Lipid bilayer thickness effects on spectroscopic measurements

Lipid bilayer membranes are biological structures that bound cells and organelles and are vital to several cellular processes, such as endocytosis and action potential propagation. Therefore, it is crucial to understand how the material properties of membranes govern their dynamics across multiple lengthscales. Dynamics on the order of membrane thickness are commonly described by phenomenological models and are attributed to competition between bending relaxation and intermonolayer friction. In this work, we provide a theory that explicitly accounts for finite membrane thickness effects, and we show that the linear response at these lengthscales is dominated by a non-equilibrium shear mechanism different from the commonly assumed intermonolayer friction. Using these insights, we calculate the dynamic structure factor and compare it to the relaxation spectra of lipid bilayer membranes obtained through Neutron Spin Echo spectroscopy. We discuss the effects of the inclusion of membrane thickness on the interpretation of the observed relaxation dynamics and the necessity of phenomenology.