Effects of As(III) binding on alpha-helical structure

As(III) displays a wide range of effects in cellular chemistry. Surprisingly, the structural consequences of arsenic binding to peptides and proteins are poorly understood. This study utilizes model alpha-helical peptides containing two cysteine (Cys) residues in various sequential arrangements and spatial locations to study the structural effects of arsenic binding. With i, and i + 1, i + 2, or i + 3 arrangements, CD spectroscopy shows that As(III) coordination causes helical destabilization when Cys residues are located at central or C-terminal regions of the helix. Interestingly, arsenic binding to i, i + 3 positions results in the elimination of helical structure and the formation of a relatively stable alternate fold. In contrast, helical stabilization is observed for peptides containing i, i + 4 Cys residues, with corresponding pseudo pairwise interaction energies (Delta G(pw) degrees) of -1.0 and -0.7 kcal/mol for C-terminal and central placements, respectively. Binding affinities and association rate constants show that As(III) binding is comparatively insensitive to the location of the Cys residues within these moderately stable helices. These data demonstrate that As(III) binding can be a significant modulator of helical secondary structure.     

Histidine in enzyme active centers.

The presence of histidine in the active center of an enzyme can be demonstrated by kinetic measurements, chemical modification, NMR spectroscopy or X-ray structure analysis. Histidine is the only naturally occurring amino acid to contain an imidazole residue as a side chain. The role of histidine in enzyme catalysis depends, inter alia, upon the special features of the imidazole residue: it thus tends to form hydrogen bonds, combines donor and acceptor properties and can take part in either nucleophilic or base catalysis. In some of these enzymes the position of each atom is known; however, the theories as to how the catalysis proceeds at a molecular level are controversial.     

Analyse geringer Substanzmengen mit Hilfe der R?ntgenfluorescenzspektrometrie

Zusammenfassung Eine Methode wird beschrieben, die die Analyse sehr geringer Substanzmengen ermöglicht. Eine maximale Einwaage von 2 mg wird in 1 g Borax geschmolzen, und die so erhaltene Boraxscheibe direkt zur röntgenfluorescenzanalytischen Bestimmung der verschiedenen Elemente eingesetzt. Für eine Reihe von Elementen werden die Nachweisgrenzen angegeben und ein Beispiel für die Reproduzierbarkeit der Methode angegeben. Verschiedene Anwendungsbeispiele dieser Methode werden beschrieben und die Ergebnisse mitgeteilt.

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Analysis of very small quantities of substances by X-ray fluorescence spectrometry

A maximum sample weight of 2 mg is molten in 1 g borax und the disk obtained is directly analysed by X-ray fluorescence spectrometry. For a number of elements the limits of detection are given and by one example the reproducibility of the method is shown. Different examples of application are described and the results are presented.

     

Stereoselective Syntheses of Tetrahydroesterastin-β -Lactam Analogues

A total synthesis of the optically active tetrahydroesterastin β -lactam analogue 2 using Miller's hydroxamate approach is described (Scheme 2). Significant modification of published procedures has resulted in a short and facile stereospecific preparation of the N-[(benzyloxycarbonyl)methyl]-β -lactam 17 starting from the readily available D -serine. This material served as intermediate for the preparation of a variety of N-[(benzyloxycarbonyl)methyl]tetrahydroesterastin β-lactam analogues (Scheme 5).     

Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes

Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.     

The host-guest chemistry of resorcinarenes

Conformations, acid-base and supramolecular properties of phenolic metacyclophanes obtained from the condensation of resorcinol with aldehydes are discussed, including the mechanisms involved in the formation of these macrocycles. The strong binding of choline-type compounds and the inhibition of acetylcholine hydrolysis with therccc stereoisomers is mechanistically evaluated; arctt isomer shows strong conformational coupling for, e.g., choline binding and simultaneous proton release. The presence of larger alkyl residues at the bottom of therccc macrocycle leads to an additional binding site for small lipophilic substrates, which is independent of the upper complexation center for positively charged substrates. Substitution at the upper rim by carboxylic groups at the 2-position of the phenyl rings yields receptors for, e.g., , -diammonium ions with alternate equatorial and axial arylunits. Positively charged substituents at the upper rim, introduced by aminoalkylation, lead to little change of complexation as a result from their orientation away from the binding center. Aminoacid substituents, for the same reason, do not lead to enantioselective complexation, but allow particularly for strong binding of transition metal ions. Preliminary studies show that resorcinarenes bearing a wide array of positive charges are potent groove binders to ds-DNA without intercalative contributions.This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes.     

Connection Formulas for Second Order Differential Equations with a Complex Parameter and Having an Arbitrary Number of Turning Points

We consider the differential equation on a finite interval I, where I contains m turning points, that is here, zeros of ?. Using asymptotic estimates proved by R. E. LANGER for solutions of (*) for intervals containing only one turning point we derive asymptotic estimates (for ρ → ) for a special fundamental system of solutions of (*) in I. The results obtained are fundamental for the investigation of eigenvalue problems defined by (*) and suitable boundary conditions.     

Global existence via Ginzburg-Landau formalism and pseudo-orbits of Ginzburg-Landau approximations

The so-called Ginzburg-Landau formalism applies for parabolic systems which are defined on cylindrical domains, which are, close to the threshold of instability, and for which the unstable Fourier modes belong to non-zero wave numbers. This formalism allows to describe an attracting set of solutions by a modulation equation, here the Ginzburg-Landau equation. If the coefficient in front of the cubic term of the formally derived Ginzburg-Landau equation has negative real part the method allows to show global existence in time in the original system of all solutions belonging to small initial conditions, inL . Another aim of this paper is to construct a pseudo-orbit of Ginzburg-Landau approximations which is close to a solution of the original system up tot=. We consider here as an example the socalled Kuramoto-Shivashinsky equation to explain the methods, but it applies also to a wide class of other problems, like e.g. hydrodynamical problems or reaction-diffusion equations, too.