Conjugated Polyene Fatty Acids as Fluorescent Probes: Binding to Bovine Serum Albumin

The binding of the conjugated polyene fatty acids, cis-parinaric acid (9,11,13,15-cis, trans, trans, cis-octadecatetraenoic acid) and cis-eleostearic acid (9,11,13-cis, trans, trans, cis-ociadecatetraenoic acid), to bovine serum albumin has been studied by using absorption and fluorescence properties of the polyene chromophores. These studies demonstrate the utility of polyene fatty acids as probes of lipid-protein interactions. Shifts in the absorption spectrum, enhancement of parinaric acid fluorescence, induced circular dichroism in the polyene chromophore, and energy transfer between the tryptophan residues and the bound chromophores all provide information about the binding of these fatty acids to albumin. Strong reversible binding of parinaric acid and eleostearic acid to bovine serum albumin is observed. Fluorescence and absorption measurements demonstrate that there are binding sites for approximately five fatty acid molecules with binding constants ∼10⁶ to 10⁸ M^-1, similar to previous results for oleic acid. An induced circular dichroism arises when the polyene is bound to approximately four of these tightest binding sites. This circular dichroism is complex when more than one parinaric acid molecule is bound to the protein and is attributed to excitonic ligand-ligand interactions. Fluorescence quenching of the albumin tryptophan residues by parinaric acid is more efficient than quenching by eleostearic acid, consistent with expectations for singlet-singlet Förster transfer. The availability of two homologous polyene fatty acids with verydifferent spectral overlap integrals with tryptophan enhances the reliability of intramolecular distance determinations by the energy transfer technique. A procedure is given for the decomposition of the total energy transfer efficiency into the contributions for each of the two tryptophan donors.