Human serum albumin (HSA) plays important roles in transport of fatty acids and binding a variety of drugs and organic compounds in the circulatory system. This protein experiences several conformational transitions by the change of pH, and the resulting conformations were essential for completing the physiological roles in vivo. Steady-state and time-resolved fluorescence spectroscopy was applied to single tryptophan residue solely arranged in HSA to study subtle conformational change around single tryptophan residue in HSA at various pH. The results showed the characteristic feature of local conformation around tryptophan residue in domain II responding to the change in entire structure. The study of time-resolved area-normalized fluorescence emission spectra (TRANES) also showed the peculiar dielectric property of water molecule trapped nearby tryptophan residue depending on pH. These results suggested that microenvironment around tryptophan residue was tightly packed at acidic and basic pH although entire conformation was loosened.
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