Binding of Fluorescent 7-Amino-4-nitrobenzoxadiazole Derivatives to Bovine Serum Albumin

The quantum yield and binding to bovine serum albumin (BSA) were investigated with various fluorescent 7-alkylamino (bearing 2-12 carbon atoms), 7-benzylamino, and 7-p-methoxybenzyl-amino derivatives of 4-nitrobenzoxadiazole (NBD). Among them, alkylamino derivatives gave high quantum yield in both ethanol and BSA solution, and greater affinity to BSA was found with those having a longer alkyl chain. In particular, those with C8-C12 alkyl chains may be useful in research on hydrophobic regions as fluorescent probes. In the binding of these probes to BSA, the logarithm of the primary association constant increased linearly with increase in the length of the side chain; a hydrophobic cleft similar to that on the human serum albumin molecule was suggested to exist also on the BSA molecule. In the investigation of the binding displacement of 7-amino-NBD derivatives based on Sudlow's classification, almost no displacement was observed with site II drugs, ibuprofen and flufenamic acid. However, diazepam (a site II drug) showed different behavior from other site II drugs. It was particularly interesting that the difference of the displacing behavior between diazepam and other site II drugs increases with increase of the side chain length of the alkylamino-NBD probes.