UNDERSTANDING SOLUTION-STATE NONCOVALENT INTERACTIONS BETWEEN XENOBIOTICS AND NATURAL ORGANIC MATTER USING ~(19)F/~1H HETERONUCLEAR SATURATION TRANSFER DIFFERENCE NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

摘要

A combination of forward and reverse heteronuclear (~(19)F/~1H) saturation transfer difference (STD) nuclear magnetic resonance (NMR) spectroscopic techniques were applied to characterize the noncovalent interactions between perfiuorinated aromatic xenobiotics and dissolved humic acid. These NMR techniques produce detailed molecular-level descriptions of weak noncovalent associations between components in complex environmental mixtures, allowing the mechanisms underlying these interactions to be explored; ~(19)F observed heteronuclear STD (H-STD) is used to describe the average molecular orientation of the xenobiotics during their interactions with humic acid, whereas ~1H observed reverse-heteronuclear STD (RH-STD) is used to both identify and quantify preferences exhibited by xenobiotics for interactions at different types of humic acid moieties. First, by using H-STD, it is shown that selected aromatic organofiuorides orient with their nonfluorine functional groups (OH, NH_2, and COOH) directed away from humic acid during the interactions, suggesting that these functional groups are not specifically involved. Second, the RH-STD experiment is shown to be sensitive to subtle differences in preferred interaction sites in humic acid and is used here to demonstrate preferential interactions at aromatic humic acid sites for selected aromatic xenobiotics, C_(10)F_7OH, and C_6F_4X_2, (where X = F, OH, NH_2, NO_2, or COOH), that can be predicted from the electrostatic potential density maps of the xenobiotic.