Interactions of dissolved organic matter with xenobiotic compounds: Molecular modeling in water

An hypothesis for the structure of dissolved organic matter (DOM) in water is proposed. It is based on previously published humic acid and soil organi c matter (SOM) models. Personal computer (PC)-based molecular modeling and geometry optimization of DOM and humic/xenobiotic complexes in vacuo and wa ter were performed using modern PC software in order to determine low energ y conformations and to simulate site-specific processes such as trapping an d binding of biological and anthropogenic substances. Nanochemistry (10(-9) m level) allows the evaluation of atomic and molecular space requirements, voids, inter- and intramolecular hydrogen bonds, and interactions with wat er, metal cations, and xenobiotics. The described modeling approach in gene ral allows hydrophilic and hydrophobic reactions to be examined. Structural , molecular, and environmental properties of DOM and its xenobiotic complex es were determined by quantitative structure-activity relationship software . Focal points were molecular properties, such as solvent accessibility as well as van der Waals surface areas and volumes, partial charges, hydration energy (peptides), hydrophobicity (log P), refractivity, and polarizabilit ies of humic/xenobiotic complexes were determined. Molecular mechanics calc ulations show that nonbonded forces (e.g., van der Waals) and hydrogen bond s were the main reasons for temporary immobility of xenobiotic substances r etained in DOM. Preliminary experiments to simulate the acidity of water mo lecules by protonation-enhanced reactions with polar xenobiotics (e.g., hyd roxyatrazine) but left nonpolar substances (e.g., DDT) unchanged.