Formation of spanning water networks on protein surfaces via 2D percolation transition

The formation of spanning hydrogen-bonded water networks on protein surfaces by a percolation transition is closely connected with the onset of their biological activity. To analyze the structure of the hydration water at this important threshold, we performed the first computer simulation study of the percolation transition of water in a model protein powder and on the surface of a single protein molecule. The formation of an infinite water network in the protein powder occurs as a 2D percolation transition at a critical hydration level, which is close to the values observed experimentally. The formation of a spanning 2D water network on a single rigid protein molecule can be described by adapting the cluster analysis of conventional percolation studies to the characterization of the connectivity of the hydration water on the surface of finite objects. Strong fluctuations of the surface water network are observed close to the percolation threshold. Our simulations also furnish a microscopic picture for understanding the specific values of the experimentally observed hydration levels, where different steps of increasing mobility in the hydrated powder are observed.     

Bemerkungen zur Synthese von 3-Aminotoluol-5-sulfonsäure und 2-Aminotoluol-3-sulfonsäure

Some comments on the synthesis of 3-aminotoluene-5-sulfonic acid and 2-aminotoluene-3-sulfonic acid. Sulfonation of 3-nitrotoluene ( 5 ) yields predominantly the unsymetrical isomer 5-nitrotoluene-2-sulfonic acid ( 7 ), and lesser amounts of 5-nitrotoluene-3-sulfonic acid ( 6 ), previously reported as the major product. The desired 5-aminotoluene-3-sulfonic acid ( 3 ) was synthesized in preparative amounts from 6-aminotoluene-3-sulfonic acid (4) via the following sequence of reactions: diazotation and Sandmeyer replacement of 6-chlorotoluene-3-sulfonic acid ( 13 ), nitration of the sulfonyl chloride 14 under suitable conditions to give isomer free 6-chloro-5-nitrotoluene-3-sulfonyl chloride ( 15 ), hydrolysis to the sulfonic acid 16 and finally, simultaneous hydrogenolysis and reduction to 3 . The isomeric 7 was unequivocally prepared from 2-amino-5-nitrotoluene ( 9 ) via two routes: (1) diazotation, Sandmeyer thiocyanatation to 5-nitro-2-thiocyanatotoluene ( 10 ), Na₂S reduction to the di(2-methyl-4-nitro-phenyl)-disulfide ( 11 ), treatment with nitric acid and chlorine to give 5-nitrotoluene-2-sulfonyl chloride ( 12 ) and finally alkaline hydrolysis to 7 ; (2) Meerwein's SO₂ treatment of the diazonium salt derived from 9 leads directly to 12 and thence to 7 . 2-Aminotoluene-3-sulfonic acid ( 1 ) was prepared from the key intermediate 3-amino-2-nitrotoluene ( 18 ) via the same two routes used to prepare 7 from 9 . Both reaction sequences provided 2-nitrotoluene-3-sulfonly chloride, the hydrolysis product of which was reduced to 1 . Intermediate 18 was prepared in the following four steps from m-toluic acid ( 19 ): nitration to the 2-nitroderivative ( 20 ), whose acid chloride ( 21 ) was converted to 2-nitro-m-toluamide ( 22 ), and Hoffmann rearrangement to 18 .     

Ab initio studies on polymers. VI. Torsional potential in regular polyethylene chains

Ab initio crystal orbital calculations have been performed on regular polyethylene chains applying basis sets of minimal and double-zeta quality. Relative stabilities of periodic all-trans, all-gauche, and alternating trans–gauche conformers have been evaluated, including extensive geometry optimization. Potential curves for a simultaneous rotation around C? C single bonds from the all-trans to the all-gauche conformation have been computed applying the rigid-rotor approximation, the flexible-rotor approximation, and an additional reoptimization of C? C distances. A rigid-rotor potential curve from the all-trans to the alternating trans-gauche conformation has been computed as well. Results obtained are compared with ab initio calculations on butane and pentane and with semiempirical and empirical force-field studies on polyethylene.     

