Effect of configuration and conformation on the spin multiplicity in xylylene type biradicals

The singlet-triplet splitting energy gap ΔEs-T= Es - ET is calculated for the ortho-, meta-, and para-xylylenes and their heteroatomic analogous by means of AM1-CI approach. It is shownthat when the radical centers R(R=H2C- ,H2N - or HN'- ) are twisted sufficiently far out ofconjugation with the benzene ring, ΔEs-T tends to zero or is negative, i.e. ortho-, meta-, and para-phenylenes turn into weak ferromagnetic or antiferromagnetic coupling unit, while they are strong ferromagnetic (meta-isomers) or antiferromagnetic (ortho-, para-isomers) coupling units under planar conformation. It is suggested that serious twisted conformation is not recommended candidate for the design of novel high-spin molecules with stable high-spin ground states by ortho-or para-phenylene coupling unit.     

Detection limits of thermal conductivity and photoionization detectors in high speed narrow bore CGC

Not only the required input band width and the availability of compatible instrumentation limit the reduction of column diameter but so do the column sample capacity and detector characteristics, such as volume and sensitivity. In this paper the scope and limitations of thermal conductivity and photoionization detection at atmospheric and reduced pressures for capillary gas chromatography are discussed at length. It is shown that the sensitivity of a thermal conductivity detector is inversely proportional to the pressure. Reduction of cell volume and decrease of pressure appear equivalent with respect to minimum detectable amount and peak broadening. This results in femtogram detection limits for column diameters as low as 10 μm. For photoionization detectors the sensitivity did not improve at reduced pressures, so that the effect of reduced pressure and addition of make-up gas on the detection limits is the same. Nevertheless, the applicability of a low volume photoionization detector (40 μl) for capillary columns with a diameter as low as 50 μm will be demonstrated by series coupled PID and FID detectors and illustrated with various applications.     

顺、反式结构对MPV异构体的分子构型和吸收光谱的影响

采用量子化学B3LYP／6－31G方法，分别优化；计算了MPV三种异构体的几何和电子结构，结果表明，在异构体中反式结构为平面构型，顺式结构为非平面构型，反式结构增加可使异构体的UV最大吸收峰有规则红移，解释了实验现象，并对烷氧基取代聚对苯乙炔中的顺、反异构其对相关性质的影响进行了有益的讨论。     

Crystal and molecular structures of two sulfonium ylides: Influence of secondary S...O interactions on the molecular conformation and packing

Results are reported of an x-ray structure study of two sulfonium ylides formed by reaction of dimethylacetylenedicarboxylate with DMSO. It is demonstrated that one of these forms inter- and the other intramolecular secondary S...O interactions. In the first case they affect the molecular packing in the crystal; in the second, the molecular conformation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 81–87, January, 1991.The authors thank V. A. Palyulin for calculating the geometry of the cyclic fragment using the Kramer-Pople method.     

Structure and conformation in molecular peroxides

Molecular structures and energies have been calculated in the MINDO approximation for fifteen neutral and anionic peroxides: fully optimized torsional potential functions have been calculated for twelve of these, and torsional potential functions, subject to constrained optimizations, for a further two peroxides. Bond dissociation energies D(R₁O—OR₂) were also calculated. Equilibrium structures and energies were calculated for the polyoxo species H₂O_n, HO_nF, F₂O_n, HO_n and FO_n (n ? 4), and a complete set of bond dissociation energies derived for H₂O_n, HO_nF, and F₂O_n.     

Influence of cetyltrimethylammonium bromide on phosphatidylcholine-coated capillaries

Large unilamellar vesicles of egg-phosphatidylcholine (eggPC), a naturally occurring phospholipid, were used in capillary electrophoresis (CE) for semi-permanent coating of fused silica capillaries. The stability of the phospholipid coating was tested at different cetyltrimethylammonium bromide (CTAB) concentrations with and without CaCl₂ present in the coating solution. The effect of physical factors influencing the coating stability (e.g. duration of the coating time, storage temperature of the coating solution) were also studied. Standing overnight in background electrolyte (BGE) solution did not alter the eggPC phospholipid coating noticeably. The performance of the coating was tested with a mixture of basic proteins (lysozyme, ribonuclease A and -chymotrypsinogen A). Highest efficiencies (over 200,000 plates m^–1) were achieved when the capillary was filled for 15 h with a liposome solution containing both CTAB and CaCl₂.Electronic Supplementary Material Supplementary material is available for this article at     

Activation energy associated with the electromigration of oligosaccharides through viscosity modifier and polymeric additive containing background electrolytes

