Jacques Courtieu

Enantiomeric excess measurements in weakly oriented chiral liquid crystal solvents through 2D 1H selective refocusing experiments

Abstract In this article, a simple and robust method is proposed for simplifying the analysis of proton spectra of molecules dissolved in weakly oriented chiral media. The NMR approach investigated is based on the use of proton selective refocusing 2D experiments (SERF) to measure proton–proton dipolar couplings from unresolved lines. This technique is applied to the case of enantiomers dissolved in chiral polypeptide liquid crystals. It is shown that an accurate determination of enantiomeric excess is possible within a short experimental time.

Lipid composition and structure of commercial parenteral emulsions

In order to study the influence of the phospholipid/triacylglycerol (PL/TG) ratio of parenteral emulsions on the distribution and the physico-chemical properties of their fat particles, commercial 10, 20 or 30% fat formulas were fractionated by centrifugation into an upper lipid cake (resuspended in aqueous glycerol) and a subnatant or mesophase, from which a PL-rich subfraction (d = 1.010-1.030 g/l) was purified by density gradient ultracentrifugation. Chemical and 31P-NMR analyses of these fractions indicated that at least two types of fat particles coexist in parenteral emulsions: (i) TG-rich particles (mean diameter: 330, 400, 470 nm in the 10, 20, 30% emulsion) which contain practically all the TG and esterified phytosterols of native emulsions, but only a fraction of their PL, unesterified cholesterol and phytosterols, and other minor lipids; (ii) PL-bilayer particles or liposomes (mean diameter: 80-100 nm) which are constituted with the remaining PL and relatively very small amounts of TG and other lipids. The higher the oil content of the emulsion, the lower the amount of these PL-rich particles, which represent the major particle population of the mesophase. Indeed, minute amounts of TG-rich particles (probably the smallest ones) are also present in the mesophase, even in the PL-rich subfraction which contains the bulk of liposomal PL. Since the PL-rich particles of the infused emulsion generate lipoprotein X-like particles, only the large TG-rich particles can be considered as true chylomicron counterparts.

Nuclear Magnetic Resonance Using a Spatial Frequency Encoding: Application to J-Edited Spectroscopy along the Sample

Nuclear magnetic resonance provides chemists with a unique insight into molecular structure and dynamics over very wide spatial and time ranges. Unfortunately, in most of the recent applications of NMR, the size of the spin networks that are investigated often remain challenging, even for spectrometers that operate at very high fields, since the overlap of signals, as well as the complexity of their multiplet structure, overwhelm the resolution of the NMR spectra. In this context, considerable methodological developments have been devoted to the design of multidimensional experiments that aim at simplifying the analytical process. For instance, COSY and J-resolved experiments illustrate the extent to which chemical shift and scalar coupling information can be separated, thus allowing a rapid assignment of each proton site, and then a measurement of H–H couplings. Unfortunately, as hard pulses are used in such experiments, they often give overcrowded spectra as soon as the size of the molecular system increases. One way to reduce the number of interactions that contribute to the structure of NMR spectra involves the use of semiselective pulses, which allow use of coherences that involve a single spin nucleus; for instance, selective refocusing experiments have opened the way to a site-specific measurement of each interaction from unresolved spectra. However, in this latter case, up to n(n 1)/2 selective experiments need to be recorded in order to extract all the couplings out of a network of n fully coupled spins, thus requiring a significantly longer experimental time. In order to overcome the above-mentioned difficulty, we proposed to run different selective experiments on different parts of the sample, that is, we carried out a parallel acquisition of different experiments using a single-receivercoil system. For this purpose, we created a spatial frequency encoding of the sample, in a manner similar to magnetic resonance imaging techniques. Application of the principle to menthol is shown in Figure 1. We simultaneously applied a semiselective p/2 pulse and a pulsed field gradient along the NMR tube (i.e., a z gradient). This irradiation scheme can be described as an rf field with a position-dependent offset, and results in the polarization of spin nuclei with different resonance frequencies in different cross sections of the sample. Although the same signals are observed on the gradient-encoded (Figure 1a) and on the broadband excitation spectra of menthol (Figure 1b), the gradient-encoded experiment should actually be compared to the semiselective spectrum (Figure 1c), where one single proton nucleus is excited by the application of a soft pulse over the whole sample. Indeed, the spatial encoding allows this selective experiment to be carried out on each of the proton spins in separate “slices”. We note that each line of the resulting spatially encoded spectrum arises from a restricted cross section of the sample: if the spatial frequency sweep that is induced by the pulsedfield gradient is adjusted so that the width of the spectrum (ca. 3 ppm on menthol) exactly matches the height of the sample, then each signal (whose width can be roughly Figure 1. a) Proton spectrum of l-( )-menthol. The spectrum was acquired using a semiselective pulse (H channel) applied together with a pulsed z-field gradient (PFG channel). An NMR tube is drawn along the spectrum in order to illustrate the spatial encoding of proton lines according to their resonance frequencies. b) Broadband excitation spectrum, recorded on the same compound. c) Spectrum that results from the application of the semiselective pulse used in (a), at the resonance frequency of proton H. The corresponding pulse sequence is shown for each spectrum).

