CHARACTERIZATION OF REGION-IC IN SITE-I ON HUMAN SERUM-ALBUMIN - MICROENVIRONMENTAL ANALYSIS USING FLUORESCENCE SPECTROSCOPY

Characteristics of region Ic among at least three overlapping binding regions (regions Ia, Ib and Ic) in site I on human serum albumin (HSA) were analysed using n-alkyl p-aminobenzoates (n-alkyl p-ABEs), all of which are specific fluorescent probes for region Ic. In the interacti on processes between rr-alkyl p-ABEs and HSA, hydrophobic interaction, van der Waals interaction and local structural changes in region Ic w ere found to be involved based on the results obtained by analyses of the fluorescence spectra, structure-activity relationships and thermod ynamic parameters. In addition, comparison of the fluorescence spectra of n-alkyl p-ABEs in HSA and detergents indicated that the possibilit y of a hydrophobic region Ic around which an amino acid with cationic charge locates could not be denied because of the similarity of fluore scence spectra between n-alkyl p-ABEs in HSA and in neutral and cation ic detergents. The deviation of n-alkyl p-ABEs with long alkyl chains (C-9- C-12) in the relationships between association constants and phy sicochemical properties of a series of n-alkyl p-ABEs (C-1- C-12) sugg ested that region Ic possess an optimal depth. A conformational change of HSA with increasing pH (pH 6-9) generated an increase in hydrophob icity and adaptability of the binding region and made interaction easy , with an increase in adaptability of the binding region Ic; consequen tly, it enhanced the binding of n-alkyl p-ABEs to HSA.