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Rigidification of Neutral Lipid Bilayers in the Presence of Salts

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

^* Institute of Biophysics and Nanosystems Research, Austrian Academy of Sciences, Graz, Austria; and Laboratory of Biophysics, "Joef Stefan" Institute, Ljubljana, Slovenia

Correspondence: Address reprint requests to Georg Pabst, Institute of Biophysics and Nanosystems Research, Austrian Academy of Sciences, Schmiedlstr. 6, A-8042 Graz, Austria. Tel.: 43-316-4120-342; Fax: 43-316-4120-390. E-mail: Georg.Pabst{at}oeaw.ac.at.

We studied the influence of sodium and calcium chloride on the global and local membrane properties of fluid palmitoyl-oleoyl phosphatidylcholine bilayers, applying synchrotron small-angle x-ray diffraction, spin-labeling electron paramagnetic resonance spectroscopy, and 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, whereas 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 were able to reconcile previous controversies between molecular dynamics simulations and x-ray diffraction experiments regarding the effect of salts on neutral lipid bilayers.

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