High pressure 1D and 2D NMR experiments on model biomembranes

Abstract

We developed a high-pressure high-resolution probe for NMR experiments on biological systems which enabled us to study the phase behaviour of model biomembranes by 1D and 2D NMR experiments at temperatures up to 65°C and at pressures up to 3000 bar.     

A Multifunctional Fluorescence Probe for the Detection of Cations in Aqueous Solution: the Versatility of Probes Based on Peptides

We synthesized a tetra-functional fluorescence probe based on dansyl and peptide motif, dansyl-Gly-Trp (DGT, 1), that efficiently bound several metal ions and showed distinguishing optical properties. The probe 1 could respond to Hg²⁺ with enhanced and blue-shifted fluorescence emission but to Cu²⁺ with obvious fluorescence quenching. In addition, 1 was sensitive to pH ranging from 2.0 to 5.0 and precipitated in the presence of Pb²⁺ at neutral conditions. The combination of these intrinsic properties with the selective responses to different chemical inputs allows this system to be implemented as an ionic switch. Furthermore, 1 could penetrate the cell membrane and accumulated well in intracellular region. The underlying mechanisms of the probe to different kind of metal ion were explored successfully by using either ¹H NMR, NOESY, electron paramagnetic resonance (EPR) or FT-IR spectra. In addition, to investigate the binding model of 1/Hg²⁺ and 1/Cu²⁺, simulations were also performed by using density functional theory (DFT) and reasonable binding configurations were achieved for these two complexes.     

NMR Studies of Hindered Rotation and Magnetic Anisotropy: The Diels–Alder Adducts of Phencyclone with N‐Phenylmaleimide and N‐(2‐Trifluoromethylphenyl)maleimide. Ab Initio Calculations for Optimized Structures

Abstract

N‐Phenylmaleimide, 2, and N‐(2‐trifluoromethylphenyl)maleimide, 3, were separately added to phencyclone, 1, to yield the corresponding phencyclone Diels–Alder adducts, 4 and 5. The resulting adducts (and some precursors) have been characterized by one‐ and two‐dimensional ¹H and ¹³C NMR at 300 and 75 MHz, and by ¹⁹F NMR at 282 MHz, at ambient temperatures. The NMR data are consistent, for both adducts, with: (a) hindered rotation of the bridgehead unsubstituted phenyl groups about the C(sp²)–C(sp³) bonds, based on slow exchange limit (SEL) spectra and (b) endo adduct configuration based on magnetic anisotropic effects in the ¹H NMR. The NMR spectra of the phencyclone adduct, 4, of N‐phenylmaleimide, indicate free rotation on the NMR timescales (fast exchange limit, FEL spectra) about the N‐phenyl bond. The spectra for the adduct, 5, of N‐(2‐trifluoromethylphenyl)maleimide are interpreted as consistent with SEL regimes, for the N‐aryl rotations, with a single rotamer present in which the trifluoromethyl group is directed “out of” the adduct cavity, and away from the phenanthrenoid moiety. This conclusion is based, in part, on NMR data suggesting the apparent slow N‐aryl bond rotation in a pair of atropisomers corresponding to the acetic acid addition products from the N‐(2‐trifluoromethylphenyl)maleimide. Evidence of magnetic anisotropic effects due to the phenanthrenoid moiety and proximal carbonyls is discussed. ¹H, ¹³C, and ¹⁹F assignments are presented and interpreted. Molecular modeling calculations at the Hartree–Fock level, 6‐31G* basis set, were performed to provide geometry optimizations for energy‐minimized structures of selected compounds.     

<Superscript>15</Superscript>N–<Superscript>13</Superscript>C Dipole Couplings in Smectic Mesophase of a Thermotropic Ionic Liquid

Unique combination of ionic conductivity and anisotropic physical properties in ionic liquid crystals leads to new dynamic properties exploited in modern technological applications. Structural and dynamics information at atomic level for molecules and ions in mesophases can be obtained by nuclear magnetic resonance (NMR) spectroscopy through the measurements of dipole–dipole spin couplings. While ¹³C–¹H and ¹⁵N–¹H dipolar NMR spectra can be routinely acquired in samples with natural isotopic abundance, recording ¹⁵N–¹³C dipolar NMR spectra is challenging because of the unfavourable combination of two rare isotopes. In the present study, an approach to measure ¹⁵N–¹³C dipole-dipole NMR spectra in static liquid crystalline samples with natural abundance is introduced. We demonstrate that well-resolved spectra can be recorded within 10 h of experimental time using a conventional NMR probe and a moderately strong magnetic field. The technique is applied to a thermotropic smectic mesophase formed by an ionic liquid with imidazolium-based organic cation.     

