7-Alkylaminocoumarin-4-acetic Acids as Fluorescent Probe for Studies of Drug-Binding Sites on Human Serum Albumin

7-Alkylaminocoumarin-4-acetic acids I-IX having alkylamino groups different in alkylchain length were synthesized as fluorescence probes for characterization of drug-binding sites on human serum albumin (HSA). The fluorescences of I-IX were quenched or enhanced in the presence of HSA with shifts of the emission maxima to shorter wavelength. The binding constants and the number of binding sites were determined by the spectral changes of the probes I-IX bound to HSA through analysis of Scatchard's and Job's plots. The primary binding sites of the tested probes were found to be site 2 (diazepam site) on HSA from the results of competitive displacement studies. The polarity of site 2 was estimated from the relationship between the emission maximum of the probe of IV and Z-values, and was found to be comparable to that of acetonitrile. Simple attempts to estimate the site 2 region from the molecular size of the probe of VIII obtained using the Corey-Pauling-Koltun molecular model suggest that the hydrophobic cleft at site 2 is about 21-25 Å in depth. The distance between the lone tryptophan residue in HSA and probes bound to site 2 was estimated to be 15-17 Å using Forster's equation on the basis of fluorescence energy transfer. The present data suggest that I-IX are useful as fluorescence probes for the characterization of site 2 on HSA.