Enantiomeric visualization using proton-decoupled natural abundance deuterium NMR in poly(γ-benzyl-l-glutamate) liquid crystalline solutions

Abstract We report the first visualization of chiral molecules oriented in a polypeptide liquid crystalline system (PBLG) using proton-decoupled natural abundance deuterium NMR. The chiral discrimination is observed through measurements of the quadrupolar splitting differences and we demonstrate that the sensitivity of natural abundance deuterium NMR is sufficient to measure the differential ordering effects (DOEs) without the need for isotopic enrichment. The feasibility and the potential of this novel method were investigated using a 5.87 T spectrometer (proton frequency 250 MHz). Several examples of chiral discrimination are presented and particular emphasis is given to demonstrate the potential of this approach.

Theoretical and experimental aspects of enantiomeric differentiation using natural abundance multinuclear nmr spectroscopy in chiral polypeptide liquid crystals

Liquid crystalline organic solutions of poly-γ-benzyl-L-glutamate generate a sufficient differential ordering effect to visualize enantiomers using multinuclear high-resolution NMR spectroscopy at natural isotopic abundance levels. Chiral discrimination can be observed through a difference in the order-sensitive NMR observables, namely: proton–proton, carbon–proton and carbon–carbon residual internuclear dipolar couplings, carbon chemical shift anisotropies, and deuterium quadrupolar splittings. In most cases, the enantiodifferentiation is large enough to allow determation of the enantiomeric excesses satisfactorily. All theoretical considerations and significant experimental parameters that affect the efficiency of this methodology are presented and discussed. The various possible anisotropic NMR techniques provide a very reliable and powerful alternative to the current analytical techniques which operate in the isotropic phase.

A single‐coil triple resonance probe for NMR experiments

A single‐coil triple‐tuned NMR probe designed to simultaneously stimulate 1H, 2H and 13C resonances is described. Isolation between the 1H and 13C channels is higher that obtained by the cross‐coil system and the 1H decoupling efficiency is significantly increased. The basic idea can be applied to design probes for observing resonances of various other nuclei either in liquid or solid state.

Application of a 1H δ‐resolved 2D NMR experiment to the visualization of enantiomers in chiral environment, using sample spatial encoding and selective echoes

We present the application of a 2D broadband homodecoupled proton NMR experiment to the visualization of enantiomers. In a chiral environment, the existence of diastereoisomeric intermolecular interactions can yield—generally slight—variations of proton chemical shifts from one enantiomer to another. We show that this approach, which relies on a spatial encoding of the NMR sample, is particularly well suited to the analysis of enantiomeric mixtures, since it allows, within one single 2D experiment, to detect subtle chemical shift differences between enantiomers, even in the presence of several couplings. This sequence, which uses semiselective radio‐frequency (rf) pulses combined to a z‐field gradient pulse, produces different selective echoes in various parts of the sample. The resulting homonuclear decoupling provides an original δ‐resolved spectrum along the diagonal of the 2D map where it becomes possible to probe the chiral differentiation process through every proton site where the resulting variation in the chemical shift is detectable. We discuss the advantages and drawbacks of this approach, regarding other experiments which provide homodecoupled proton spectra. This methodology is applied to the observation of enantiomers of (1) ( ± )2‐methyl‐isoborneol coordinated to europium (III) tris[3‐(trifluoromethyl‐hydroxymethylene)‐(+)‐camphorate] in isotropic solution, and (2) ( ± )3‐butyn‐2‐ol dissolved in a chiral liquid‐crystal solvent, in order to show the robustness of this pulse sequence for a wide range of chiral samples. Copyright

The relationship between molecular symmetry and second-rank orientational order parameters for molecules in chiral liquid crystalline solvents

From group theoretical arguments, we demonstrate that the effective molecular symmetry is reduced in a chiral liquid crystalline phase. This reduction changes the location of the principal axes of the orientational order matrices for four molecular point groups, namely Cs, C2v, S4, and D2d. These symmetries correspond to compounds which have prochiral faces, groups, or directions. The change in effective molecular symmetry can be detected by NMR spectroscopy, and this is illustrated by the example of acenaphthene dissolved in a chiral nematic solvent.

