Measurements of Solute Polarizabilites Affecting Lipid Membrane Interactions

Abstract

The van der Waals attraction between lipid membranes depends on the polarizability of solute molecules present in the aqueous space between neighboring membranes [1]. In particular, zwitterionic molecules (such as common pH buffers) have been shown to affect van der Waals forces more strongly than monovalent saltions [2]. Experimentally, changes in van der Waals forces are detected by x-ray scattering measurements of multilamellar lipid vesicles through the sensitivity of lamellar repeat distances to solution polarizabilities. In this respect, a direct determination of solute polarizabilities would lend support to the interpretation of x-ray data. Here we use a recently developed method [3] to quantify the polarizabilities of zwitterionic pH buffers including TAPS, TES, MES, MOPS, and HEPES. This method calculates a solution function r(c) which gives the polarizability of hydrated solutes as a function of concentration (c) by combining mass density and index of refraction measurements into a dimensionless quantity. We obtain that the zwitterionic molecules considered are significantly more polarizable than NaCl used as reference. These results are in agreement with x-ray measurements of lipid membrane interactions in the presence of buffers and monovalent salt solutions. Further, measurements of solutions 1:1 mixtures by volume have shown the polarizability is additive in agreement with theoretical models.