Investigating the Binding of Acridine,Acridine Orange,and Acridine Yellow G to Humic Acid through Fluorescence Quenching

Abstract

A fluorescence quenching method was used to determine the equilibrium binding constants for the association of acridine, acridine orange, and acridine yellow G to humic acid. The fluorescence of each polycyclic aromatic nitrogen heterocycle (PANH) was monitored as aliquots of humic acid were added, and a Stern–Volmer plot was produced in which the slope is the equilibrium constant of the binding reaction. The quenching experiments were performed at temperatures of 30, 35, 40, and 45 °C. A van't Hoff plot generated from the equilibrium binding constants as a function of temperature for a given PANH resulted in a linear plot. Calculation of the ΔH_binding, ΔG_binding, and ΔS_binding for each PANH leads to the conclusion that the equilibrium binding constant, and ΔG_binding, may be predictors of bioavailability. The other thermodynamic quantities, ΔH_binding and ΔS_binding, are helpful in understanding the relative binding of the compounds. For example, acridine yellow G appears to be the least bioavailable of the three PANHs studied because of its strong ΔH_binding = −29.8 kJ/mol, which leads to ΔG_binding = −0.71 kJ/mol. While acridine orange and acridine have similar ΔH_binding values, acridine orange is more likely to bind to humic acid because the ΔS_binding for the process is less negative. Thermodynamic values and equilibrium binding constants for all three compounds are reported.