The activation energy related to the electromigration of oligosaccharides can be determined from their measured electrophoretic mobilities at different temperatures. The effects of a viscosity modifier (ethylene glycol) and a polymeric additive (linear polyacrylamide) on the electrophoretic mobility of linear sugar oligomers with α1–4 linked glucose units (maltooligosaccharides) were studied in CE using the activation energy concept. The electrophoretic separations of 8‐aminopyrene‐1,3,6‐trisulfonate‐labeled maltooligosaccharides were monitored by LIF detection in the temperature range of 20–50°C, using either 0–60% ethylene glycol (viscosity modifier) or 0–3% linear polyacrylamide (polymeric additive) containing BGEs. Activation energy curves were constructed based on the slopes of the Arrhenius plots. With the use of linear polyacrylamide additive, solute size‐dependent activation energy variations were found for the maltooligosaccharides with polymerization degrees below and above maltoheptaose (DP 7), probably due to molecular conformation changes and possible matrix interaction effects.     

Influence of solvents on the conformation of benzoin

The conformation of benzoin in several organic solvents is investigated by infrared spectrometry and dipolometry. The frequencies, intensities, and band shapes of the ν(OH), ν(CO), and aromatic ring vibrations indicate that in solvents of low proton acceptor ability, the cis conformer with intramolecular OH···O hydrogen bonding is preserved. In solvents of large proton acceptor ability there is equilibrium between the cis and trans conformers. The dipole moments are less sensitive to conformational changes, but indicate the same trends. The results are discussed as a function of the specific solvation of the O atoms or OH groups of benzoin.     

Variety of the molecular conformation in Peptide nanorings and nanotubes

Possible molecular conformations in peptide nanorings and nanotubes were theoretically investigated by a mathematical conformation analysis as well as ab initio Hartree-Fock calculations. The mathematical analysis predicts not only the conventional nanorings having an extended-type (E-type) backbone (trans zigzag) but also the novel ones having bound-type (B-type) backbones with a smaller internal diameter. Ab initio calculations for the amino acid substitution reveal that all 20 encoded residues can form both types of the above nanorings as a local minimum. However, the energetically stable type is determined in accordance with the kind of the replaced side chains. Moreover, the present work theoretically reveals that both types of nanorings stack to form nanotubes through inter-ring hydrogen bonds, i.e., larger E-type nanotubes and smaller B-type nanotubes. Electronically, the HOMO and LUMO states of the nanoring and nanotube backbones are formed by the in-plane pi state. The replacement by the appropriate residues is furthermore predicted to intrude additional levels in the energy gap and to form the frontier states localized at the side chains.     

HPLC separations of meropenem and selected pharmaceuticals using a polar endcapped octadecylsilane narrow bore column

Summary Stability indicating high performance liquid chromatography methods have been developed for the assay of meropenem in combination with either dopamine (A), aminophylline (B), metoclopramide (C) or ranitidine (D) in intravenous fluid mixtures. Separations B, C and D were performed on a polar endcapped ODS column (150×2 mm) with aqueous, pH 3.0—acetonitrile (89∶11, 88∶12, and 92∶8) eluent and detection at 270, 290, 317 nm respectively. Meropenem was linear over the concentration ranges 126.88–507.50, 131.25–525, and 131.25–525 gmg mL⁻¹. Aminophylline, metoclopramide and ranitidine were linear over the concentration ranges 13–52, 37.5–150, and 25–100 μg mL⁻¹. Separation A was performed on a conventional ODS column (150×2.1 mm) with aqueous, pH 3.0—acetonitrile (85∶15) eluent and detection at 280 nm. Meropenem and dopamine were linear in the 61.25–245 and 10–40 μg mL⁻¹ ranges, respectively. Accuracy and precision for all methods were 0.20–3.30% and 0.10–1.58%, respectively. Accelerated stability studies have been carried out on each drug by exposure to acid, base, H₂O₂, and heat for different time periods.     

Modeling of molecular packing and conformation in oligofluorenes

We describe the application of molecular modeling to study problems related to the packing and conformation of oligofluorene molecules in the solid state. First of all, we describe an improved force field for oligofluorenes. The model is based on the MM3 force field for the intramolecular degrees of freedom, but it relies on ab initio calculations for the torsion potential between two monomers and the electrostatic interactions. We also report ab initio calculations of the interaction potentials between fluorene and fluorenone units. The force field has been tested on the crystal structures of a fluorene monomer, a dimer, and a pentamer containing a fluorenone at the center. It has then been employed to study conformational defects of the chains, both in vacuo and in the bulk. We find that certain modes of inversion from right-handed to left-handed helices are also possible within the constraining environment of the crystals. The effect of the presence of two different types of side chains has been also addressed. Finally, the possibility of having two fluorene units parallel and close to each other has been investigated as a model of a ground-state precursor of an excimer. Our simulations show that this configuration is sterically and energetically unfavorable so that formation of an excimer following optical excitation appears to be unlikely.     