Is Enantiomer Assignment Possible by NMR Spectroscopy Using Residual Dipolar Couplings from Chiral Nonracemic Alignment Media?—A Critical Assessment

The discovery of Jean-Baptiste Biot in 1815 that optical activity is not a property bound to a certain aggregation state of matter but a property of the constituting molecules themselves, has had an enormous influence on the structural models that chemists developed at the end of the 19th century, long before the description of the chemical bond was based on quantum mechanics. Pasteur achieved the first separation of enantiomers in 1847, namely by crystallization of a racemic tartrate mixture that separated the two enantiomers into enantiomorphic crystals, solutions of which rotated the plane of linearly polarized light in opposite directions. Not until 1951, when Bijvoet used anomalous X-ray diffraction, it was possible to assign the absolute configuration to a specific enantiomer. However, anomalous X-ray diffraction has not put the problem of assigning absolute configurations to rest, because many chemical compounds cannot be crystallized. Moreover, anomalous X-ray diffraction of molecules that consist exclusively of lightweight atoms commonly lacks the needed accuracy to allow unambiguous assignment of absolute configurations. An alternative method for resolving enantiomers is to convert them to diastereoisomers, either by chemical derivatization with chiral nonracemic moieties or by intermolecular coordination with chiral nonracemic reagents. In this way it is possible to determine absolute configuration from NMR observables, most commonly chemical shifts. The use of chiroptical spectroscopies such as optical rotatory dispersion, and electronic or vibrational circular dichroism is well established, sometimes in combination with ab initio calculations. Further methods are conceivable but impractical momentarily. Yet, there is currently not a simple and universally applicable approach to determine the absolute configuration of molecules with few stereogenic centers. Two recent papers published in 2007 and 2011 have therefore created a lot of excitement in the chemistry and NMR spectroscopy communities. Their titles are: “Stereochemical Identification of (R)and (S)-Ibuprofen Using Residual Dipolar Couplings, NMR, and Modeling”, henceforth called “article 1”, and more recently: “Spin-Selective Correlation Experiment for Measurement of Long-Range J Couplings and for Assignment of (R/S) Enantiomers from the Residual Dipolar Couplings and DFT”, henceforth called “article 2”. Both articles describe the assignment of the absolute configuration of the chiral molecules, ibuprofen 1 (article 1) and 4-methyl-1,3-dioxolan-2-one 2 (article 2), using NMR spectroscopy in chiral nonracemic alignment media (Figure 1). Under chiral nonracemic conditions, the authors measured residual dipolar couplings (RDCs), a NMR parameter only visible in oriented samples, such as in liquid crystals, but not in isotropic solvents. The interaction of the enantiomers with the chiral nonracemic alignment medium gives rise to diastereomorphic associates for which reason the authors indeed found different sets of anisotropic parameters for each enantiomer, in total

Deuterium NMR stereochemical analysis of threo–erythro isomers bearing remote stereogenic centres in racemic and non-racemic liquid crystalline solvents

Abstract We report the proton-decoupled deuterium NMR study of labelled diastereomers with remote stereogenic carbons dissolved in various mixtures of poly-γ-benzyl- l -glutamate (PBLG) and poly-γ-benzyl- d -glutamate (PBDG) liquid crystalline solutions. The evolution of quadrupolar splitting as well as the diastereomeric and the enantiomeric discrimination versus the proportion of PBLG and PBDG in the liquid crystalline phase is studied. It is shown that racemic liquid crystalline solutions of PBLG and PBDG may be used to measure diastereomeric excess ( de ). Thereafter the spectrum in PBLG solution allows for measuring the enantiomeric excess ( ee ) of each diastereomer. These first results suggest substantial prospects in the field of the analysis of diastereomers with remote stereogenic carbons.