Environment-related properties of polyhydroxylated dibenzo-p-dioxins

摘要

Polyhydroxylated dibenzo-p-dioxins (PHODDs) are important metabolic and synthetic products of poly chlorinated dibenzo-p-dioxins (PCDDs). Two types of hydrogen bonds exist in PHODD molecules: one be tween a hydroxyl group (HO) and an oxygen atom of the ether bond, and the other between two ortho hydroxyls of a benzene ring. By fully optimized calculation with density functional theory (DFT), their bond energies were ascertained to be approximately 9-14 kJ/mol and 15-19 kJ/mol respectively by the com parison of standard Gibbs energy of formation (△_fG~θ) between different molecules, which was experimental ly verified. The two types of hydrogen bonds affect the hydrophilicity and stability of the molecules. The torsional potential of hydroxyls and the orientation making the congener most stable were obtained. The octanol-water partition coefficients (logKows) were calculated based on the group contribution method, and the standard state entropy (S~θ), standard enthalpy (△_fH~θ) of formation and △_fG~θ were obtained from the combination of DFT calculation and isodesmic reaction for the stable PHODD congeners. The number and position of hydroxyl substitution (NPHos) were employed as descriptors to establish quantitative struc ture-property relationship (QSPR) models. Although the hydrophilicity of PHODDs increases with the num ber of hydroxyl groups, it is impaired by the intramolecular hydrogen bonds. The logKows of PHODDs are much smaller than those of PCDDs, and the variation trend with the number of substituents is different. In addition, the relative stability order of PHODD congeners was theoretically proposed, which is quite different from that of PCDDs. Considering the ionization in water, first-order ionization constants of PHODDs were cal culated according to the results of SMD method of Self-Consistent Reaction Field Theory (SCRF), and they were influenced by the hydrogen bonds.