G. Le Bas

The Respective Benefits of X-ray Crystallography and NMR for the Structural Determination of the Inclusion Complex Between Butyl-isothiocyanate and Alpha-cyclodextrin

Isothiocyanates are natural products extracted from plants. These molecules which exhibit very interesting antifungal properties, are insoluble in water. To increase their solubility, we have prepared inclusion complexes with different cyclodextrins. Among all the isothiocyanates studied, we have investigated in more detail the structure of one complex: butyl-isothiocyanate and alpha-cyclodextrin, using two different techniques. Firstly, 1H NMR experiments were performed and revealed the inclusion phenomenon. In parallel, crystals of butyl-isothiocyanate–alpha-cyclodextrin were grown and their crystallographic structure determined. This confirmed the inclusion of the ITC molecules and allowed us to determine the exact position of the guest. Finally, we showed that even though the complex structure was determined separately in solution and in the solid state, the structural characterisations obtained with these two techniques are complementary, enhancing the respective benefits of X-ray crystallography and NMR.

Chiral Conformations Induced by Cyclodextrin

Studies of chiroptical properties need separation or at least enrichment of enantiomers. The separation is difficult with chiral conformers of molecules with very low barriers of internal rotation. However, in association with cyclodextrin, these labile molecules can exhibit a strong Cotton effect in solution: one chiral conformer is favoured by complexation. The cyclodextrins have the advantage to yield inclusion complex in solution as well as crystalline clathrates. Therefore, the absolute configuration of the guest can be obtained by determining the structure of the cyclodextrin. In this work, the first CD spectrum of 4-helicene is recorded and crystal structures of several clathrates of labile molecules are studied.

Different building modes of α-cyclodextrin/monoalkyl amphiphile complexes

In this study, the impact of the length of the guest molecule alkyl chain and the crystallization conditions on the structural parameters of α-cyclodextrin (α-CD)/monoalkyl complexes was determined. Several procedures to crystallize those complexes were developed for different alkylalcohols as model guest molecules, as a function of temperature. Three different crystalline structures were identified depending on the alkyl chain length, using synchrotron X-ray diffraction (LURE, Orsay, France). In all cases, complexes crystallize in channel-type structures, where α-CD molecules are stacked like coins in a roll and the alkyl chain of the guest compound is embedded in the tubular cavity of the α-CDs. However, depending on the length of the chains and the crystallization conditions, the channels are organized differently. C6-C8 chains give rise to a pseudo-hexagonal lattice, a packing mode already observed for polyiodide complexes [1]. C10-C12 chains crystallize in a triclinic pseudo-monoclinic C2 lattice, while longer chains up to C18 form hexagonal crystals with R3 symmetry. These two novel crystal structures are described. Understanding these structures opens new routes to nanotube formation through amphiphile-driven crystallization of cyclodextrin templates.