Exploring the enantiomeric 13C position-specific isotope fractionation: challenges and anisotropic NMR-based analytical strategy

Trying to answer the intriguing and fundamental question related to chiral induction/amplification at the origin of homochirality in Nature: "Is there a relationship between enantiomeric and isotopic fractionation of carbon 13 in chiral molecules?" is a difficult but stimulating challenge. Although isotropic ¹³C-PSIA NMR is a promising tool for the determination of (¹³C/¹²C) ratios capable of providing key ¹³C isotopic data for understanding the reaction mechanisms of biological processes or artificial transformations, this method does not provide access to any enantiomeric ¹³C isotopic data unless mirror-image isomers are first physically separated. Interestingly, ¹³C spectral enantiodiscriminations can be potentially performed in situ in the presence of enantiopure entities as chiral-europium complexes or chiral liquid crystals (CLCs). In this work, we explored for the first time the capabilities of the anisotropic ¹³C-{¹H} NMR using PBLG-based lyotropic CLCs as enantiodiscriminating media in the context of the enantiomeric position-specific ¹³C isotope fractionation (EPSIF), within the requested precision of the order of the permil. As enantiomeric NMR signals are discriminated on the basis of a difference of ¹³C residual chemical shift anisotropy (¹³C-RCSA) prior to being deconvoluted, analysis of enantiomeric mixtures becomes possible. The analytical potential of this approach when using poly-γ-benzyl-L-glutamate (PBLG) is presented, and the preliminary quantitative results on small model chiral molecules obtained at 17.5 T with a cryogenic NMR probe are reported and discussed. A promising analytical approach based on anisotropic irm-¹³C-NMR spectrometry to potentially reveal the natural ¹³C/¹²C isotopic enantiofractionation effects in organic chiral molecules is proposed and discussed.

Multinuclear NMR in polypeptide liquid crystals: Three fertile decades of methodological developments and analytical challenges

NMR spectroscopy of oriented samples makes accessible residual anisotropic intramolecular NMR interactions, such as chemical shift anisotropy (RCSA), dipolar coupling (RDC), and quadrupolar coupling (RQC), while preserving high spectral resolution. In addition, in a chiral aligned environment, enantiomers of chiral molecules or enantiopic elements of prochiral compounds adopt different average orientations on the NMR timescale, and hence produce distinct NMR spectra or signals. NMR spectroscopy in chiral aligned media is a powerful analytical tool, and notably provides unique information on (pro)chirality analysis, natural isotopic fractionation, stereochemistry, as well as molecular conformation and configuration. Significant progress has been made in this area over the three last decades, particularly using polypeptide-based chiral liquid crystals (CLCs) made of organic solutions of helically chiral polymers (as PBLG) in organic solvents. This review presents an overview of NMR in polymeric LCs. In particular, we describe the theoretical tools and the major NMR methods that have been developed and applied to study (pro)chiral molecules dissolved in such oriented solvents. We also discuss the representative applications illustrating the analytical potential of this original NMR tool. This overview article is dedicated to thirty years of original contributions to the development of NMR spectroscopy in polypeptide-based chiral liquid crystals.

Multi-nuclear NMR of axially chiral biaryls in polypeptide orienting solvents: spectral discriminations and enantiorecognition mechanisms

The analytical potential of multinuclear NMR using chiral liquid crystals to discriminate between enantiomers of axially chiral biaryls is explored.