Conformational analyses of 2,3-dihydroxypropanoic acid as a function of solvent and ionization state as determined by NMR spectroscopy

Vicinal (1)H--(1)H coupling constants were used to determine the conformational preferences of 2,3-dihydroxypropanoic acid (1) (DL-glyceric acid) in various solvents and its different carboxyl ionization states. The stereospecific assignments of J(12) and J(13) were confirmed through the point-group substitution of the C-3 hydrogen with deuterium, yielding rac-(2SR,3RS)-[3-(2)H]-1, and the observation of only J(13) in the (1)H NMR spectra. While hydrogen bonding and steric strain may be expected to drive the conformational equilibrium, their role is overshadowed by a profound gauche effect between the vicinal hydroxyl groups that mimics other substituted ethanes, such as 1,2-ethanediol and 1,2-difluoroethane. At low pH, the conformational equilibrium is heavily weighted toward the gauche-hydroxyl rotamers with a range of 81% in DMSO-d(6) to 92% in tert-butyl alcohol-d(10). At high pH, the equilibrium exhibits a larger dependence upon the polarity and solvating capability of the medium, although the gauche effect still dominates in D(2)O, 1,4-dioxane-d(8), methanol-d(4), and ethanol-d(6) (96, 89, 85, and 83% gauche-hydroxyls respectively). The observed preference for the gauche-hydroxyl rotamers is believed to stem primarily from hyperconjugative sigma(C--H) --> sigma*(C--OH) interactions.     

Conformational distribution of a ferroelectric liquid crystal revealed using fingerprint vibrational spectroscopy and the density functional theory

The authors have investigated the conformational structure of the ferroelectric liquid crystal compound 4-3-methyl-2-chloropentanoyloxy-4"-hexyloxy-biphenyl also known under the abbreviations 3M2CPHOB and C6 using vibrational (IR and Raman) spectroscopy. The measured spectra exhibit two bands corresponding to the C=O stretching vibration that are separated by 20 cm(-1). In contrast, the molecular structure comprises only one such group. They assigned the two bands to different conformers that coexist in a temperature range between 25 and 65 degrees C covering the entire mesophase of this material. This assignment is strongly confirmed by calculated vibrational spectra based on the density functional theory.     

Order and dynamics of a liquid crystalline dendrimer by means of 2H NMR spectroscopy

A complete Deuterium NMR study performed on partially deuterated liquid crystalline carbosilane dendrimer is here reported. The dendrimer under investigation shows a SmA phase in a large temperature range from 381 to 293 K, and its mesophasic properties have been previously determined. However, in this work the occurrence of a biphasic region between the isotropic and SmA phases has been put in evidence. The orientational order of the dendrimer, labeled on its lateral mesogenic units, is here evaluated in the whole temperature range by means of (2)H NMR, revealing a peculiar trend at low temperatures (T < 326 K). This aspect has been further investigated by a detailed analysis of the (2)H NMR spectral features, such as the quadrupolar splitting, the line shape, and the line-width, as a function of temperature. In the context of a detailed NMR analysis, relaxation times (T(1) and T(2)) have also been measured, pointing out a slowing down of the dynamics by decreasing the temperature, which determines from one side the spectral changes observed in the NMR spectra, on the other the observation of a minimum in the T(1).     

Conformations of banana-shaped molecules studied by 2H NMR spectroscopy in liquid crystalline solvents

ClPbis11BB and Pbis11BB, two banana-shaped mesogens differing by a chlorine substituent on the central phenyl ring, show a nematic and a B2 phase, respectively. To obtain information on the structural features responsible for their different mesomorphic behavior, a study of the preferred conformations of these mesogens has been performed by NMR spectroscopy in two nematic media (Phase IV and ZLI1167), which should mimic the environment of the molecules in their own mesophases, avoiding problems of sample alignment by a magnetic field. To this aim, 2H NMR experiments have been performed on selectively deuterated isotopomers of ClPbis11BB and Pbis11BB and of two parent molecules, ClPbisB and PbisB, assumed as models in previous theoretical and experimental conformational studies. We found that only a limited number of conformations is compatible with experimental data, often very different from those inferred from theoretical calculations in vacuo, indicating a strong influence of the liquid crystalline environment on molecular conformation. No significant differences between chlorinated and non-chlorinated molecules were found, this suggesting that chlorine does not change the molecular conformational equilibrium, as previously proposed.     

