Excess molar internal pressures and changes in refractive indices of acetone + methanol + (2-methyl-1-propanol or 3-methyl-1-butanol) at 298.15 K

This study aims at extending the characterisation by refractive index of ternary systems containing potential separation agents for extractive distillation of minimum azeotropes. The systems considered are acetone?+?methanol?+?(2-methyl-1-propanol or 3-methyl-1-butanol), which have been studied at 298.15?K and atmospheric pressure for the whole composition diagram. Parameters of polynomial equations, which represent the molar fraction dependence of the refractive index and derived property, are gathered. Based on the variations of the derived values with composition, conclusions about the molecular interactions and their dependence on branched alcohol structure were drawn.     

Are Fulleranes Responsible for the 21 Micron Feature?

Recent detections of Cbegin{document}$ _{60} $end{document}, Cbegin{document}$ _{70} $end{document}, and Cbegin{document}$ _{60} $end{document}begin{document}$ ^+ $end{document} in space induced extensive studies of fullerene derivatives in circumstellar environments. As the promising fullerene sources, protoplanetary nebulae (PPNe) shows a number of unidentified bands in their infrared spectra, among which a small sample exhibits an enigmatic feature at begin{document}$ sim $end{document}21 begin{document}$ mathtt{μ} $end{document}m. Hydrogenation converts fullerenes into fulleranes, which breaks the symmetry of fullerene molecules and produces new infrared bands. In this work, we investigate the possibility of fulleranes (Cbegin{document}$ _{60} $end{document}Hbegin{document}$ _m $end{document}) as the carrier of the 21 begin{document}$ mathtt{μ} $end{document}m feature in terms of theoretical vibrational spectra of fulleranes. The evidences favoring and disfavoring the fullerane hypothesis are presented. We made an initial guess for the hydrogen coverage of Cbegin{document}$ _{60} $end{document}Hbegin{document}$ _m $end{document} that may contribute to the 21 begin{document}$ mathtt{μ} $end{document}m feature.     

Water-soluble aminocalix[4]arene receptors with hydrophobic and hydrophilic mouths

We report the new water-soluble aminocalix[4]arene hosts 1 and 2 with deep hydrophobic cavity facilitating hydrophobic mouth and hydrophilic mouth, respectively. The ¹H NMR titrations revealed that host 1 shows high selectivity for neutral guests 9 and 10, with log K of 4.2 and 4.6, respectively. The host 2 shows log K of 4.9 for binding with guest 15. Moreover, the binding ability of the host 2 for guest 14 is stronger by a factor of 1000 than that of the host 1.     

Implementation of the IMOMM methodology for performing combined QM/MM molecular dynamics simulations and frequency calculations

A technique for implementing the integrated molecular orbital and molecular mechanics (IMOMM) methodology developed by Maseras and Morokuma that is used to perform combined quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations, frequency calculations and simulations of macromolecules including explicit solvent is presented. Although the IMOMM methodology is generalized to any coordinate system, the implementation first described by Maseras and Morokuma requires that the QM and MM gradients be transformed into internal coordinates before they are added together. This coordinate transformation can be cumbersome for macromolecular systems and can become ill-defined during the course of a molecular dynamics simulation. We describe an implementation of the IMOMM method in which the QM and MM gradients are combined in the cartesian coordinate system, thereby avoiding potential problems associated with using the internal coordinate system. The implementation can be used to perform combined QM/MM molecular dynamics simulations and frequency calculations within the IMOMM framework. Finally, we have examined the applicability of thermochemical data derived from IMOMM framework. Finally, we have examined the applicability of thermochemical data derived from IMOMM frequency calculations. Received: 11 May 1998 / Accepted: 14 August 1998 / Published online: 16 November 1998     

A new “hydrophobic template” method detects segments forming transmembrane α-helical bundles in ion channels

