Complex formation of trivalent americium with salicylic acid at very low concentrations

For the first time, the complexation of americium(III) with salicylic acid was studied at trace metal concentrations using a 2.0 m Long Path Flow Cell for UV–vis spectroscopy. The detection limit of Am(III) in aqueous solution at pH 3.0 was found to be 5 × 10⁻⁹ M. Two Am(III)-salicylate complexes were formed at pH 5.0 in 0.1 M NaClO₄, indicated by a clear red shift of the absorption maximum. The absorption spectra obtained from spectrophotometric titration were analyzed by means of factor analysis and complex stabilities were calculated to be log β₁₁₀ = 2.56 ± 0.08 and log β₁₂₀ = 3.93 ± 0.19.     

Towards Control of Aggregational Behaviour of α-Lactalbumin at Acidic pH

α-Lactalbumin (α-La) undergoes considerable structural changes upon loss of bound Ca²⁺ at acidic pH, leaving α-La in a molten globule structure. Using fluorescence the present work provides more insight into the structural transition of α-La at acidic pH leading to protein aggregation, most likely caused by a combination of hydrophobic and electrostatic interactions. The rate of aggregation is determined by the protein concentration and temperature applied. Availability of Ca²⁺ stabilises the protein, and thus prevent aggregation at pH values as low as pH 2.9. In contrast, presence of Cu²⁺ induces a destabilisation of the protein, which can be explained by a binding to the Zn²⁺ binding site in α-La, possibly resulting in structural alterations of the protein. In general, presence of anions destabilise α-La at pH values below pI, with SO₄ ²⁻ exhibiting the strongest effect on the protein stability, thus correlating well with the Hofmeister series. At more acidic pH values far from pI, α-La becomes more stable towards ion induced aggregation, since higher ion activity is required to efficiently screen the charges on the protein surface. The results presented in this paper provide detailed knowledge on the external parameters leading to aggregation of α-La at acidic pH, thus permitting rational design of the aggregation process.     

7Li and 13C solid-state NMR spectra of lithium cuprates

7Li and 13C solid-state MAS NMR spectra of three lithium cuprates with known X-ray structures--lithium([12]crown-4)2 dimethyl and diphenyl cuprate (1,2) and lithium(thf)4-[tris(trimethylsilyl) methyl]2 cuprate (3)--have been measured and analysed with respect to the quadrupolar coupling constants of lithium-7, chi(7Li), and the asymmetry parameters of the quadrupolar interactions, eta(7Li), as well as the 6, 7Li and 13C chemical shifts. The chi(7Li) values of 23, 30, and 18 kHz for 1, 2 and 3, respectively, are in line with the high symmetry around the lithium nucleus in the solvent-separated structures and may be used as reference data for this structural motif. Calculations based on charges derived from ab initio 6-31 G* HF computations using the point charge model (PCM) and the program GAMESS support the experimental findings.     

A pseudo-ligand approach to virtual screening

A virtual screening method is presented that is grounded on a receptor-derived pharmacophore model termed "virtual ligand" or "pseudo-ligand". The model represents an idealized constellation of potential ligand sites that interact with residues of the binding pocket. For rapid virtual screening of compound libraries the potential pharmacophore points of the virtual ligand are encoded as an alignment-free correlation vector, avoiding spatial alignment of pharmacophore features between the pharmacophore query (i.e., the virtual ligand) and the candidate molecule. The method was successfully applied to retrieving factor Xa inhibitors from a Ugi three-component combinatorial library, and yielded high enrichment of actives in a retrospective search for cyclooxygenase-2 (COX-2) inhibitors. The approach provides a concept for "de-orphanizing" potential drug targets and identifying ligands for hitherto unexplored or allosteric binding pockets.     

Elliptic operators on planar graphs: Unique continuation for eigenfunctions and nonpositive curvature

This paper is concerned with elliptic operators on plane tessellations. We show that such an operator does not admit a compactly supported eigenfunction if the combinatorial curvature of the tessellation is nonpositive. Furthermore, we show that the only geometrically finite, repetitive plane tessellations with nonpositive curvature are the regular and tilings.

     

Phosphorylation state-responsive lanthanide peptide conjugates: a luminescence switch based on reversible complex reorganization

[reaction: see text] A luminogenic probe for peptide dephosphorylation has been developed. It consists of a serine-/tyrosine-containing peptide modified on the N-terminus with a tryptophan residue and a DTPA chelate capable of binding Tb(3+). We propose a mechanistic model for the luminescence enhancement based on the interconversion of monomeric and dimeric lanthanide species, which is affected by the phosphorylation state of the serine or tyrosine residue. The optical switch reports effectively on phosphatase-catalyzed dephosphorylation in vitro.     

Exploring the limits of polarization transfer efficiency in homonuclear three spin systems

The limits of polarization transfer efficiency are explored for systems consisting of three isotropically coupled spins 1/2 in the absence of relaxation. An idealized free evolution and control Hamiltonian is studied, which provides an upper limit of transfer efficiency (in terms of transfer amplitude and transfer time) for realistic homonuclear spin systems with arbitrary Heisenberg-type coupling constants J12, J13, and J23. It is shown that optimal control based pulse sequences have significantly improved transfer efficiencies compared to conventional transfer schemes. An experimental demonstration of optimal polarization transfer is given for the case of the carbon spin system of fully 13C labelled alanine at 62.5 MHz Larmor frequency.     

Extremely Electron-Poor Bis(diarylmethylium)-Substituted Ferrocenes and the First Peroxoferrocenophane

We have prepared and studied extremely electron-poor, deeply colored dicationic 1,1'-bis(diarylmethylium)-substituted ferrocenes [(η⁵-C₅H₄-CAr₂)₂Fe]²⁺ with various aryl substituents as their [B{C₆H₃(CF₃)₂-3,5}₄]^– salts. Due to the strong acceptor substitution, the redox potential for the ferrocene-based oxidation of the anisyl- or 2-methylanisyl-substituted congeners 1b²⁺ and 1c²⁺ is close to or even surpasses that of the second oxidation of parent ferrocene, i.e. the Cp₂Fe^+/2+ couple. The strongly Lewis-acidic character of these complexes is manifest through strong interactions with donor solvents, which lead to a significant reduction of the intensities of the charge-transfer bands in their electronic spectra and to solvatochromism. The reduced forms of the complexes tend to dimerize or oligomerize as revealed by EPR spectroscopy. Direduced 1b selectively reacts with molecular oxygen to form a peroxo-bis(diarylmethyl)[4]ferrocenophane, which was also characterized by X-ray crystallography.