Dielectric spectroscopy of liquid crystalline dispersions

We describe dielectric spectroscopy measurements on dispersions of two thermotropic liquid crystals (5CB and 8CB) in a poly(dimethylsiloxane) matrix. 5CB exhibits nematic and isotropic phases, while 8CB exhibits smectic, nematic, and isotropic phases. The spectra of the dispersions exhibit a temperature-dependent dielectric relaxation in the interval from 100 to 1000 Hz, with relaxation times that depend strongly on whether the dispersed phase is isotropic, nematic, or smectic. The dielectric relaxation times also depend on the viscosity of the matrix fluid. These results suggest a coupling between the electric field and the mechanics of the interface that affects the spectrum of the dispersed phase and shifts the Maxwell-Wagner interfacial polarization peak.     

The molecular structure and unusual orientation of phenol in a liquid crystal solevent as determined by NMR spectroscopy

The NMR spectrum of phenol, dissolved in a nematic solvent, is explainable in terms of two averaging planar structures for each of which the principle axis of orientation in the ring is on the opposite side of the oxygen atom from the hydroxyl proton. External hydrogen bonding to the solvent seems to be an important factor in determining the orientation. The molecular structure obtained is compared with those found by microwave spectroscopy and X-ray diffraction.     

Extreme enantiomeric discrimination of fluoroalkanes using deuterium NMR in chiral liquid crystalline media

The enantiomeric assay of fluoroalkanes using 2H-NMR in a chiral liquid crystalline medium is demonstrated, and at its limit the enantiomers of [5-(2)H]-5-fluorodecane were successfully resolved.     

Structure,dynamics and phase separation in liquid crystalline polymers,block copolymers and blends as revealed by solid state NMR spectroscopy

A short survey is given of possibilities solid state NMR techniques offer for the study of functional polymers. Three examples are described explicitly, photoinduced reorientation of liquid crystalline polymers considered for information storage, phase separation and the mobility in the interfacial regions in segmented model polyurethanes composed of hard and soft segments and determination of domain sizes in incompatible polymer blends.     

Broadband dielectric spectroscopy on a discotic liquid crystalline polymer

Broadband dielectric spectroscopy is employed for the first time to study the molecular dynamics in discotic liquid crystalline polymers. One dielectric relaxation process is found which is strongly broadened and asymmetric. It is assigned a local hindered rotation of the ester groups attaching the spacer to the discotic mesogen. This assignment is supported by NMR measurements on the identical substance.Dedicated to Prof. E. W. Fischer on the occasion of his 60th birthday.     

Conformational changes in a photosensory LOV domain monitored by time-resolved NMR spectroscopy

Phototropins are light-activated kinases from plants that utilize light-oxygen-voltage (LOV) domains as blue light photosensors. Illumination of these domains leads to the formation of a covalent linkage between the protein and an internally bound flavin chromophore, destabilizing the surrounding protein and displacing an alpha-helix from its surface. Here we use a combination of spectroscopic tools to monitor the kinetic processes that spontaneously occur in the dark as the protein returns to the noncovalent ground state. Using time-resolved two-dimensional (2D) NMR methods, we measured the rate of this process at over 100 independent sites throughout the protein, establishing that regeneration of the dark state occurs cooperatively within a 1.6-fold range of observed rates. These data agree with other spectroscopic measurements of the kinetics of protein/FMN bond cleavage and global conformational changes, consistent with these processes experiencing a common rate-limiting step. Arrhenius analyses of the temperature dependence of these rates suggest that the transition state visited during this regeneration has higher energy than the denatured form of this protein domain despite the fact that there is no global unfolding of the domain during this process.     