We present a “hydrophobic template” method enabling recognition of α-helix bundles in membrane channels from sequence analysis. Inspection of hydrophobic properties of pore-forming helices in proteins with known structure (A-B₅ toxins) permits delineation of a common polarity motif: two hydrophobic surface stretches separated by polar areas. The bundles are stabilized by nonpolar interhelical contacts. A number of transmembrane segments were checked for presence of this motif, and it was detected for pore-forming helices of several ion transporters (segments M2 of acetylcholine and GABA_A receptors, α5 peptide of δ-endotoxin), which reveal five α-helix bundle architecture. Applications of the method to modeling of membrane channels are discussed. Received: 24 April 1998 / Accepted: 3 September 1998 / Published online: 7 December 1998     

Future perspectives of radiation chemistry

Future perspectives of radiation chemistry are discussed by the analysis of the related information in detail as obtained from our recent surveys of publications and scientific meetings in radiation chemistry and its neighboring research fields, giving some examples, and are summarized as follows. (1) Traditionally important core-parts of radiation chemistry should be activated more. The corresponding research programs are listed in detail. (2) Research fields of physics, chemistry, biology, medicine, and technology in radiation research should interact more among them with each other. (3) Basic research of radiation chemistry should interact more with its applied research. (4) Interface research fields with radiation chemistry should be produced more with mutually common viewpoints and research interests between the two. Interfaces are not only applied research but also basic one.     

Assessment of conformation and energetics of the N-terminal part of elafin via computer simulations

Elafin, a specific inhibitor of elastase, is thought to play a regulatory role in inflammation. An NMR-derived solution structure of recombinant elafin has been reported [Francart et al. (1997) J Mol Biol 268:666 ], although the conformation of its flexible N-terminal part is not established. There is experimental evidence that the N terminus (residues 1–15) of elafin interacts with the cell membrane. To explore the conformational preferences of residues in this region, we have performed Monte Carlo simulations of the peptide in water, in cyclohexane, and in a model membrane. Additionally, 3.7-ns molecular dynamics with explicit water was carried out. The main results were that the hydrophobic environment stabilizes an α helix in the region 6–11, the peptide is unordered in water, and it is attached to the membrane via the amphiphilic α-helix 6–11, which inserts with its N terminus forming an angle of about 60° to the membrane plane. We therefore assume that in nonpolar media the N-terminal part of elafin forms a short α helix which might act as a membrane anchor. Received: 5 July 2000 / Accepted: 4 October 2000 / Published online: 28 February 2001     

Peptides in membranes: assessment of environmental effects via simulations using an implicit solvation model

A recently developed implicit solvation model is applied to Monte Carlo simulations of peptides in bilayer-mimetic and polar environments. The model employs the formalism of atomic solvation parameters and reproduces experimental data. Solvent effects on the␣structure of the following peptides were studied: 20-residue poly-Leu and poly-Val, transmembrane helix A of bacteriorhodopsin, magainin2. It was shown that a␣membrane-like environment considerably promotes α-helix formation (all the peptides were found to be α-helical), while simulations in water reveal helix distortion. Consistency of the results with experimental data and further implications of the model are discussed. Received: 24 April 1998 / Accepted: 3 September 1998 / Published online: 10 December 1998     

Semiempirical quantum chemical calculations of molecular π- complexes

A method is suggested to calculate the geometrical and thermodynamical characteristics of organic π- complexes. The interaction energy is considered as a sum of two terms: the specific quantum chemical interaction of the π- electron systems depending essentially on the chemical nature of reactants, and the nonspecific atom-atom (van-der-Waals) interaction depending only on the kind of atoms belonging to the fragments of a complex. An attractive quantum chemical interaction is described in terms of the PPP-method; the van-der-Waals interaction is expressed in terms of the empirical exp-6-potential. The geometries of complexes are found by the complete energy minimization with respect to six parameters characterizing the mutual orientation of the complex fragments. Energies, entropies and equilibrium constants of several tetracyanoethylene π-complexes are calculated by this method. The results agree satisfactorily with experimental data.     