13C-NMR studies of marine natural products II. Total assignment of the 13C-NMR spectrum of asperdiol

¹³C-nmr spectral assignments for asperdiol are made with the aid of model compounds and T₁ relaxation behavior. Overall isotropic tumbling was inferred from the latter providing a supplemental means of multiplicity determination. Utilization of the T₁ relaxation times as an assignment criterion for select resonances in asperdiol is also described.     

Microchemical characterization of paintings — A case study

Summary The fundamental importance of scientific research for the preservation and restoration of works of art is uncontestable. The methods described in this paper, such as light microscopy, microchemical tests, emission spectroscopy, chromatography, scanning electron analysis, electron microprobe analysis and infrared spectroscopy, are of special significance in this filed. Nevertheless a better understanding of the problems that arise out of new special questions of art history can be obtained only by the right interpretation of the data obtained by scientific methods.

---

Mikrochemische Charakterisierung von Gemälden

Zusammenfassung Die grundlegende Bedeutung wissenschaftlicher Forschung für die Erhaltung und Restaurierung von Kunstwerken ist unbestritten. Die hier beschriebenen Methoden (Lichtmikroskopie, mikrochemische Reaktionen, Emissionsspektroskopie, Chromatographie, Rasterelektronen-Analyse und Infrarotspektroskopie) sind hierfür von zunehmender Bedeutung. Sie ermöglichen ein besseres Verständnis neuerer kunsthistorischer Probleme nur durch die richtige Interpretation der Ergebnisse wissenschaftlicher Methoden.

     

Wave operators and similarity of certain non-normal one-parameter groups

This paper starts from a self-adjoint Schrödinger operator H(0) for three particles. If the interaction is dilation-analytic, H(0) has an analytic continuation H() (>0). G(t,) (–(±,a,) defined as strong limits, when t±, of t-dependent operators. The wave operators establish transformations under which the subgroups are similar to unitary groups. The scattering matrix determined by G(t,) is diagonal with respect to a.This work was supported in part by the National Science Foundation under grant DMS-8301096.     

Research on the aromas. On the aroma of coffee. I

The analysis of a coffee concentrate is described: 202 constituents have been identified, 154 of which for the first time. Some of these compounds are new chemicals; their synthesis is described. Most identifications are based upon mass spectrometry combined in many cases with IR. spectrometry and comparison with reference compounds. MS. and IR. data are given in detail. The analysis shows that a large number of minor constituents is present, many of which could not be identified.     

Synthesis of N-heterocyclic carbene precursors bearing biphenyl units and their use in ruthenium-catalyzed ring-opening metathesis polymerization

A range of new imidazolium and imidazolinium chlorides bearing biphenyl units on their nitrogen atoms was synthesized. They differed by the electron-withdrawing or -donating nature and the steric bulk of the substituents on their aromatic rings. These various N-heterocyclic carbene (NHC) precursors were combined with the [RuCl₂(p-cymene)]₂ dimer and potassium tert-butoxide to generate the corresponding ruthenium-arene complexes [RuCl₂(p-cymene)(NHC)] in situ. The catalytic activity of these species was investigated in the photoinduced ring-opening metathesis polymerization (ROMP) of cyclooctene. The results obtained confirmed the necessity of blocking the ortho-positions of the phenyl rings in the vicinity of the metal center in order to attain high catalytic efficiencies. They also showed that changing the steric and electronic properties of the substituents on the remote phenyl rings of the biphenyl units had no significant influence on the outcome of the polymerization.     

The modulating possibilities of dicarbollide clusters: optimizing the Kharasch catalysts

exo-Cluster dicarbollides substitution has allowed tuning of the E degrees (Ru(II)/Ru(III)) potential to obtain the best-performing Kharasch catalyst. We postulate that this is possible through the to-and-fro electron movement between the boron cluster and the sulfonium moieties.