Phase diagrams of new lamellar liquid crystalline systems based on 2H NMR spectroscopy data

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Structure and phase transitions of a liquid crystalline polymer

Summary The structures, textures as well as thermodynamic properties of a side chain polymer exhibiting a liquid crystalline phase in addition to a partially crystalline state and the isotropic fluid state were investigated. Furthermore the kinetics of phase transitions between these states were analyzed. It was found that the properties of this polymer are intermediate between that of low molecular weight liquid crystals and common polymers. In particular it was observed that the relation between the liquid crystalline texture and the structure is different from that of low molecular weight liquid crystals and that the properties of the crystalline and liquid crystalline state depend strongly on the tacticity of the polymer.

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Zusammenfassung Für ein Seitenkettenpolymeres, das zusätzlich zum teilkristallinen Zustand und zur isotropen Schmelze einen flüssig-kristallinen Zustand aufweist, wurden die Strukturen und Texturen untersucht sowie thermodynamische Eigenschaften. Außerdem wurde die Kinetik der Phasenumwandlungen analysiert. Es zeigt sich, daß die Eigenschaften zwischen denen üblicher Polymerer und denen von niedrigmolekularen flüssigen Kristallen liegen. Insbesondere zeigte sich, daß die Beziehung zwischen molekularer Struktur und Textur von der bei niedrigmolekularen flüssigen Kristallen abweicht, und daß die Taktizität der Kette einen starken Einfluß auf die Eigenschaften nicht nur der kristallinen Phase sondern auch der flüssig-kristallinen Phase ausübt.

     

Conformational analysis of ochratoxin a by NMR spectroscopy and computational molecular modeling

Two-dimensional NMR spectroscopy has been used for a complete assignment of the proton and carbon-13 spectra of the metabolite from Aspergillus ochraceus, ochratoxin A. In addition, phase-sensitive nuclear Overhauser effect spectrometry experiments and computational molecular modeling (MM2 and MMFF force field programs) have been employed to examine the conformational properties of ochratoxin A in chloroform solutions. Particular attention has been given to intramolecular hydrogen-bonding formation involving the phenolic group on dihydroisocoumarin, which may be responsible for the toxic mechanism of ochratoxin A.     

Photo-polymerization of liquid crystalline monomer in oriented liquid crystal phase

<正>A new approach to synthesize liquid crystalline polymer with narrow polydispersity index(PDI) was developed.Photopolymerization of 4-cyanophenyl-4'-(6-acryloyloxyhexyloxy)benzoate(RM23) in nematic liquid crystals with macroscopic orientation was studied.The effects of the monomer concentration on the molecular weight and PDI of the resulting polymers were studied through gel permeation chromatography(GPC) and polarized optical microscopy.The low PDI of 1.19 and 1.22 was obtained in the reverse and normal modes,respectively.The PDI and molecular weight increased with monomer concentration.     

Conformational enthalpy and volume changes in bromocyclohoexane determined by Raman spectroscopy

Raman spectra of bromocyclohexane have been recorded as a function of temperature between 22 and 135°C and as a function of pressure from 1 bar to 7.0 kbar. The integrated intensities of the bands at 657 and 687 cm⁻¹ have been used to monitor the relative populations of the axialand equatorial forms respectively. From these data, enthalpy and volume differences of 1.1 ± 0.3 kJ mol⁻¹ and −2.2 ± 0.3 cm³ mol⁻¹ respectively were determined for the equatorial—axial equilibrium.     

