Protein binding of benz[a]anthracene and benzo[a]pyrene.

Fluorometric studies on the binding of benz[a]anthracene and benzo[a]pyrene to human serum albumin are described. The protein molecule appears to have one binding site for the hydrocarbons, but all of the sites on the protein are not fully occupied even in relatively large hydrocarbon concentrations. Equilibrium studies showed that both hydrocarbons bind to the protein to the same extent. Evidence for the energy transfer from the tryptophan residue of the protein to bound hydrocarbons is examined. By using Förster's theory, the mean distance between the tryptophan residue and bound ligand was found to be 15,2 A for benz[a]anthracene and 19.6 A for benzo[a]pyrene. It is concluded that the two hydrocarbons may bind to the same general area on the protein molecule near the tryptophan residue but at different sites. The structural differences of the hydrocarbons, which may greatly affect their orientations on the protein molecule, affect mainly the selection of the binding site rather than the binding equilibrium.