HR: 0830h
AN: B21C-0723    [PDF]
TI: Molecular Simulations of a Model Humic Substance
AU: * Sutton, R
EM: rsutton@nature.berkeley.edu
AF: Division of Ecosystem Sciences, UC Berkeley, \#3110 Hilgard Hall, Berkeley, CA 94720-3110 United States
AU: Sposito, G
EM: gsposito@nature.berkeley.edu
AF: Division of Ecosystem Sciences, UC Berkeley, \#3110 Hilgard Hall, Berkeley, CA 94720-3110 United States
AU: Schulten, H
EM: hansrolf_schulten2003@yahoo.de
AF: University of Rostock, Institute of Soil Science and Plant Nutrition, Justus-von-Liebig-Weg 6, Rostock, 18059 Germany
AB: Atomistic simulations can be used to explore the molecular scale interactions that govern the formation of organo-mineral complexes in soils, complexes that in turn control the size and turnover of soil C pools. A suitable model organic molecule, featuring properties and behavior typical of the recalcitrant portion of soil C in humic substances, is needed for these simulations. The latest iteration of the Schulten model of dissolved organic matter (DOM) was modeled under both dry and hydrated conditions, and as a carboxyl-deprotonated, Na- or Ca-saturated complex, using the COMPASS force field with energy minimization and molecular dynamics algorithms. This large molecule (10,419.3 Da) possesses a flexible, porous structure and a distribution of functional groups appropriate to humic substances. When dry and densely packed, the DOM molecule has a bulk density value within the range measured for natural humic substances, but its Hildebrand solubility parameter lies just outside the range of experimental estimates. A model IR spectrum calculated from this structure was comparable to experimental spectra. When surrounded by water molecules, the DOM molecule goes through conformational adjustments, resulting in the concentrating of polar groups in exterior regions, as has been predicted for pseudomicellar structures. In order to simulate humic substances under more typical aqueous conditions, the carboxyl groups of the DOM molecule were deprotonated, and hydrated Na$^{+}$ or Ca$^{2+}$ was added to balance charge. These model metal-humic complexes were more porous, had greater solvent-accessible surface areas, and formed more H-bonds with water than the protonated, hydrated DOM molecule, due to intrusion of the cation hydrates. Our simulations indicated that, relative to Na$^{+}$, Ca$^{2+}$ is both more strongly bound to carboxylate groups and more fully hydrated, evidently due to the higher charge of the divalent cation. The Ca-DOM complex also featured fewer H-bonds than the Na-DOM complex, perhaps because of reduced orientational freedom of both the organic moieties and the water molecules after interaction with Ca$^{2+}$. The DOM molecule mimics the behavior of natural humic substances under the conditions investigated, and therefore, may be considered suitable for use in simulations of organo-mineral complexes.
DE: 0400 Biogeosciences
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1806 Chemistry of fresh water
DE: 4805 Biogeochemical cycles (1615)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting