Induced circular dichroism and chiral discrimination of racemates revisited: bilirubins as illustrative examples

The scope and limitation of the circular dichroism of racemates induced by an optically active cosolute (ICD) are elaborated. Accordingly, the interpretation of ICD spectra depends upon the barrier to interconversion of enantiomers, the degree of complexation, the availability of reference values and population and spectral differences of the species involved. The distinct nature of these factors may occasionally prevent a meaningful application of this technique in the field of molecular and chiral recognition. The relevance of the more general considerations is outlined and eventual implications for the ICD spectra of bilirubin-IXα(1) reported in the literature are provided by the bridged analogues 2, 3a and 3b.