The development of thermodynamic models for actinide species in mixed solvent systems: application to binding in microbial membranes.
EMSL Project ID
4199
Abstract
Recent molecular dynamics simulations of the binding of metal ions in the outer lipopolysaccharide (LPS) layer of gram-negative bacteria have suggested unique metal binding mechanisms via cross-linking among the different LPS chains (Lins and Straatsma 2001). These cross-linking mechanisms involve multiple binding of phosphate and carboxylate groups to Ca and other metals that can stabilize the outer microbial membrane. Unfortunately, there are no current thermodynamic models available for calculating the stability of these cross-linked structures nor any capability for predicting how actinide ions and other species can bind or displace other metals from these structures. The problem is further complicated by the fact that the water content in the microbial membrane is highly variable, from essentially full solvation in the outer membrane to essentially no water in the inner lipid core. Thermodynamically, the microbial membrane therefore represents an effective mixed organic-water ?solvent? of variable composition. In order to address these issues we propose to develop the initial experimental data and thermodynamic models to accurately predict the binding of metal ions (i.e. Ca) and actinide analogs (i.e. Eu) in the microbial outer membrane. To achieve this objective we will modify an existing thermodynamic model (GMIN, Felmy 1995) to include the recently developed mixed solvent model of Wang, Anderko, and Young (2002). This thermodynamic model is the first model with the capability of accurately representing mixed solvents (including multiple solutes and solvents), liquid-liquid equilibria, and multi-component high ionic strength solutions in a self-consistent thermodynamic formalism. At the same time, we will be developing the necessary thermodynamic data to include Ca and Eu in the mixed solvent framework. We will begin these studies examining the aqueous phase reactions of these components with the most important analog groups that are predicted by the molecular simulations to be the principal groups binding the metal ions. These analog structures are expected to be present in the sugar phosphates (i.e. glucose-1-phosphate, glucose-6-phosphate, ribose 1,5 diphosphate) and sugar carboxylates (D-galacturonic acid). Plans then call for the extension of these speciation studies to organic-water mixed solvent systems to more accurately represent the microbial membrane. These studies initially will be designed to assess the order of magnitude of the effect of moving the aqueous phase stability data into solvents with increasing organic content. The objective will be to demonstrate the need for such information in the mixed solvent system, not to develop a final model, or simulant, for the microbial membrane.
References:
Felmy, A.R. (1995). ?GMIN: A Computerized Chemical Equilibrium Model Based Upon a Constrained Minimization of the Gibbs Free Energy.? Chapter 18. In: Soil Science Society of America Special Publication 42:377-407.
Lins, R.D. and T.P.Straatsma (2001). Computer simulation of the rough lipopolysaccharide membrane of Pseudomonas aeruginosa, Biophysical J., 81, 1037-1046.
Wang P., A. Anderko, and R.D. Young (2002). A speciation-based model for mixed-solvent electrolyte systems. Fluid Phase Equilibria 203, 141-176.
Project Details
Project type
Exploratory Research
Start Date
2003-08-06
End Date
2006-08-13
Status
Closed
Released Data Link
Team
Principal Investigator