Parallel-plate RF resonator imaging of chemical shift resolved capillary flow

Magnetic resonance imaging has been introduced to study flow in microchannels using pure phase spatial encoding with a microfabricated parallel-plate nuclear magnetic resonance (NMR) probe. The NMR probe and pure phase spatial encoding enhance the sensitivity and resolution of the measurement. In this paper, ¹H NMR spectra and images were acquired at 100 MHz. The B₁ magnetic field is homogeneous and the signal-to-noise ratio of 30 μl doped water for a single scan is 8×10⁴. The high sensitivity of the probe enables velocity mapping of the fluids in the micro-channel with a spatial resolution of 13×13 μm. The parallel-plate probe with pure phase encoding permits the acquisition of NMR spectra; therefore, chemical shift resolved velocity mapping was also undertaken. Results are presented which show separate velocity maps for water and methanol flowing through a straight circular micro-channel. Finally, future performance of these techniques for the study of microfluidics is extrapolated and discussed.     

Parallelizing acquisitions of solid-state NMR spectra with multi-channel probe and multi-receivers: Applications to nanoporous solids

A five-channel (¹H, ¹⁹F, ³¹P, ²⁷Al, ¹³C) 2.5 mm magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) probe is used in combination with three separate receivers for the parallel acquisitions of one (1D) and two-dimensional (2D) NMR spectra in model fluorinated aluminophosphate and porous Al-based metal-organic framework (MOF). Possible combinations to record simultaneously spectra using this set-up are presented, including (i) parallel acquisitions of quantitative 1D NMR spectra of solids containing nuclei with contrasted T₁ relaxation rates and (ii) parallel acquisitions of 2D heteronuclear NMR spectra. In solids containing numerous different NMR-accessible nuclei, the number of NMR experiments that have to be acquired to get accurate structural information is high. The strategy we present here, i.e. the multiplication of both the number of irradiation channels in the probe and the number of parallel receivers, offers one possibility to optimize this measurement time.     

Structural Assignment of Isomeric S‐ and S,N‐1‐Alkoxycarbonylalkylated 6‐Amino‐2‐thiouracil Derivatives by Means of 1H and 13C NMR Spectroscopy

Abstract

The structures of new isomeric 2‐alkoxycarbonylalkylthio‐ and 2‐alkoxy‐ carbonylalkylthio‐1‐alkoxycarbonylalkyl‐6‐aminouracils (1–21) have been established on the basis of the ¹H NMR and ¹³C NMR spectroscopic data. The ¹H NMR and ¹³C NMR spectra of 1–21 have been fully assigned by a combination of two‐dimensional experiments [heteronuclear multiple quantum coherence (HMQC) and heteronuclear multiple bond correlation (HMBC)]. The ¹³C NMR spectra have been shown to be able to differentiate between isomers.     

Design and construction of a versatile dual volume heteronuclear double resonance microcoil NMR probe

Improved NMR detection of mass limited samples can be obtained by taking advantage of the mass sensitivity of microcoil NMR, while throughput issues can be addressed using multiple, parallel sample detection coils. We present the design and construction of a double resonance 300-MHz dual volume microcoil NMR probe with thermally etched 440-nL detection volumes and fused silica transfer lines for high-throughput stopped-flow or flow-through sample analysis. Two orthogonal solenoidal detection coils and the novel use of shielded inductors allowed the construction of a probe with negligible radio-frequency cross talk. The probe was resonated at ¹H–²D (upper coil) and ¹H–¹³C (lower coil) frequencies such that it could perform 1D and 2D experiments with active locking frequency. The coils exhibited line widths of 0.8–1.1 Hz with good mass sensitivity for both ¹H and ¹³C NMR detection. ¹³C-directly detected ²D HETCOR spectra of 5% v/v ¹³C labeled acetic acid were obtained in less than 5 min. Demonstration of the probe characteristics as well as applications of the versatile two-coil double resonance probe are discussed.     