Influence of pressure on protein conformation

The pressure effects in aqueous solutions of biopolymers are presented. It is shown, that pressure in the range of 300 to 800 MPa leads for small monomeric proteins to irreversible denaturation. Larger oligomeric enzymes and structural proteins dissociate reversibly in the pressure range around 100 MPa. The sign of the volume effects of the various noncovalent interactions contributing to the observed reaction and activation volumes is discussed.     

Influence of silaproline on peptide conformation and bioactivity

The analogue gamma-(dimethylsila)-proline, denoted silaproline (Sip), was synthesized in both enantiomerically pure forms by diastereoselective alkylation of a chiral glycine equivalent with use of Sch?llkopf's bis-lactim ether method. The effect of replacing a proline residue in model peptides by this new proline surrogate has been examined in the crystal state by X-ray diffraction and in solution by IR absorption and NMR techniques. Silaproline and proline-containing sequences exhibit very similar conformational properties. Silaproline was also substituted for proline in a neurotensin (8-13) analogue that retained biological activity and exhibited enhanced resistance to biodegradation.     

Complementary molecular orbital investigations on the conformation of choline derivatives

Quantum-mechanical computations by the PCILO method, applied previously to the study of the conformational properties of acetylcholine and its derivatives modified in the central part of this molecule, are extended to modifications involving its cationic head and its ester terminal. The replacement of the methyl groups of the cationic head by hydrogens or ethyl groups leads to a steep decline in parasympathomimetic activity. It is shown that the triethyl derivative conserves the gauche form as the most stable one. The redistribution of the electronic charges at the onium group implies, however, a transition from an ionic to a hydrophobic binding. The replacement of the methyls by two or three hydrogens leads to a different preferred gauche-gauche conformation. The replacement of the methyl group at the ester terminal by a phenyl ring enables a comparison with the conformational properties of local anesthetics. The study brings about evidence, substantiated by NMR spectroscopy, that acetylcholine analogs and protonated local anesthetics are conformationally similar. Choline ethers also show a general preference for a gauche conformation. Nevertheless, biological studies do not indicate a constant correlation between conformation and biological potency. Conformational analogies or discrepancies alone cannot thus account for the fine details of the biological activity which must depend also on the electronic structure.This work was supported by the A.T.P. N A 655-2303 of the C.N.R.S.     

Effects of molecular conformation on inner-shell ionization energies

Experimental evidence for an effect of molecular conformation on inner-shell ionization energies has been observed for the first time. Examples are seen in the carbon 1s spectra of butyronitrile, 1-fluoropropane, and propanal, and other similar molecules. At room temperature these exist in two different conformations, with different distances and, hence, different Coulombic interactions between the negatively charged electronegative group and the methyl carbon. The experimental results are in accord with theoretical predictions with respect to both ionization energies and populations of the different conformers.     

Rapid analysis of environmental samples using solid-phase microextraction (SPME) and narrow bore capillary columns

The application of SPME, a solventless extraction procedure, is demonstrated for two environmental applications. Extraction of VOCs by SPME is coupled with analysis on short narrow bore capillary gas chromatography columns. The technique is shown both as a fast screening tool and as part of an analytical procedure when combined with a mass spectrometer. Data show the linear range of the procedure. The extraction of chlorinated pesticides from hazardous wastewater and drinking water by SPME is also described in this paper. SPME is compared to traditional extraction procedures with respect to cost, time, ease of use, solvent usage, and sample usage.     

Influence of a charged graphene surface on the orientation and conformation of covalently attached oligonucleotides: a molecular dynamics study

Molecular dynamics (MD) simulations of single-stranded (ss) and double-stranded (ds) oligonucleotides anchored via an aliphatic linker to a graphene surface were performed in order to investigate the role of the surface charge density in the structure and orientation of attached DNA. Two types of interactions of DNA with the surface are crucial for the stabilisation of the DNA-surface system. Whereas for a surface with a zero or low positive charge density the dispersion forces between the base(s) and the surface dominate, the higher charge densities applied on the surface lead to a strong electrostatic interaction between the phosphate groups of DNA, the surface and the ions. At high-charge densities, the interaction of the DNA with the surface is strongly affected by the formation of a low-mobility layer of counterions compensating for the charge of the surface. A considerable difference in the behaviour of the ds-DNA and ss-DNA anchored to the layer was observed. The ds-DNA interacts with the surface at low- and zero-charge densities exclusively by the nearest base pair. It keeps its geometry close to the canonical B-DNA form, even at surfaces with high-charge densities. The ss-DNA, owing to its much higher flexibility, has a tendency to maximise the attraction to the surface exploiting more bases for the interaction. The interaction of the polar amino group(s) of the base(s) of ss-DNA with a negatively charged surface also contributes significantly to the system stability.