Binding of a chiral drug to a protein: an investigation of the 2-(3-benzoylphenyl)propionic acid/bovine serum albumin system by circular dichroism and fluorescence.

A combined approach using global analysis of circular dichroism multiwavelength data and time resolved fluorescence was applied to investigate the interaction of R-(-)- and S-(+)-ketoprofen with bovine serum albumin in buffer solution at neutral pH. A characterization of the most stable drug : protein adducts of 1 : 1 and 2 : 1 stoichiometry, as individual chemical species, was obtained. The stability constants and the absolute circular dichroism spectra of the diastereomeric complexes were determined. The spectra of the 1 : 1 conjugates are opposite in sign, those of the 2 : 1 complexes are both negative, but different in shape from each other (peaks at 358 and 342 nm for S-(+)- and R-(-)-ketoprofen, respectively). A tryptophan residue was shown to be involved in the binding of the drug, in the primary site for the R-(-) and in the secondary site for the S-(+) enantiomer, thereby showing that chiral recognition by the protein causes the site of highest affinity being not the same for both optical antipodes.