Role of the Methoxy Groups in Cryptophanes for Complexation of Xenon: Conformational Selection Evidence from 129 Xe-1 H NMR SPINOE Experiments.

Patrick Berthault,Erwann Jeanneau,Thierry Brotin,Céline Boutin,Estelle Léonce

doi:10.1002/cphc.201700266

Abstract

We report the laser-polarized 129 Xe and 1 H NMR spectra of a series of cryptophane derivatives that differ only by the number of methoxy groups attached on their benzene rings and the syn or anti arrangement of the linkers (compounds 6 a-s, 9 a-s, 12 a-s). All these compounds bind xenon even though the characteristic signal of the gas encapsulated in the cavity of the cage-molecule cannot always be detected. Interestingly, the exchange dynamics of xenon strongly depends on the degree of substitution and is different from that of the cryptophane derivatives studied previously. In solution, the 1 H NMR spectra of these derivatives show the presence of different conformations in a slow exchange regime that can be explained by a decrease of the flexibility of their skeleton. Thanks to 129 Xe-1 H dipolar cross-relaxation (SPINOE) spectra, we demonstrate that a single conformation present in solution can bind xenon.

Highlights

Xenon is known to show good affinity for organic structures such as the hydrophobic pockets of proteins or hollow organic compounds capable of accommodating substrates in their cavities.[1,2,3,4] For instance, it has been reported that xenon presents a good affinity for cryptophane-A (1), a synthetic organic compound with a roughly spherical cavity.[5]
We report a study of the Xe@6a-s, 9a-s and 12a-s complexes by 1H NMR and 129Xe NMR spectroscopy
These compounds differ from their congeners 1-3 by the number of methoxy substituents attached on the cryptophane-222 skeleton and the arrangement syn or anti of the linkers

Summary

Introduction

Xenon is known to show good affinity for organic structures such as the hydrophobic pockets of proteins or hollow organic compounds capable of accommodating substrates in their cavities.[1,2,3,4] For instance, it has been reported that xenon presents a good affinity for cryptophane-A (1), a synthetic organic compound with a roughly spherical cavity.[5]. The possibility to modify the host structure in order to graft a biological site that can recognize a biological target represents another advantage for these systems.[9] From a more fundamental point of view, 129Xe NMR spectroscopy appears as the tool of choice for studying cryptophane derivatives with small cavities. The xenon atom present within the cavity of cryptophane-A congeners shows a chemical shift significantly modified with respect to that of xenon present in the bulk. In 1,1,2,2-tetrachloroethane-d2 the 129Xe NMR spectrum of the Xe@cryptophane-A complex shows a chemical shift [c] E. Jeanneau Centre de Diffractométrie Henri Longchambon Université de Lyon 1, 5 rue la Doua 69100 Villeurbanne, France

Methods

Results

Conclusion