Unusual Magneto-Structural Features of the Halo-Substituted Materials [FeIII(5-X-salMeen)2]Y: a Cooperative [HS-HS]↔[HS-LS] Spin Transition

X-ray structures of the halo-substituted complexes [Fe^III(5-X-salMeen)₂]ClO₄ (X=F, Cl, Br, I) [salMeen=N-methyl-N-(2-aminoethyl)salicylaldiminate]at RT have revealed the presence of two discrete HS complex cations in the crystallographic asymmetric unit with two perchlorate counter ions linking them by N−H_amine⋅⋅⋅O_perchlorate interactions. At 90 K, the two complex cations are distinctly HS and LS, a rare crystallographic observation of this coexistence in the Fe^III-salRen (R=alkyl) spin-crossover (SCO) system. At both temperatures, crystal packing shows dimerization through C−H_imine⋅⋅⋅O_phenolate interactions, a key feature for SCO cooperativity. Moreover, there are noncovalent contacts between the complex cations through type-II halogen-halogen bonds, which are novel in this system. The magnetic profiles and Mössbauer spectra concur with the structural analyses and reveal 50 % SCO of the type [HS-HS]↔[HS-LS] with a broad plateau. In contrast, [Fe^III(5-Cl-salMeen)₂]BPh₄⋅2MeOH is LS and exhibits a temperature-dependent crystallographic phase transition, exemplifying the influence of lattice solvents and counter ions on SCO.     

Application of Soczewiński-type equation to study molecular interactions in liquid adsorption chromatography

Summary The possible application of the Soczewiński-type linear relationship is considered for the estimation of molecular interactions in chromatographic systems. The physical meaning of the slope of this relationship is discussed. The influence of solvent adsorption, solvent-solvent and solute-solvent interactions on the slope are shown. Theoretical conceptions are verified for several solutes and five binary mixed mobile phases.     

Effect of Temperature and Composition on the Density and Viscosity of Binary Mixtures of Ionic Liquid with Alcohols

Density and viscosity were determined for the binary mixtures containing the ionic liquid 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]) and 1-alcohols (1-butanol, 1-pentanol and 1-hexanol) at six temperatures (298.15, 308.15, 318.15, 328.15, 338.15 and 348.15) K and ambient pressure. The density and viscosity correlation for these systems was tested by an empirical second-order polynomial and by the Vogel-Fucher-Tammann equation in wide ranges of temperatures. Excess molar volumes were described by the Redlich-Kister polynomial expansions. The density and viscosity variations with composition were described by polynomials. A qualitative analysis of the trend of the properties with solvent and temperature was performed. The obtained results indicate that ionic liquid interactions with 1-alcohols are strongly dependent on the special trend of packing of this ionic liquid into hydroxylic solvents. As previously observed, an increase of the 1-alcohol carbon-chain length leads to lower interactions on mixing. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Influence of column characteristics on retention and selectivity in RP-HPLC

Summary Eight commercially available reversed-phase (RP) columns of widely different characteristics were evaluated and compared using the linear solvation energy relationships (LSER). Retention factors of 32 solutes of different types were determined under isocratic conditions using an acetonitrile-water (30∶70) mobile phase. Stationary phase properties were compared by the fitting coefficients of the LSER-based regression equations which are characteristic of the individual stationary phases and represent the extent of various molecular interactions contributing to the retention process. The good agreement between the calculated and measured logk values for different type of compounds support the adequacy and applicability of the LSER model to describe chromatographic retention. Characterization of column performance for the separation of various type of compounds was established by the determination of the different selectivity factors representing hydrophobic selectivity, polar selectivity and specific selectivity.     

Second-order quantum similarity measures from intracule and extracule densities

The calculation of quantum similarity measures from second-order density functions contracted to intracule and extracule densities obtained at the Hartree-Fock level is presented and applied to a series of atoms, (He, Li, Be, and Ne), isoelectronic molecules (C₂H₂, HCN, CNH, CO, and N₂), and model hydrogen-transfer processes (H₂/H⁺, H₂/Hot, H₂/H⁻). Second-order quantum similarity measures and indices are found to be suitable measures for quantitatively analyzing electron-pair density reorganizations in atoms, molecules, and chemical processes. For the molecular series, a comparative analysis between the topology of pairwise similarity functions as computed from one-electron, intracule, and extracule densities is carried out and the assignment of each particular local similarity maximum to a molecular alignment discussed. In the comparative study of the three hydrogen-transfer reactions considered, second-order quantum similarity indices are found to be more sensitive than first-order indices for analyzing the electron-density reorganization between the reactant complex and the transition state, thus providing additional insights for a better understanding of the mechanistic aspects of each process. Received: 7 July 1997 / Accepted: 29 October 1997     

