Determination of equilibrium constants for binding of acridine orange and its 10-alkyl derivatives to dissolved humic substances by a fluorescence quenching method.

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Fluorescence intensities of Acridine Orange and its 10-alkyl derivatives were reduced by the presence of commercial humic acid or dissolved humic substances obtained from river water, seawater and pond water samples. Synchronous changes in the absorption spectra and the virtually unchanged fluorescence lifetime of the 10-dodecyl derivative suggested that the fluorescence quenching could be explained on the basis of a static quenching mechanism, in which the fluorescence dyes were bound to humic substances to form non-fluorescent dye aggregates. Curve fitting procedures for the upwardly-curved Stern-Volmer plots yielded the equilibrium binding constants normalized to organic carbon content. The binding constants at pH 7.0 for fractions eluting at pH 2.4-7.5 in a pH-gradient chromatographic separation of the aquatic humic substances were larger than those for fractions eluting at pH 8.2- 11.4. Some chemical properties of the binding have also been discussed in terms of the electrostatic interaction and/or the hydrophobic interaction.
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