Reduced dimensionality hyphenated NMR experiments for the structure determination of compounds in mixtures.

Justinas Sakas,Nicholle G. A. Bell

doi:10.1039/c9fd00008a

Justinas Sakas, Nicholle G. A. Bell

Open Access

https://doi.org/10.1039/c9fd00008a

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Abstract

For the structure determination of molecules in mixtures using NMR spectroscopy, the dispersion of 13C chemical shifts provides much needed separation of resonances in the indirectly detected dimension of 2D heterocorrelated NMR experiments. This separation is crucial for establishing networks of coupled spins by hyphenated techniques that combine hetero- and homonuclear polarisation transfers. However, as the sample complexity increases, 13C chemical shifts stop being unique, hindering spectral interpretation. The resulting ambiguities can be removed by adding another dimension to these experiments. However, the spectra obtained from complex samples are riddled with overlapped signals, meaning that another dimension will only reduce the spectral resolution and prevent structure determination. A promising solution is to stay in two dimensions and use the combined 13C and 1H chemical shifts to separate signals. We have developed a suite of (3,2)D reduced dimensionality hyphenated NMR experiments that preserve the information content of 3D spectra but offer all of the advantages of 2D spectra - high resolution and ease of manipulation with only a mild sensitivity penalty. The proposed experiments complement the existing (3,2)D HSQC-TOCSY and include a (3,2)D HSQC-NOESY/ROESY, (3,2)D HSQC-CLIP-COSY and (3,2)D HSQC-HSQMBC. The new experiments represent a set of NMR techniques typically employed in the structure determination of complex compounds and have been adopted here for use on mixtures. The resolving power of these experiments is illustrated on the analysis of hot water extracts of green tea.

Highlights

NMR structure determination of compounds without their puri cation from mixtures of varying complexity is a challenging task
The new experiments represent a set of NMR techniques typically employed in the structure determination of complex compounds and have been adopted here for use on mixtures
Chemical modi cation in the form of molecular tagging with fully enriched NMR active isotopes seems to be the only option to unambiguously determine structures, here the main limitation is the potential reach from a given tag and only parts of the molecules can be studied at one time

Summary

Introduction

NMR structure determination of compounds without their puri cation from mixtures of varying complexity is a challenging task. Different approaches can be employed to achieve this, such as spreading the signals into multiple dimensions, exciting only a fraction of the nuclei, utilising nuclei with larger chemical shi dispersion (usually 13C), reducing the footprint of individual signals by collapsing their multiplets, or “separating” the molecules by their size in an NMR tube through molecular diffusion.. Chemical modi cation in the form of molecular tagging with fully enriched NMR active isotopes seems to be the only option to unambiguously determine structures, here the main limitation is the potential reach from a given tag and only parts of the molecules can be studied at one time. Different approaches can be employed to achieve this, such as spreading the signals into multiple dimensions, exciting only a fraction of the nuclei, utilising nuclei with larger chemical shi dispersion (usually 13C), reducing the footprint of individual signals by collapsing their multiplets, or “separating” the molecules by their size in an NMR tube through molecular diffusion. For very complex mixtures, chemical modi cation in the form of molecular tagging with fully enriched NMR active isotopes seems to be the only option to unambiguously determine structures, here the main limitation is the potential reach from a given tag and only parts of the molecules can be studied at one time.

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