Rigidification of neutral lipid bilayers in the presence of salts

Georg Pabst ^1*, Aden Hodzic ¹, Janez trancar ², Sabine Danner ¹, Michael Rappolt ¹ and Peter Laggner ¹

¹ Austrian Academy of Sciences
² Jozef Stefan Institute

^* To whom correspondence should be addressed. E-mail: georg.pabst{at}oeaw.ac.at.

Submitted on May 11, 2007
Revised on June 13, 2007
Accepted on 15 June 2007

	Abstract

We have studied the influence of sodium and calcium chloride on the global and local membrane properties of fluid palmitoyl-oleoyl phosphatidylcholine (POPC) bilayers applying synchrotron small angle x-ray diffraction, spin-labeling electron paramagnetic resonance spectroscopy, differential scanning calorimetry, as well as simultaneous density and acoustic measurements. The salt concentration was varied over a wide range from 0 to 5 M. We found that NaCl leads to a continuous swelling of the bilayers, while the behavior of the bilayer separation d_W in the presence of CaCl₂ is more complex showing an initial large d_W value, which decreased upon further addition of salt and finally increased again in the high concentration regime. This can be understood by a change of balance between electrostatic and van der Waals interactions. We were further able to show that both salts lead to a significant increase of order within the lipid bilayer leading to a decrease of bilayer elasticity and shift of main phase transition temperature. This effect is more pronounced for Ca²⁺, and occurs mainly in the high salt concentration regime. Thus, we have been able to reconcile previous controversies between molecular dynamics simulations and x-ray diffraction experiments regarding the effect of salts on neutral lipid bilayers.

Key Words: combined dilatometry and velocimetry, electron paramagnetic resonance spectroscopy, interactions, ion-specific effects, membrane structure, small angle x-ray diffraction

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