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

✍ Scribed by Ping-Chieh Hsieh; Shih-Han Hsu; Chon-Lin Lee; Peter Brimblecombe

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 181 KB
• Volume: 29
• Category: Article
• ISSN: 0730-7268
• DOI: 10.1002/etc.220

✦ Synopsis

Abstract

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. Environ. Toxicol. Chem. 2010; 29:1696–1702. © 2010 SETAC