The first high-pressure high-resolution NMR probe for a normal bore cryomagnet1

Abstract

We describe a high-pressure high-resolution NMR probe fitting into a 4.7 Tesla normal bore (50 mm Ø) cryomagnet used with a Bruker AC-200 spectrometer. The probe is designed for a pressure range from 0.1 to 200 MPa and can be used in a temperature domain from -50 to + 120°C. Eguipped with a double tuned frequeney adapter circuit the probe is used for ¹H (200 MHz) observation and ²H (30.7 MHz) field locking for easy homogeneity adjustment. The resolution obtained routinely is better than 1 Hz (< 5′10^?9). Other frequencies, e.g. 27.1 MHz for ¹⁷O observation, can easily be obtained by changing and according the frequency adapter box.     

1H and 13CNMR Study of a New Class of Bipyridinium Liquid Crystals

Abstract

¹H and ¹³C NMR, as well as UV and IR, spectra of a new class of bipyridinium salts made by double quaternization of trans?1, 2-bis(4-pyridyl) ethene unit with n-alkyl salts are reported. The effect of quaternization on the ¹ and ¹³C NMR spectra of the salts is discussed.     

Infrared Spectra of Crystalline,Glassy and Liquid Methanol at High Pressures

Abstract

Mid-infrared spectra in the range 400–1800 cm^?1 of methanol samples in diamond anvil cells at ambient temperature and pressures up to 11 GPa are reported. The freezing pressure is confirmed to be 3.6 GPa, and the spectra of the resulting metastable glass are very similar to those of the liquid. When maintained at high pressure, the glass spontaneously transforms to an ordered crystalline phase which is stable over the range 3.6 to 11 GPa. Small changes in peak wavenumbers for 14 internal modes as a function of pressure are observed, indicating that distortion of the molecules is minimal. A slight decrease for the C-O-H bending mode is attributed to charge transfer from the molecular 0-H bond to the strengthening intermolecular hydrogen bond.     

NMR Studies Of Drugs. Chiral Solvating Agents for Direct Determination of Enantiomeric Excess of the Cardiac Antiarrhythmic,Mexiletine

The 400 MHz ¹H NMR spectra of the cardiac antiarrhythmic, mexiletine, 1, have been studied with different chiral solvating agents (CSA) to obtain a very promising method for direct determination of enantiomeric excess (e.e.) with limited amounts of 1. The methods included the use of β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), α-methoxy-α-(trifluoromethyl)phenylacetic acid (MTPA), and 2,2,2-trifluoro-1-(9-anthryl)ethanol (TFAE). Use of TFAE in CDCl₃ with the free base of 1 appeared to give the best results, with enantiomeric shift differences observed for the signals of the sidechain methyl, CH ₃CH, and the aryl methyls.     

Proton Nuclear Magnetic Resonance Spectroscopy: Significant Barriers to Rotation About N-N Bonds in 3-Acylaminoquinazolin-4(3H)-One Derivatives

Abstract

The proton NMR spectra of several 3-acylaminoquinazolin-4(3H)-one derivatives have been studied as a function of temperature. The changes, which were found to occur in the spectra at high temperatures, are discussed in terms of hindered rotation about the nitrogen-nitrogen bond. The free energies of activation for the rate-determining stereochemical process were calculated to be as high as (14.7～20.6 Kcal mol^?1) for hydrazine derivatives.     

NMR Spectroscopic Study of Camphanic Acid,Rh2[Camphanate]4, and its Adducts with 1,4-Benzodiazepines

The H and ¹³C NMR spectra of chiral camphanic acid, lactonc of (1S,3R)-1-hydroxy-2,2,3-trimethyl-cyclopentan-1,3-dicarboxylic acid (1), and dirhodium tetracamphanate complex (2) were completely assigned on the basis of one- and two-dimensional NMR experiments. The NMR spectra of the adducts of dirhodium tetracamphanate 2 with 5-pyrido-1, 4-benzodiazepin-2-ones 3 and 4, complexes with catalytic activity, support the assignation and also revealed two different types of axial complexation of these nitrogen ligands.     

Infrared Spectra of Liquid and Crystalline Ethanol at High Pressures

Abstract

Mid-infrared spectra in the ranges 400–1800 and 2700–4600 cm^?1 of ethanol samples in diamond anvil cells at ambient temperature and pressures up to 11 GPa are reported. The freezing pressure is confirmed to be 1.8 GPa, and, unlike methanol, the resulting solid is crystalline rather than glassy. No further phase transitions are observed in this pressure range. The wave number shifts of 30 selected peaks with pressure are deduced, and their small magnitudes indicate that only minor distortions of the molecules occur. The effects of the strengthening of the intermolecular hydrogen bonds with pressure on the internal modes are briefly discussed.     

