The presence of a chiral or chirally perturbed chromophore in the molecule under investigation
is a fundamental requirement for the appearance of a circular dichroism (CD) spectrum.
For native and for most of the substituted cyclodextrins, this condition is not applicable,
because although chiral, cyclodextrins lack a chromophore group and therefore have
no characteristic CD spectra over 220 nm. The reason this method can be used is that
if the guest molecule has a chromophore group and this is in the right proximity to
the cyclodextrin, it becomes chirally perturbed. As a result, the complex will now
provide a CD signal, and this phenomenon is called induced circular dichroism (ICD).
The appearance of the ICD spectrum is clear evidence of the formation of the complex,
and the spectral sign and intensity is a good predictor of the structure of the complex.
By varying the concentration of cyclodextrin, the ICD signal changes, resulting in
a saturation curve, and from these data, the stability constant can be calculated
for a 1:1 complex. This article compares ICD and NMR spectroscopic and molecular modeling
results of cyclodextrin complexes of four model compounds: nimesulide, fenbufen, fenoprofen,
and bifonazole. The results obtained by the different methods show good agreement,
and the structures estimated from the ICD spectra are supported by NMR data and molecular
modeling.
