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Proceedings Paper

Specific interaction of 7-hydroxyquinoline with Trp-214 in the drug-binding site IIA of human serum albumin
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Paper Abstract

The drug-binding site subdomain IIA of human serum albumin (HSA) was characterized by absorption and fluorescence spectroscopy using 7-hydroxyquinoline (7-HQ) as a local reporter. The spectra of 7-HQ in solution indicate that a ztitterionic tautomer is stabilized by water in the ground state and produces a unique absorption peak at 400 nm and a fluorescence peak at 510 nm. By examining the spectral change in binary mixtures of water and 1,4-dioxane, three water molecules were estimated to stabilize this tautomer through direct interactions with the polar regions of the molecule. When 7-HQ is mixed with HSA, a reduction in the absorbance of the zwitterionic tautomer was observed which indicates a less polar environment around the molecule. The 7-HQ molecule is found to specifically bind in subdomain IIA of HSA and causes a reduction in the fluorescence intensity of the Trp-214 residue which is located in the same binding site. The reduction in the fluorescence of Trp-214 is due to energy transfer from the Trp-214 residue to the 7- HQ probe. The distance between Trp-214 and the probe was calculated using Förster theory for energy transfer to be 1.95 nm. This distance and the calculated quenching rate constant using a Stern-Valmer plot (kq = 3.04 x 1012 M-1s-1) both point to a static quenching mechanism. The binding constant and the number of binding sites of the complex were also estimated and the calculations show that the 7-HQ probe binds only in subdomain IIA. The change in the fluorescence intensity of HSA in the presence of the probe indicates that the 7-HQ molecule selectively interacts with the Trp-214 residue which results in partial unmasking of the fluorescence due to the Tyr-263 residue (located in the same site). A much stronger fluorescence from Tyr-263 is observed when HSA is chemically unfolded by 6.0 M GdnHCl. 7- HQ is found to still bind in subdomain IIA in the unfolded state of HSA and causes a reduction in the fluorescence intensities of both Trp-214 and Tyr-263. The present results propose 7-HQ as a useful photophysical probe in studying binding sites in proteins and exploring their hydrophobic environment.

Paper Details

Date Published: 16 February 2010
PDF: 11 pages
Proc. SPIE 7576, Reporters, Markers, Dyes, Nanoparticles, and Molecular Probes for Biomedical Applications II, 757618 (16 February 2010); doi: 10.1117/12.840485
Show Author Affiliations
Osama K. Abou-Zied, Sultan Qaboos Univ. (Oman)
Najla Al-Lawatia, Sultan Qaboos Univ. (Oman)

Published in SPIE Proceedings Vol. 7576:
Reporters, Markers, Dyes, Nanoparticles, and Molecular Probes for Biomedical Applications II
Samuel Achilefu; Ramesh Raghavachari, Editor(s)

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