom associated with the interior of each subdomain and solve iteratively the resulting saddle point Schur complement using a hybrid preconditioner based on a coarse mixed elasticity problem and local mixed elasticity problems with natural and essential boundary conditions. A polylogarithmic bound in the local number of degrees of freedom is proven for the condition number of the preconditioned operator in the constant coefficient case. Parallel and serial numerical experiments confirm the theoretical results, indicate that they still hold for systems with discontinuous coefficients, and show that our algorithm is scalable, parallel, and robust with respect to material heterogeneities. The results on heterogeneous general problems are also supported in part by our theory. Mathematics Subject Classification (1991):65N55, 65N30, 65N35, 65F10, 65Y05This work was supported by a scholarship of CNPq, of the Ministry for Science and Technology of Brazil, under process 201205/97-1. The work was developed in part at MCS/ANL-DOE, under a Givens Research Associate appointment in Summer 2001.This work was supported in part by the National Science Foundation under Grant NSF-CCR-9732208 and in part by MIUR.This work was supported in part by the National Science Foundation under Grants qNSF-CCR-9732208, and in part by the U.S. Department of Energy under contracts DE-FC02-01ER25482 and DE-FG02-92ER25127.