Lyotropic liquid crystalline phase behaviour in amphiphile-protic ionic liquid systems

Approximate partial phase diagrams for nine amphiphile-protic ionic liquid (PIL) systems have been determined by synchrotron source small angle X-ray scattering, differential scanning calorimetry and cross polarised optical microscopy. The binary phase diagrams of some common cationic (hexadecyltrimethyl ammonium chloride, CTAC, and hexadecylpyridinium bromide, HDPB) and nonionic (polyoxyethylene (10) oleyl ether, Brij 97, and Pluronic block copolymer, P123) amphiphiles with the PILs, ethylammonium nitrate (EAN), ethanolammonium nitrate (EOAN) and diethanolammonium formate (DEOAF), have been studied. The phase diagrams were constructed for concentrations from 10 wt% to 80 wt% amphiphile, in the temperature range 25 °C to >100 °C. Lyotropic liquid crystalline phases (hexagonal, cubic and lamellar) were formed at high surfactant concentrations (typically >50 wt%), whereas at <40 wt%, only micelles or polydisperse crystals were present. With the exception of Brij 97, the thermal stability of the phases formed by these surfactants persisted to temperatures above 100 °C. The phase behaviour of amphiphile-PIL systems was interpreted by considering the PIL cohesive energy, liquid nanoscale order, polarity and ionicity. For comparison the phase behaviour of the four amphiphiles was also studied in water.     

Discrimination of enantiomers by means of NMR spectroscopy using chiral liquid crystalline solution: application to triazole fungicides, uniconazole and diniconazole

An NMR method for discriminating among enantiomers by using a chiral liquid crystalline solution was applied to chiral triazole compounds, uniconazole (1) and diniconazole (2), which exhibit antifungal and plant growth regulating activities. These chiral compounds were dissolved in PBLG (poly-gamma-benzyl-L-glutamate)--CDCl3 chiral liquid crystalline solvent for measurements of 13C NMR. The enantiomeric separations were primarily observed in the signals of aromatic carbons owing to differences in chemical shift anisotropies. The enantiomeric excess (ee) was determined from the integral scale of the separated peaks. The resulting ee values are in fair agreement with the actual values. The extrasplittings due to residual dipolar couplings were also measured using Het2DJ spectra for 1S and R, and 2R, and the results are discussed.     

Conformational analysis of a tetrasaccharide based on NMR spectroscopy and molecular dynamics simulations

The conformational preference of the human milk oligosaccharide lacto-N-neotetraose, beta-d-Galp-(1 --> 4)-beta-d-GlcpNAc-(1 --> 3)-beta-d-Galp-(1 --> 4)-d-Glcp, has been analyzed using (1)H,(1)H T-ROESY and (1)H,(13)C trans-glycosidic J coupling experiments in isotropic solution and (1)H,(13)C residual dipolar couplings (RDCs) obtained in lyotropic liquid crystalline media. Molecular dynamics simulations of the tetrasaccharide with explicit water as the solvent revealed that two conformational states are significantly populated at the psi glycosidic torsion angle, defined by C(anomeric)-O-C-H, of the (1 --> 3)-linkage. Calculation of order parameters, related to the molecular shape, were based on the inertia tensor and fitting of experimental RDCs to different conformational states showed that psi(+) > 0 degrees is the major and psi(-) < 0 degrees is the minor conformation in solution, in complete agreement with a two-state analysis based on the T-ROESY data. Attention was also given to the effect of salt (200 mM NaCl) in the anisotropic medium, which was a ternary mixture of n-octyl-penta(ethylene glycol), n-octanol, and D(2)O.     

Conformational dynamics of bistable RNAs studied by time-resolved NMR spectroscopy

The structural transition between two alternate conformations of bistable RNAs has been characterized by time-resolved NMR spectroscopy. The mechanism, kinetics, and thermodynamics underlying the global structural transition of bistable RNAs were delineated. Both bistable RNA conformations and a partial unstructured RNA of identical sequence could be trapped using photolabile protecting groups. This trapping allowed for an investigation of the initial folding from an unfolded RNA to one of the preferred conformations of the bistable RNA and of the structural transitions involved. Folding of the secondary structure elements occurs rapidly, while the global structural transition of the bistable RNA occurs on a time scale of minutes and shows marked temperature dependence. Comparison of these results with bistable systems previously investigated leads to the prediction of activation enthalpies (DeltaH++) associated with global structural transitions in RNA.