Chiral discrimination between tyrosine and β-cyclodextrin revealed by cryogenic ion trap infrared spectroscopy

Complexes of permethylated β-cyclodextrin (β-MCD) with the two enantiomers of protonated tyrosine (l- and d-TyrH⁺) are studied by cryogenic ion trap infrared photo-dissociation spectroscopy. The vibrational spectra in the OH/NH stretch and fingerprint regions are assigned based on density functional theory calculations. The spectrum of both l- and d-TyrH⁺ complexes contains features characteristic of a first structure with ammonium and acid groups of the amino acid simultaneously interacting with the β-MCD, the phenolic OH remaining free. A second structure involving additional interaction between the phenolic OH and the β-MCD is observed only for the complex with d-TyrH⁺. The larger abundance of the d-TyrH⁺ complex in the mass spectrum is tentatively explained in terms of (1) better insertion of d-TyrH⁺ within the cavity with the hydrophobic aromatic moiety less exposed to hydrophilic solvent molecules and (2) a stiff structure involving three interaction points, namely the ammonium, the phenolic OH and the carboxylic acid OH, which is not possible for the complex with l-TyrH⁺. The recognition process does not occur through size effects that induce complementarity to the host molecule but specific interactions. These results provide a comprehensive understanding of how the cyclodextrin recognises a chiral biomolecule.