Synthesis and Chiral Recognition Properties of Novel Fluorescent Chemosensors for Amino Acid

The charge neutral chiral optical sensors 1a∼d containing thiourea and amide groups were synthesized by simple steps in good yields and their structures were characterized by IR, ¹H NMR, ¹³C NMR, MS spectra and elemental analysis. The enantioselective recognition for α-phenylglycine and phenylglycinol was examined by fluorescence emission and UV-vis spectra. The fluorescence and UV-vis spectra changes of 1a were obvious when the enantiomers of α-phenylglycine anion were added, which exhibited that 1a has good enantioselective recognition ability towards α-phenylglycine. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

EASY: A simple tool for simultaneously removing background,deadtime and acoustic ringing in quantitative NMR spectroscopy—Part I: Basic principle and applications

Elimination of Artifacts in NMR SpectroscopY (EASY) is a simple but very effective tool to remove simultaneously any real NMR probe background signal, any spectral distortions due to deadtime ringdown effects and -specifically- severe acoustic ringing artifacts in NMR spectra of low-gamma nuclei. EASY enables and maintains quantitative NMR (qNMR) as only a single pulse (preferably 90°) is used for data acquisition. After the acquisition of the first scan (it contains the wanted NMR signal and the background/deadtime/ringing artifacts) the same experiment is repeated immediately afterwards before the T1 waiting delay. This second scan contains only the background/deadtime/ringing parts. Hence, the simple difference of both yields clean NMR line shapes free of artefacts.In this Part I various examples for complete ¹H, ¹¹B, ¹³C, ¹⁹F probe background removal due to construction parts of the NMR probes are presented. Furthermore, ²⁵Mg EASY of Mg(OH)₂ is presented and this example shows how extremely strong acoustic ringing can be suppressed (more than a factor of 200) such that phase and baseline correction for spectra acquired with a single pulse is no longer a problem. EASY is also a step towards deadtime-free data acquisition as these effects are also canceled completely. EASY can be combined with any other NMR experiment, including 2D NMR, if baseline distortions are a big problem.     

Hydrogen Bond and 19F NMR Chemical Shift Akisotropy in Apatite

Abstract

The fluoroapatite minerals, Ca₅-F(PO₄)₃ usually contain some hydroxyis, instead of fluorine. From infrared spectra it was concluded, that there are weak hydrogen bonds OH … F along apatite hexagonal axis¹. In order to receive more detailed information about the character of F^?and OH^?bonds with the nearest atoms in apatites we studied the nuclear magnetic resonance (NMR) spectra of protons and fluorines in natural apatite single crystal, in which approximately 15% of fluorines is substituted by hydroxyis.     

Spinning Sidebands in Slow-Magic-Angle-Spinning NMR Spectra Arising from TightlyJ-Coupled Spin Pairs

Complex spinning sidebands are observed in magic-angle-spinning (MAS) NMR spectra arising from isolated tightlyJ-coupled spin pairs under slow spinning conditions. Such spinning sidebands are sensitive to the magnitude and relative orientation of the chemical-shift tensors, the dipolar-coupling tensor, and the sign of the indirect spin–spin (J) coupling. We show that it is possible to extract information concerning such NMR parameters from an analysis of the observed spinning sidebands. As an example, numerical simulations are carried out to reproduce observed³¹P MAS NMR spectra of a phosphole tetramer (1) ando-bis(diphenylphosphino)benzene (2), so that invaluable information concerning the orientations of the phosphorus chemical-shift tensors and the sign ofJ(³¹P,³¹P) can be deduced. Simulations are carried out by numerically evaluating the spin-density matrix of the spin system.     

Application of a New Program,H1MACH,for Prediction of Iridoid Skeletons

Abstract

A procedure is presented that utilizes ¹H NMR for prediction of the skeleton of iridoids. A new program was developed, named H1MACH, that presents a database with 800 data points from the ¹H NMR spectra of iridoids. This program was widely tested for the prediction of the skeleton of 40 compounds and compared with other programs in the expert system SISTEMAT. The results obtained show that H1MACH is very useful for the prediction of the skeleton of iridoids, especially for the iridane skeleton.