The partitioning of PAHs to egg phospholipids facilitated by copper and proton binding via cation-π interactions

The partitioning to lipid-containing solids (cell membranes, natural organic matters) plays an important role in the fate of organic pollutants. We herein studied sorption of a series of aromatic compounds from aqueous solution to gel-phase egg phospholipids. The regression line describing the free-energy relationship between lipid-water distribution coefficient (K-d) and n-octanol - water partition coefficient (K-OW) for the highpolar compounds (phenolics, dinitrobenzene, trinitrobenzene) is displaced upward relative to the low-polar compounds (chlorobenzenes, polycyclic aromatic hydrocarbons (PAHs), nitrobenzene, dichlorobenzonitrile), suggesting additive polar extra-interactions besides hydrophobic effects in sorption. Binding of Cu2+ or decreasing pH increases sorption of the three and fourring PAHs but not the rest compounds. These results led us to propose a specific sorption mechanism, cation-pi bonding between PAHs and complexed metal ions or protonated amine groups of phospholipids. The Cu2+-PAH complexation in solution was supported by the observation that PAHs enhance the saturated solubility of CUSO4 in chloroform, and the enhancement correlates with pi-donor strength of PAH (pyrene > phenanthrene > naphthalene). The electron coupling between the protonated amine groups of phospholipids and PAHs in chloroform was verified by the electronic deshieldinginduced downfield chemical shifts of phenanthrene at low pH in the H-1 NMR spectrum.