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 interaction processes between n-alkyl p-ABEs and HSA, hydrophobic interaction, van der Waals interaction and local structural changes in region Ic were found to be involved based on the results obtained by analyses of the fluorescence spectra, structure-activity relationships and thermodynamic parameters. In addition, comparison of the fluorescence spectra of n-alkyl p-ABEs in HSA and detergents indicated that the possibility of a hydrophobic region Ic around which an amino acid with cationic charge locates could not be denied because of the similarity of fluorescence spectra between n-alkyl p-ABEs in HSA and in neutral and cationic detergents. The deviation of n-alkyl p-ABEs with long alkyl chains (C9-C12) in the relationships between association constants and physicochemical properties of a series of n-alkyl p-ABEs (C1-C12) suggested that region Ic possess an optimal depth. A conformational change of HSA with increasing pH (pH 6-9) generated an increase in hydrophobicity and adaptability of the binding region and made interaction easy, with an increase in adaptability of the binding region Ic; consequently, it enhanced the binding of n-alkyl p-ABEs to HSA.