Electrostatic contribution to the surface pressure of charged monolayers containing polyphosphoinositides
Ilya Levental 1, Paul A. Janmey 1* and Andrejs Cebers 2
1 University of Pennsylvania
2 University of Latvia
* To whom correspondence should be addressed. E-mail: janmey{at}mail.med.upenn.edu.
Submitted on November 27, 2007
Revised on February 22, 2008
Accepted on 12 March 2008
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Abstract |
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Structural and functional studies of lateral heterogeneity in biological membranes have underlined the importance of membrane organization in biological function. Most inquiries have focused on steric determinants of membrane organization, such as head group size and acyl chain saturation. This manuscript reports a combination of theory and experiment that shows significant electrostatic contributions to surface pressures in monolayers of phospholipids where the charge spacing is smaller than the Bjerrum length. For molecules with steric cross-sections typical of phospholipids in the cell membrane (~50 Å2), only polyphosphoinositides achieve this threshold. The most abundant such lipid is phosphatidylinositol bisphosphate (PIP2), which has between 3 and 4 charged groups at physiological conditions. Theory and experiment show that surface pressure increases linearly with PIP2 net charge and reveal crossing of high and low ionic strength pressure-area isotherms, due to opposing effects of ionic strength in compressed and expanded monolayers. Theory and experiment show that electrostatic effects are negligible for monolayers of univalent lipids, emphasizing the unique importance of electrostatic effects for lateral organization of polyphosphoinositides. Quantitative differences between theory and experiment suggest that attractive interactions between polyphosphoinositides, possibly mediated by hydrogen bonding, can lessen the effect of electrostatic repulsions.
Key Words:
Bjerrum length, PIP2, electrostatics
