EFFECTS OF MONOVALENT ANIONS OF THE HOFMEISTER SERIES
ON DPPC LIPID BILAYERS. PART II. MODELLING THE PERPENDICULAR AND LATERAL EQUATION-OF-STATE
Epameinondas Leontidis 1*, Andria Aroti 1, Luc Belloni 2, Monique Dubois 2 and Thomas Zemb 2
1 Department of Chemistry, University of Cyprus
2 CEA/SACLAY, LIONS at Service de Chimie Moléculaire, 91191-Gif-sur-Yvette Cedex, France
* To whom correspondence should be addressed. E-mail: psleon{at}ucy.ac.cy.
Submitted on March 19, 2007
Revised on April 13, 2007
Accepted on 23 April 2007
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Abstract |
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The effects of Hofmeister anions on the perpendicular and lateral Equation of State of the DPPC lamellar phase discussed in the previous paper are here examined using appropriate free energy models for the intra- and interbilayer interactions. Minimizing the free energy with respect to the two basic geometrical parameters of the lamellar phase, which are the interbilayer water thickness, dw, and the lipid headgroup area, aL, provides the perpendicular (osmotic pressure balance) and lateral EOS. Standard models were used for the hydration, undulation and VdW attractive force between the bilayers in the presence of electrolytes while two alternative treatments of electrostatic interactions were used in order to obtain "binding" or "partitioning" constants of anions to the lipid bilayers both in the absence and in the presence of sodium binding. The computed binding constants depend on anion type and follow the Hofmeister series, but were found to increase with electrolyte concentration, implying that the local binding approximation cannot fit bilayer repulsion data. The partitioning model was also found inadequate at high electrolyte concentrations. The fitting attempts revealed two additional features worthy of future investigation. First, at maximum swelling in the presence of electrolytes the osmotic pressure of the bilayer system cannot be set equal to zero. Second, at high salt concentrations an additional repulsion appears to come into effect in the presence of strongly adsorbing anions such as I- or SCN-. Both these phenomena may reflect an inconsistent treatment of the ion-surface interactions, which have an impact on the osmotic pressure. Alternatively, they may arise from bulk solution nonidealities that cannot be handled by the classical Poisson-Boltzmann formalism. The inability of current models to explain the "lateral" Equation of State by fitting the area per lipid headgroup as a function of salt type and concentration shows that current understanding of phospholipid-ion interactions is still very incomplete.
Key Words:
DPPC bilayers, Hofmeister series, bilayer interaction model, osmotic-stress method, specific ion effect
