Anion Specific Partitioning in Two Phase Finite Volume Systems: Possible Implications for Mechanisms of Ion Pumps
M. Boström, E. Lima, E. C. Biscaia Jr, F. W. Tavares, P. Lo Nostro, D. F. Parsons, V. Deniz, and B. W. Ninham
Specific ion effects occur everywhere in biological systems. A classical example is the highly ion specific Donnan equilibria of red blood cells. The attribution of such ion specificity to active ion pumps is a central tenet of biology. Our group has studied, theoretically (MB et al.) and experimentally (PLN), a model system consisting of two phases in equilibrium with each other (one with salt and one with lipids and salt). There in this system no ion pumps that could give rise to an ion specific Donnan equilibrium. However, we find that in this two phase finite volume systems the ratio of the electrolyte concentration in the upper phase and in the lower phase depend on the ionic species (see Figure a below).. The reason is that one phase has lots of surfaces available that can attract polarizable ions. This attraction between ions and available lipid surfaces is due to ionic dispersion forces that lead to specific ion binding. The mechanism may also provide a contribution to active ion pumps not previously considered (a thought provoking fact is that there is lots of hemoglobin available inside the red cell that can adsorb polarizable cations).
Figure a. The variation of the concentration ratio between the upper and lower phase for the investigated anions as a function of the ion polarizability in solution. Comparison between experimental results (taken from Lo Nostro and co-workers) and our theoretical results (using ab initio calculated ion polarizabilities).
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Last updated: 09/18/09