Tilting operator for phospholipidic molecular domains at the liquid–gas interface

We derive a coordinate independent operator expression for the tilting operator of molecular domains at the liquid–gas interface. The domains are made up of phospholipidic molecules modeled as spherocylinders. The molecules of the domain are oriented parallel to each other. The centers of symmetry of the molecules form a lattice. The tilting operator keeps track of the deformations suffered by this lattice as the domain molecules are tilted relative to the normal to the interface. The results obtained are important for dynamic calculations of inclination dependent collective film characteristics, as in the simulation of surface density versus surface pressure curves in a Langmuir film. The tilting operation can be decomposed into three separate simple operations: a global rotation, a local oblique realignment, and a global vertical translation. This revised version was published online in July 2006 with corrections to the Cover Date.     

The o-chlorotoluene photodissociation at 266 nm

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Methylcorrinoids Methylate Radicals-Their Second Biological Mode of Action?

An unprecedented methylation of alkyl radicals is possible with the diamagnetic vitamin B(12) derivative methylcobalamin (see schematic representation). Methylcorrinoids can thus also be considered as methylating cofactors in radical C-methylations.     

Second critical micelle concentration of dodecyldimethylbenzylammonium chloride in aqueous solution at 25 °C

Two breaks have been found on the conductivity, refractive index, density and sound velocity against molality plots for aqueous solutions of dodecyldimethylbenzylammonium chloride at 25 °C, in the absence of any additive. The first break corresponds to the critical micelle concentration, cmc. The second, less distinct break, occurring in the molality range of 0.082 to 0.104 mol kg^-1, in dependence on the technique applied, has been ascribed to the second critical micelle concentration, 2nd cmc, responsible for structural transitions of spherical micelles. Values of cmc and 2nd cmc have been also estimated conductometrically for tetradecyltrimethylammonium bromide and dodecylpyridinium chloride. On the basis of available conductometric data it has been shown that the 2nd cmc/cmc ratio varies in the range of 2 to 10 in dependence on the type of 1:1 ionic surfactant. It has been also shown that for a given class of surfactants, the logarithm of 2nd cmc varies linearly with the number of carbon atoms in the alkyl chain ( n= 12, 14 and 16). Both empirical regression coefficients depend upon the class of surfactants considered.     

The Binding Pocket at the Interface of Multimeric Telomere G-quadruplexes: Myth or Reality?

Human telomeric DNA with hundreds of repeats of the 5’-TTAGGG-3’ motif plays a crucial role in several biological processes. It folds into G-quadruplex (G4) structures and features a pocket at the interface of two contiguous G4 blocks. Up to now no structural NMR and crystallographic data are available for ligands interacting with contiguous G4s. Naphthalene diimide monomers and dyads were investigated as ligands of a dimeric G4 of human telomeric DNA comparing the results with those of the model monomeric G4. Time-resolved fluorescence, circular dichroism, isothermal titration calorimetry and molecular modeling were used to elucidate binding features. Ligand fluorescence lifetime and induced circular dichroism unveiled occupancy of the binding site at the interface. Thermodynamic parameters confirmed the hypothesis as they remarkably change for the dyad complexes of the monomeric and dimeric telomeric G4. The bi-functional ligand structure of the dyads is a fundamental requisite for binding at the G4 interface as only the dyads engage in complexes with 1 : 1 stoichiometry, lodging in the pocket at the interface and establishing multiple interactions with the DNA skeleton. In the absence of NMR and crystallographic data, our study affords important proofs of binding at the interface pocket and clues on the role played by the ligand structure.     

Solid water at room temperature?

Previously, we have shown that water adjacent to many hydrophilic substances excludes colloidal and molecular solutes. It was labelled “exclusion zone” (EZ) or “fourth phase” water. A salient feature is its characteristic light absorbance at or near 270 nm. In this study, EZ water formed against three chemically distinct surfaces, Nafion, ghee, and Whatman-5 filter paper was extracted, characterized by UV–Visible absorbance spectroscopy, and solidified either by lyophilizing or evaporation in an oven. The resulting highly stable solid was dissolved in water and confirmed as EZ water by its characteristic absorbance at 270–280 nm. We used mass spectroscopy to verify the absence of ionizable contaminants that could reproduce the characteristic “signature EZ” spectra in the three liquid preparations, or in the solids formed from desiccated EZ water that had been reconstituted in deionized water. Hence, a solid form of EZ water may, indeed, exist at room temperature.     

The three-dimensional structure of trichosanthin refined at 2.7resolution

The three-dimensional structure of trichosanthin crystallizing in space group C2 has beenrefined at 2.7 resolution from a previously reported starting model at 3 resolution based on asolvent flattened map and the revised primary structure consisting of 247 amlno-acids.The finalR-factor is 19.2% with the root mean-square deviations of 0.018 from ideal bond lengths andof 2.2° from ideal bond angles.Trichosanthin molecule is composed of two domains,the largedomain consisting of 181 amino-acld residues starting from N-terminus and the small domain con-sisting of the rest of the amino-acid residues.The molecule contains eight α-helices,five β-sheetsmade of sixteen β-strands,and some reverse turns.It is noteworthy that some of the α-helicesand β-sheets show irregular hydrogen bonding patterns.Six of the thirteen residues absolutelyconserved in eleven ribosome-inactivating proteins are located in a cleft near the interface ofthe two domains and they are likely to be active sites.Three additional conservative residueslocated in the cleft region might make some functional contribution as well.     

Investigation of the Second Harmonic Generation at the Water–Vacuum Interface by Using Multi-Scale Modeling Methods

The Sequential Quantum Mechanics/Molecular Mechanics scheme has been enacted to perform a systematic investigation of the polarizability (α) and first hyperpolarizability (β) responses at the water–vacuum interface. After performing classical molecular dynamics simulations to provide snapshots of the structures, quantum chemistry calculations of the linear and nonlinear optical responses have been performed for clusters of five water molecules at the time-dependent DFT level in combination with different embedding schemes, ranging from point charges to polarizable point charges, with and without local field effects. When going from the bulk to the interface, the main observations of these calculations encompass i) a modest increase of the average polarizability but an increase by about a factor of two of its anisotropy, ii) an increase by about 20 % of the β_HRS response, accompanied by a small increase of its depolarization ratio, and iii) a net increase of the component of the β tensor normal to the interface (β_zzz) as well as of β_//. Globally, the interfacial effects on β are localized at the first molecular layer while they are observed up to the fourth molecular layer on α.     

The Second Virial Coefficient and “Liquid-Vapour” Phase Transition in Sterically Stabilized Colloidal Dispersions

Abstract

The observed temperature dependence of the second virial coefficient in dispersions of sterically stabilized colloids is explained using a macroparticle interaction potential that allows for a decrease of the effective hard core diameter with increasing temperature. Such assumptions also account for the “liquid-vapour” phase transition.     

Deviation of the Kempster?Lipson Law from Linearity

It is shown that the larger organic molecules composed of C, H, N, and O at most crystallizing in the voluminous unit cells tend to be more loosely packed than those that crystallize in the unit cells with small volumes. This is demonstrated by the deviation from the linear relationship (Kempster? Lipson law) between the unit‐cell volumes and the sum of the non‐H‐atoms in the unit cell. This deviation can be distinguished from the effect of the varying number of H‐atoms in the described compounds: the ratio of the non‐H‐atom to the H‐atoms converges to a constant value close to 1 towards the large unit‐cell volumes. The proportion of the disordered structures increases towards the large unit‐cell volumes and thus correlates with the less dense packing of the large molecules. The deviation of the Kempster? Lipson law from linearity can be interpreted as the manifestation of decreasing ability of a crystallization of large molecules.     

The Electrochemistry of Cytochrome c at a Viologen-thiol Self-Assembled Monolayer

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The Solubility of Hydrogen Sulfide in Acetonitrile at 25°C

Abstract

The solubility of hydrogen sulfide in acetonitrile at 25°C has been determined both gravimetrically and titrimetrically to be 0. 528 M. The effects of the presence of water and an electrolyte (LiClO₄) on this parameter are reported.     

The Crystal and Molecular Structure of Trichosanthin at 2.6 Resolution

The crystallographic refinement of trichosanthin has been performed at 2.6 resolution. The crystal and molecular structure of trichosanthin is described in detail in this paper. On summarizing the regularity of the amino acid sequences of eight kinds of ribosome inactivating proteins and combining with the crystal and molecular structure of trichosanthin, fifteen most conservative amino acid residues are analyzed. It is found that four most conservative polar amino acid residues Gln156, Glu160, Arg163 and Glu189 gather on the molecular surface on the boundary of the large and small domains, thus forming the active center of the protein molecule.     

Are thiophenes attacked at sulphur by nitrenes?

Ethyl azidoformate is shown to attack a range of thiophenes at sulphur giving transient S,N-ylides which can be trapped with acenaphthylene as [4+2] adducts; thiophene yields self-trapped products also.     

The titration of bismuth with complexone III at pH 2.0–2.8

A titration procedure for Bi is described; by adding an excess of complexone III (Versene) and back-titrating at pH 2.0–2.8 with standard thorium nitrate solution using alizarin-S as an indicator, it is possible to determine Bi with a precision of about 0.3%.By choice of the proper pH it is possible to eliminate most of the interfering elements, but at pH 2.0 Fe and Ni still interfere, and Cu only to a slight extent.Attention is drawn to the fact that it may be possible to determine Bi at pH 2.0 by direct titration with complexone III in the presence of much tartrate, using thiourea as an indicator. This method might be of value in the presence of Sn and Sb.