Photophysical Features of Protolytic Equilibria of Fluorescein Bifluorophores
摘要
The effects of pH and the solution composition on the photophysical properties of two carboxyfluorescein bifluorophores were studied along with a comparison of the fluorescence effect of these bifluorophores with that of monomer carboxyfluoresceins (5-FAM and 6-FAM) in conjugates with protein molecules. Both the bifluorophores were found to fluoresce well in an alkaline medium but the fluorescence intensity decreased at pH 6–7 and almost disappeared in acidic media. The fluorescence of meta-(6-FAM)2 is very sensitive to the solution composition, while the fluorescence of meta-(5-FAM)2 is more stable and depends only slightly on the solution properties. Upon formation of a conjugate of the bifluorophore with a protein, the fluorescence yield of the bifluorophore decreases to a greater extent in comparison with the FAM monomers but the total fluorescence yield is not less than the intensities of the two carboxyfluorescein monomer–protein conjugates due to the high extinction coefficient of the bifluorophore (~140,000 M–1·cm–1).