Separation of minor macromolecular constituents from multicomponent polymer systems by means of liquid chromatography under limiting conditions of enthalpic interactions

Minor (<1%) macromolecular constituents may significantly affect physical/utility properties of the multicomponent polymer systems. Separation and molecular characterization of the small amounts of macromolecular additives from the dominant polymer matrices represents an exacting analytical problem. Recently a series of unconventional liquid chromatographic methods was developed for separation of the constituents of polymer blends; their generic name is Liquid chromatography under limiting conditions of enthalpic interactions, LC LC. The LC LC procedures employ the difference in elution rate of the low molecular substances and the macromolecules within the column packed with porous particles. Small molecules permeate practically all pores of the packing and therefore they elute slowly. Polymer species are partially of fully pore excluded and in absence of enthalpic interactions they are rapidly transported along the column. The appropriately chosen low molecular substances promote interactions of macromolecules within the column. If eluted in front of sample, the interaction promoting low molecular substance may create a sort of slowly eluting barrier that is “impermeable” for the interacting macromolecules and efficiently decelerates their fast transport. The blocking action of a barrier differs for macromolecules of distinct nature, which elute from the column with a different rate to be mutually separated irrespectively of their molar mass. In present work, different approaches to the LC LC separations are compared from the point of view of their applicability to complex polymer systems, in which one constituent is present at very low concentration, and also in light of sample recovery. The practical examples are the two- and three-component polymer blends of polystyrenes, poly(methyl methacrylate)s and poly(vinyl acetate)s of different molar mass averages and distributions, as well as the diblock copolymers polystyrene-block-poly(methyl methacrylate) that contain their parent homopolymers.     

Aminocalix[4]arene: the effect of pH on the dynamics of gate and portals on the hydrophobic cavity

The pH of a solution shows a significant effect on the dynamics of the gate (formed by eight benzylic functions) and portal on the hydrophobic cavity of receptor. At pH 5.8 the gate closes and prohibits the entry of anionic guests. However, at pH 7.3 the gate opens and allows the entry of anionic guests into the hydrophobic cavity. It is the first time that anionic receptor efficiently recognizes anionic guests.     

Improved generator coordinate Hartree–Fock method for molecular systems: application to H2, Li2 and LiH

The improved generator coordinate Hartree–Fock (GCHF) method is extended to molecular systems. The Griffin–Hill–Wheeler–HF equations were solved by an integral discretization technique. The method is then implemented with the use of the GAMESS program and applied to the H₂, Li₂, and LiH molecules. For these molecules, sequences of basis sets of atom-centred Gaussian-type functions are employed to explore the accuracy achieved with our approach. For all systems studied, our ground-state HF total energies are better than those obtained with basis sets generated with the original GCHF method for molecules and larger even-tempered basis sets. For H₂, Li₂, and LiH, the differences between our best energies and the corresponding numerical HF results are about 2 × 10⁻², 1, and 4 × 10⁻¹ μhartree, respectively. The dipole, quadrupole, and octupole moments at the center of mass and electric field, the electric field gradient, the electrostatic potential, and the electron density at the nuclei were evaluated and compared with results reported in the literature. Received: 4 May 1999 / Accepted: 22 July 1999 / Published online: 2 November 1999     

Chemical interpretation of differences in pyrolysis—mass spectra of simulated mixtures of biopolymers by factor analysis with graphical rotation

A method is described for the separation of a set of pyrolysis-mass spectra of simulated mixtures into subspectra for each of the components and for calculation of their relative concentrations. The technique is based on factor analysis and graphical rotation and is evaluated on a data set, constructed by linear combination of pyrolysis-mass spectra of three biopolymers.     

Chemical characterization of yeasts and yeast-like fungi by factor analysis of their pyrolysis—mass spectra

A factor analysis technique is described for determining differences in the pyrolysis—mass spectra of complex biological material in terms of chemical components. The technique is based on factor analysis with subsequent graphical rotation in order to find axes, describing changes in chemical components. Three yeast species could be clearly distinguished on the basis of four classes of chemical building blocks, viz. N-acetylamino-sugar, protein, hexose and pentose.