Humic substances are important components of aqueous environmental systems as they interact with both cellular membranes and pollutants. There is an increasing amount of metal cations found in aqueous environments due to water acidification by both natural and human sources.  These metals can interact with humic substances by both electrostatic and chelating mechanisms. It is important to understand how humic acids interact with cellular membranes in the presence of cations.Fluorescence spectroscopy was used to study the passive interaction of humic acid with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) large unilamellar model biomembranes in the presence of cations. The chosen cations had a range of charges and affinities for different humic acid components. It was found that the humic acid interactions with biomembranes were diminished in the presence of cations. The cations protected the biomembranes in this order: K+1 < Na+1 < Mg+2 < Ca+2 < Mn+2 ~ Co+2 << Cd+2 << Fe+3 < Al+3. As the charge and chemical softness of the cations increased, the permeation of the biomembranes by humic acid decreased. This is attributed to the cations binding the humic acid at sites that would have otherwise been involved in interactions with the membranes.In order to determine which functional groups were responsible for the permeation of biomembranes and the binding of the cations, the humic acid was chemically edited by: (i) bleaching to remove aromatics, (ii) Soxhlet extraction to remove lipids, and (iii) acid hydrolysis to reduce O � and N- alkyl groups. The cation binding with the lipid extracted and acid hydrolyzed humic acid showed similar trends to those for the unedited humic acid; however, the bleached humic acid showed a significant increase in membrane permeation in the presence of cations.

Influence of cations on humic acid and biomembrane interactions

248th National Meeting (2014)

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