Investigations into the Role of Humic Acid in Biomembrane Permeability and the Effects of Naturally Formed Gold and Silver Nanoparticles on These Interactions

Abstract

The mechanism and kinetics of the interaction of humic acids (HAs) with a 1-palmitoyl-2-oleoyl-Sn-glycero-3-phosphatidylcholine (POPC) large unilamellar vesicle (LUV) model biomembrane system were studied by fluorescence spectroscopy. All three HAs studied induced a perturbation to the biomembrane bilayer structure at pH 4.8. Concentration dependence studies revealed that biomembrane perturbation increased with increasing HA concentrations for Suwannee River HA (SWHA) from 0 to 20 mg C/L. For both Leonardite HA (LAHA) and Florida Peat HA (FPHA) aggregation influenced biomembrane perturbation at concentrations above 5 and 7.5 mg C/L, respectively. Temperature studies over the environmentally relevant ranges of 10 to 30 °C revealed that biomembrane perturbation increased with decreasing temperature for all three HAs studied. Kinetic data established that adsorption and absorption occurred within seconds to minutes and the complex absorption process consisted of both fast and slow components. The slow component was fitted to first order kinetics; however, the fast component could not be fitted with either first or second order kinetics. A mechanism based on “lattice errors” within the biomembranes was proposed to explain the fast and slow components as well as the concentration and temperature findings. The aromatic moieties within HAs were shown to be responsible for the interactions of HAs with biomembranes and the major biomembrane disruptors for the chemically edited HAs studied. Au and Ag nanoparticles (NPs) were synthesized using the same chemically diverse and chemically edited HAs at environmentally relevant temperature and pH conditions. Stable particles with narrow size distributions for Au NPs and wider distributions for Ag NPs were formed. The NPs synthesized with HAs at pH 4.8 were polydisperse while the NPs synthesized with HAs at pH 7.6 were monodisperse. The interactions of Au and Ag NPs, synthesized and stabilized by chemically diverse HAs, with POPC LUVs contributed to greater biomembrane perturbation at pH 4.8 compared to 7.6, as observed by fluorescence leakage experiments and cryo-TEM imaging.