X-ray Scattering Experiments Support Tilt-dependent Membrane Theory

Recent molecular dynamics simulations have suggested that the traditional model for topographical fluctuations in lipid bilayers should be enriched to consider molecular tilt. We present the first experimental support for a tilt-dependent theory. X-ray scattering from a liquid crystalline stack of oriented fluid phase lipid bilayers was collected and compared to the predictions of tilt-dependent and tilt-independent membrane models. Both models satisfactorily fit the X-ray data dominated by in-plane lengths greater than membrane thickness ($>$ 100~\AA), but only the tilt-dependent model accounts for X-ray data primarily attributable to shorter length correlations. By fitting the measured X-ray scattering intensity, both the bending modulus $K_c$ = 8.3 $\pm$ 0.6 $\times$ $10^{-20}$ J and the tilt modulus $K_\theta$ = 95 $\pm$ 7 mN/m were determined for DOPC bilayers at 30 $^\circ$C. Our experimental results support the enrichment of the classic Helfrich continuum model to include an internal degree of freedom, the fluctuations of lipid directors from the local normal.