Manifestation of chiral recognition of camphor enantiomers by alpha-cyclodextrin in longitudinal and transverse relaxation rates of the corresponding 1:2 complexes and determination of the orientation of the guest inside the host capsule

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.

Spectrophotometric and Calorimetric Titration Studies on Molecular Recognition of Camphor and Borneol by Nucleobase-Modified β-Cyclodextrins

A series of modified beta-cyclodextrins with nucleobase substituents, that is, mono(6-ade-6-deoxy)-beta-cyclodextrin (2) and mono(6-ura-6-deoxy)-beta-cyclodextrin (3) as well as mono(6-thy-6-deoxy)-beta-cyclodextrin (4), were selected as molecular receptors to investigate their conformation and inclusion complexation behaviors with some chiral molecules, that is, (+)-camphor, (-)-camphor, (+)-borneol, and (-)-borneol, by spectrophotometric and microcalorimetric titrations in aqueous phosphate buffer solution (pH 7.2) at 298.15 K. Circular dichroism and NMR studies demonstrated that these nucleobase-modified beta-cyclodextrins adopted a co-inclusion mode upon complexation with guest molecules; that is, the originally self-included nucleobase substituents of the host did not move out from the beta-cyclodextrin cavity, but coexisted with guest molecule in the beta-cyclodextrin cavity upon inclusion complexation. Significantly, these nucleobase-modified beta-cyclodextrins efficiently enhanced the molecular binding ability and the chiral recognition ability of native beta-cyclodextrin, displaying enantioselectivity up to 3.7 for (+)-camphor/(-)-camphor pair by 2 and 3.5 for (-)-borneol/(+)-borneol pair by 3. The enhanced molecular/chiral recognition abilities of 2-4 toward (+/-)-camphor were mainly attributed to the increased entropic gains due to the extensive desolvation effects, while the favorable enthalpic gains originating from the good size-fit relationship as well as the hydrogen bond interactions between host and guest result in the enhanced molecular/chiral recognition abilities of 2-4 toward (+/-)-borneol.

Understanding chiral molecular micellar separations using steady-state fluorescence anisotropy, capillary electrophoresis, and NMR

Chiral separations employing four diastereomers of poly sodium N-undecanoyl leucylvalinate (p-SULV) as chiral selectors are probed by use of MEKC, steady-state fluorescence anisotropy, and NMR. By employing diastereomers and thus altering the stereochemistry of a single amino acid in a systematic way, one may control the enantiorecognition ability of the chiral selector. As a result, one can gain a better understanding of the mechanisms of chiral recognition for the two classes of neutral or anionic chiral analytes studied. Evaluation of the chiral interactions leading to chiral separations confirmed our earlier observation of a strong relationship between the selectivity (alpha) observed using a chromatographic separation technique (MEKC) and that determined from the spectroscopic parameter, beta. A linear alpha versus beta relationship was observed for the molecular micelle p-(L)-SULV with all eight analytes included in this study. However, as we earlier predicted, different groups of analytes had different slopes, i.e., values of m, suggesting different chiral separation mechanisms. Evaluation of the data allowed a grouping of the analytes according to the primary site of chiral interaction with the leucine or valine moiety of molecular micelle chiral headgroup.

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

Association constants in weak molecular complexes can be determined by analysis of chemical shifts variations resulting from changes of guest to host concentration ratio. In the regime of very fast exchange, i.e., when exchange rate is several orders of magnitude larger than the Larmor angular frequency difference of the observed resonance in free and complexed molecule, the apparent position of averaged resonance is a population-weighted mean of resonances of particular forms involved in the equilibrium. The assumption of very fast exchange is often, however, tacitly admitted in literature even in cases where the process of interest is much slower than required. We show that such an unjustified simplification may, under certain circumstances, lead to significant underestimation of association constant and, in consequence, to non-negligible errors in Gibbs free energy under determination. We present a general method, based on iterative numerical NMR line shape analysis, which allows one for the compensation of chemical exchange effects, and delivers both the correct association constants and the exchange rates. The latter are not delivered by the other mentioned method. Practical application of our algorithm is illustrated by the case of camphor-alpha-cyclodextrin complexes.

Induced Circular Dichroism and Structural Assignment of the Cyclodextrin Inclusion Complexes of Bicyclic Azoalkanes

The stoichiometries and binding constants of the host-guest complexes between the bicyclic azoalkanes 1-6 and alpha-, beta-, and gamma-cyclodextrins (CDs) and the induced circular dichroism (ICD) of the complexes were analyzed. Assisted by proximity relationships obtained from 2D ROESY NMR spectra, the signs and intensities of the ICD spectra are interpreted in terms of the solution structures (co-conformations) of the CD complexes. The ICD assignments are based on the orientation-intensity ICD rules of Harata and Kodaka, which relate the ICD signs and intensities to the relative orientation of the electric dipole transition moment of the n,pi azo chromophore to the CD axis. The influence of the size of the guest and the host is discussed and the effect of introducing an additional chromophore (either a phenyl or a second azo group) on the ICD spectra is demonstrated.

NMR studies of chiral recognition by cyclodextrins

Chiral recognition by cyclodextrins is of considerable importance, especially for pharmaceutical industry, in view of the possible side effects of the second enantiometer of chiral drugs. In general, it manifests itself in all NMR parameters (chemical shifts, coupling constants, NOE and ROE effects, and relaxation rates) on one hand. On the other hand, it allows one to determine the thermodynamic parameters characterizing diastereomeric complexes formed by cyclodextrins with enantiomeric guests. After an introduction and a general discussion of NMR manifestations of chiral recognition by cyclodextrin, the existing literature data on this problem will be discussed herein. Chirality 16:90-105, 2004.

Thermodynamics of the Molecular and Chiral Recognition of Cycloalkanols and Camphor by Modified β-Cyclodextrins Possessing Simple Aromatic Tethers

The complex stability constants (K(S)) and thermodynamic parameters (DeltaG degrees, DeltaH degrees, and TDeltaS degrees ) for 1:1 inclusion complexation of beta-cyclodextrin (beta-CD) derivatives, 6-O-phenyl-beta-CD (2) 6-O-(4-formyl-phenyl)-beta-CD (3), 6-O-(4-nitrophenyl)-beta-CD (4), 6-O-(4-bromophenyl)-beta-CD (5), 6-O-(4-chlorophenyl)]-beta-CD (6), and 6-O-(4-hydroxybenzoyl)-beta-CD (7) with representative guest molecules, cyclic alcohols (cyclopentanol, cyclohexanol, cycloheptanol, cyclooctanol), (+/-)-borneol, and (+/-)-camphor, have been determined by means of titration microcalorimetry in an aqueous phosphate buffer solution (pH = 7.20) at 298.15 K. The results obtained indicate that the introduction to beta-CD of an aromatic ring bearing different substituent groups significantly enhances the molecular binding ability and moderately alters the chiral discrimination ability for the guests examined here, displaying the highest enantioselectivity of up to 4.01 for the inclusion complexation of 6 with (+/-)-camphor. The enhanced molecular/chiral discrimination ability caused by derivatization is attributed solely to increased positive entropy changes due to the expanding hydrophobic interaction and desolvation effects. The binding modes of host-guest interactions derived from ROESY spectroscopy data show that the resulting complex of 4 and (+)-borneol possesses better induced-fit interaction as compared to (-)-borneol, which is responsible for the enhanced molecular/chiral recognition ability.