Supramolecular Association Impacts Biomolecule Adsorption onto Goethite

Formation of biomolecule-rich supramolecular complexes in dissolved organic matter (DOM) and subsequent adsorption onto minerals is important for the development of mineral-stabilized organic matter, yet the impact of supramolecular association on interfacial behavior is seldom studied. A series of supramolecular complexes of model biomolecules (deoxyribonucleic acid (DNA) and bovine serum albumin (BSA)) are synthesized, characterized, and adsorbed onto goethite. Complexes represent 0.1 mg/mL DNA mixed with BSA concentrations from 0.05 to 0.5 mg/mL in 5 mM KCl at pH = 5.0. Circular dichroism demonstrates strong binding between DNA and BSA, with DNA saturation when (BSA) ≈ 0.4 mg/mL. Dynamic light scattering and electrophoretic mobility measurements suggest DNA-BSA binding reduces DNA-DNA electrostatic repulsion. Spectroscopic studies of DNA/BSA complex adsorption show complexation hinders coordination of DNA phosphodiester groups with goethite. Increasing BSA (≤0.4 mg/mL) in DNA/BSA complexes enhances DNA adsorption, due to reduced repulsion between adsorbed DNA helices. When (BSA) > 0.4 mg/mL, however, DNA adsorption is decreased. We hypothesize this results from blocking of surface sites by fast adsorption of BSA loosely associated with DNA/BSA complexes. We posit an additional mechanism describing multilayered architecture formation of organo-mineral associations in soil, suggesting solution interactions may represent an overlooked factor when considering mineral retention of DOM.