pH dependence of binding benzo[h]quinoline and humic acid and effects on fluorescence quenching

The binding constant (K(DOC)) between humic acid and the nitrogen-containing polycyclic aromatic compound (N-PAC), benzo[h]quinoline, was measured at varying pH levels using fluorescence quenching (FQ). Because fluorescence characteristics of benzo[h]quinoline change with pH, determination required two optimum sets of excitation and emission wavelength pairs. A simple mixing model was used to eliminate the inherent fluorescence interference between benzo[h]quinoline and its protonated form, benzo[h]quinolinium, when estimating binding constants. Hydrophobic interaction is likely to control the binding between humic acid and benzo[h]quinoline and benzo[h]quinolinium, in lower and higher pH ranges (pH <3, pH >6). In contrast, cation exchange seems to control the binding affinity of benzo[h]quinolinium in the middle range of pH. The estimates of K(DOC) were up to 70% smaller after elimination of interference. This indicates that the contribution of the minor form influences estimates of the K(DOC)-pH trend for benzo[h]quinoline, and potentially explains the large discrepancy reported in the literature between results based on using FQ and those based on equilibrium dialysis methods. Previous FQ measurements overestimate K(DOC) at some pH values and lead to an underestimation of bioavailability in an aquatic environment. The application of our models appears to be necessary when using FQ for determining the K(DOC)-pH trend for organic compounds with acid-base pair analogs.