JCJC SIMI 8 - JCJC - SIMI 8 - Chimie du solide, colloïdes, physicochimie

Development of gradient spatially encoded NMR : toward a novel generation of correlation experiments. – gNMR

When spectroscopy is inspired by imaging ...

Following on the concept of a localisation of the signal in MRI, we develop an original approach based on the selective handling of nuclear spins with respect to their resonance frequency, in localised places throughout the sample. The parallel acquisition of these signals yields high resolution NMR spectra with a unique analytical content.

Understanding and controlling the concept of spatially encoded spectroscopic correlation.

Nuclear Magnetic Resonance spectroscopy is a tool of choice for the structural and dynamic characterization of molecular assemblies. But the complexity of the investigated molecules lead to spectra whose resolution is too low to give access to their analytical content. We develop an original concept inspired by the localization of the signal in Magnetic Resonance Imaging, that allows to image the spin interaction network throughout the sample.

First, we develop a program that simulates NMR pulse sequences, which allows us to describe formally the concept of a spatial frequency encoding generated by a pulsed field gradient. The semi-symbolic analysis that can be provided by this program to describe the action of any pulse sequence on a given spin system is exploited to optimise sequences that implement such kind of sample encoding, and also to develop new correlation experiments. These sequences are applied to the investigation of molecular systems whose analysis is difficult with state-of-the-art techniques (determination of enantiomeric excesses ...). Finally, we explore two techniques to enhance the sensitivity of our experiments.

We have developed a new sequence based on a spatial encoding of the sample along two directions of space, that allows a direct assignment of all the proton interaction network in a molecule, in a single spectrum. This approach allows to combine spin evolutions with the spin system so that the fine structure of each correlation is perfectly controlled. The proton network appears as a series of simplified multiplets whose analytical content is straightforwardly assignable.
We have also written a program that is dedicated to the simulation of pulse sequences, and that allows a semi-symbolic analysis of the coherences generated by the action of the sequence on a given spin system. This program allow for a direct visualization of the simulated spectra in the software used for operating the spectrometer. It is adapted to the analysis of proton spin systems corresponding to small organic molecules such as the model compounds used to implement new sequences.

The main perspective of this project is the enhancement of the resolution in correlation NMR spectra, and the simplification of their analytical content, in order to accelerate the analytical protocols, notably for molecules with fully coupled spin systems.

Giraud, N., Pitoux, D., Ouvrard, J.M. & Merlet, D., Combining J-edited and Correlation Spectroscopies Within a Multi-Dimensional Spatial Frequency Encoding: Toward Fully Resolved 1H NMR Spectra, Chemistry –A European Journal, 19: 12221-12224 (2013)

NMR spectroscopy has proved to be a unique tool for probing structure and dynamics in a wide range of molecular assemblies. Continuous methodological developments, combined with the breakthroughs which have been accomplished in probe and spectrometer hardwares have led to a variety of high-resolution experiments that have paved the way for an accurate measurement of large ensembles of spin interactions. These observables are particularly useful to constrain the most sophisticated simulations, allowing to describe complex chemical species and processes at an atomic level.
Unfortunately, in most of the systems that are of interest to the scientific community nowadays, the size or the complexity of the molecular architecture which is probed often leads to overcrowded spectra whose resolution is too low to give access to their analytical content. Neither the use of very high field spectrometers, nor the development of pulse sequences that combine broadband and selective irradiations have allowed to fully address this problem.
In this context, we have recently proposed to develop an original concept which consists in carrying out a parallel acquisition of different experiments using a single-receiver-coil system. We have successfully shown that this approach could be applied to run different selective echoes in different parts of an NMR sample, leading to a spin-spin coupling edition of the interaction network around a selected spin nucleus.
Following up these encouraging results, the research proposal aims at providing NMR spectroscopists with a novel generation of correlation experiments based on a sample spatial frequency encoding. First, a set of simulation programs will be developed, that will target the evaluation of the NMR signal, based on the analytical calculation of the evolution of spin coherences, which results from a gradient encoded pulse sequence. Second, capitalizing on this theoretical tool, we will then focus on the methodological development of new, high-resolution correlation techniques inspired by this sample gradient encoding concept. Third, since gradient encoded spectroscopy is intrinsically less sensitive than standard NMR experiments, we will aim at exploring several techniques as potential alternatives to improve sensitivity. Among them, we will notably study in what extent the use of para-hydrogen as a polarizing agent is compatible with our experimental schemes. Our ultimate goal will be to apply the high-resolution sequences that will result from this work to address challenging systems : these sequences will first be incorporated into a structural analysis protocol that will be applied to determine the stereochemistry of synthetic oligosaccharides. Then, we will evaluate the ability of the gradient encoded selective refocusing (G-SERF) spectroscopy to simplify enantiomeric visualization when it is applied to enantiomeric mixtures dissolved in chiral liquid crystals. Finally, we will focus on quantitativity (which is an essential issue in analytical chemistry), in the frame of an accurate evaluation of enantiomeric excesses.

Project coordination

Nicolas Giraud (UNIVERSITE DE PARIS XI [PARIS- SUD]) – nicolas.giraud@u-paris.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ICMMO UNIVERSITE DE PARIS XI [PARIS- SUD]

Help of the ANR 219,457 euros
Beginning and duration of the scientific project: August 2011 - 48 Months

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