Determination of association constants at moderately fast chemical exchange: complexation of camphor enantiomers by alpha-cyclodextrin

NMR studies of inclusion complexes: naphthalene and natural cyclodextrins

Inclusion complexes of naphthalene (NP) with cyclodextrins (CD) have been investigated so far using non-NMR techniques resulting in inconsistent data. Here, the first application of high-field NMR spectroscopy in combination with a precise analysis of the results has allowed us to determine accurately the stoichiometry of complexes and their association constants. Titration measurements have been performed by ¹H NMR spectroscopy in D₂O at a magnetic field B₀ of 18.8 T. NP and αCD form a 1 : 2 complex in which a single NP molecule is closed in a capsule made up of two αCD macrocycles. NP and βCD build coexisting 2 : 1 and 2 : 2 complexes with large binding constants. Larger γCD host molecules form essentially similar complexes with NP as the βCD but corresponding binding constants are smaller.

Complex formation of fenchone with α-cyclodextrin: NMR titrations

¹³C NMR titration studies of inclusion complexes of bicyclic terpenoid, fenchone enantiomers with α-cyclodextrin revealed their 1:2 guest–host stoichiometry. Sequential binding constants were determined indicating a strong binding cooperativity of two α-cyclodextrin to fenchone. The overall association constants were used to calculate the Gibbs free energies of diastereomeric complex formation, which might be used as a measure of chiral recognition of fenchone by α-cyclodextrin. These results were compared with corresponding data derived for camphor, which is an isomeric bicyclic terpenoid.

Study of the inclusion of the (R)- and (S)-camphor enantiomers in alpha-cyclodextrin by X-ray crystallography and molecular dynamics

The inclusion of (R)- and (S)-camphor compounds in alpha-cyclodextrin has been studied by X-ray crystallography. The crystal structures of the complexes reveal that one guest molecule is accommodated inside the cavity formed by a head-to-head cyclodextrin dimer. In the crystal lattice, the dimers form layers which are successively shifted by half a dimer. In both (R)- and (S)-cases, the camphor molecule exhibits disorder and occupies three major sites with orientations that can be described as either 'polar' or 'equatorial'. Molecular dynamics simulations performed for the observed complexes indicate that although the carbonyl oxygen of both (R)- and (S)-camphor switches between different hydrogen bonding partners, it maintains the observed mode of 'polar' or 'equatorial' alignment.

1H NMR titration and quantum calculation for the inclusion complexes of cis-cyclooctene, cis, cis-1, 3-cyclooctadiene and cis, cis-1, 5-cyclooctadiene with beta-cyclodextrin

The inclusion behavior of cis-cyclooctene, cis, cis-1, 3-cyclooctadiene and cis, cis-1, 5-cyclooctadiene with beta-cyclodextrin (beta-CD) was studied by using (1)H NMR method in D(2)O/CD(3)OD solution and PM3 quantum-chemical simulation in vacuum. The experimental results indicate that each guest molecule penetrates deeply into beta-CD cavity and forms equimolecular inclusion complex with the host. The association constants of the complexes were determined by non-linear least-square method on the bases of the conversion-dependent chemical shift of two protons of the host molecule. The inclusion process and the most probable structure of the inclusion complexes were simulated using PM3 energy scanning and optimization. The trend of stability of the three inclusion complexes deduced from their calculated stabilization energies agrees well with the order of their association constants obtained from NMR experiments.