Crucial Roles of Two Hydrated Mg2+ Ions in Reaction Catalysis of the Pistol Ribozyme

Pistol ribozymes constitute a new class of small self‐cleaving RNAs. Crystal structures have been solved, providing three‐dimensional snapshots along the reaction coordinate of pistol phosphodiester cleavage, corresponding to the pre‐catalytic state, a vanadate mimic of the transition state, and the product. The results led to the proposed underlying chemical mechanism. Importantly, a hydrated Mg²⁺ ion remains innersphere‐coordinated to N7 of G33 in all three states, and is consistent with its likely role as acid in general acid base catalysis (δ and β catalysis). Strikingly, the new structures shed light on a second hydrated Mg²⁺ ion that approaches the scissile phosphate from its binding site in the pre‐cleavage state to reach out for water‐mediated hydrogen bonding in the cyclophosphate product. The major role of the second Mg²⁺ ion appears to be the stabilization of product conformation. This study delivers a mechanistic understanding of ribozyme‐catalyzed backbone cleavage.     

Efficient computation of cellular materials using the finite cell method

The application of cellular materials in lightweight structures is a subject of growing interest. As meshing with surface aligned elements is very time consuming and entails a lot of degrees of freedom, we apply the finite cell method (FCM). The FCM is an extension of the known fictitious domain methods based on using high order polynomials for the Ansatz functions. In order to obtain a realistic representation of the geometry, we derive our discretization directly from the voxel data coming from the computer tomography (CT-scan) of the specimen of interest. Thus, a much coarser Cartesian grid consisting of hexahedral cells can be readily generated in a fully automated fashion. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

New Experimental Limit on the Pauli Exclusion Principle Violation by Electrons��The VIP Experiment

We present an experimental test of the validity of the Pauli Exclusion Principle for electrons based on the concept put forward a few years ago by Ramberg and Snow (Ramberg and Snow in Phys. Lett. B 238:438, 1990). In this experiment we perform a very accurate search of X-rays from the Pauli-forbidden atomic transitions of electrons in the already filled 1S shells of copper atoms. Although the experiment has a simple structure, it poses deep conceptual and interpretational problems. Here we describe the experimental method and recent experimental results, which we interpret as an upper limit for the probability to violate the Pauli Exclusion Principle. We present also future plans to upgrade the experimental apparatus using Silicon Drift Detectors.     

Electrorotation of dumb-bell shaped particles: Theory and experiment

The electrorotation of symmetrical doublets was calculated in the dipole approximation as a function of the polarizability of the single spherical constituents and as a function of their distance. The result was that a doublet in parallel orientation to the electric field plane undergoes a shift of the first characteristic frequency towards higher frequencies. The second characteristic was shifted towards lower frequencies. The opposite is true for a perpendicular orientation. The theoretical predictions were confirmed with red cells and pollen. The electrical interaction was larger than theoretically possible in the dipole approximation.     

Die Massenspektren cyclischer Acylale

Zusammenfassung Vergleichende Studien der Massenspektren von 26 cyclischen Acylalen (Derivaten derMeldrumsäure) lassen Zusammenhänge zwischen der Art der Substituenten und der Stabilität des Molekülions sowie das allgemeine Fragmentierungsschema des Acylalringes erkennen. Charakteristisch ist die Eliminierung von Aceton mit nachfolgender mehrmaliger Abspaltung von CO bzw. CO₂. Daneben tritt in bestimmten Fällen die Abspaltung von 57 Masseneinheiten (unter H-Wanderung) in den Vordergrund. In aliphatischen oder alicyclischen Acylalen tritt zusätzlich H₂O-Eliminierung auf. An ortho-substituierten Benzolderivaten wurde eine stark begünstigte Abspaltung des ortho-Substituenten beobachtet.

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Comparative studies of the mass spectra of 26 cyclic acylals (derivatives ofMeldrum's acid) revealed correlations between the nature of substituents and the stability of the molecular ion as well as a general fragmentation pattern. One of the characteristic features is the elimination of acetone, followed by multiple eliminations of CO and CO₂. In certain cases 57 mass units are eliminated predominately (with hydrogen migration). Aliphatic and alicylic acylals eliminate H₂O. Among ortho-substituted benzenederivatives a very favoured loss of the ortho-substituent is observed.

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Mit 4 Abbildungen

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Herrn Prof. Dr. Dr. h. c.F. Wessely in dankbarer Verehrung zum 70. Geburtstag gewidmet.     

MNDO Analysis of Regio- and Stereoselectivity in Hydroboration

The origin of regioselectivity in hydroboration of olefins has been analyzed by MNDO methodology. With a standard transition structure derived from full MNDO optimizations, a linear correlation between calculated and experimental regioselectivities has been found. A similar approach applied to asymmetric inductions of hydroboration with chiral hydroborating agents lead also to an acceptable linear correlation between calculated and experimental results. The MNDO method has been used for a search of alternative hydroborating agents.     

Shape transformation of poly(butadiene)‐b‐poly(ethyleneoxide) plus DTAB compound micelles in aqueous solutions

The micellar shape of Poly(butadiene)‐b‐poly(ethyleneoxide) (PB‐PEO) plus Dodecyltrimethylammoniumbromide (DTAB) compound micelles was investigated by light scattering, small‐angle X‐ray scattering and small‐angle neutron scattering in dependence of the molar ratio between block copolymer and surfactant. The given block copolymer forms cylindrical micelles in binary aqueous solution, which transform to spherical aggregates upon the addition of a sufficiently high amount of DTAB. It is interesting to note that the micellar shape seems to be a bistable feature, in the sense that it depends not only on the molar ratio of BCP and DTAB but also in a predictable manner on the preparation procedure of the solution.