Propagating Subterahertz Longitudinal Phonon-like Modes in a Lipid Bilayer in Aqueous Medium

Abstract

It has been revealed recently that the subterahertz/terahertz vibrational motions in enzymes and DNA immersed in aqueous solutions can be underdamped. Importantly, these motions are associated with coherent delocalized modes that control functional processes. Analogous propagating phonon-like modes have been found in free hydrated lipid bilayers. In the present work, subterahertz (frequencies of the order of tens and hundreds of gigahertz) longitudinal acoustic oscillations in a bilayer lipid membrane immersed in aqueous medium are investigated theoretically. We consider driven oscillations excited by tangential mechanical tensions at the bilayer surfaces and thermally induced phonon modes. The analysis is based on: (i) a generalized hydrodynamic model of two-dimensional lipid bilayer in aqueous medium; (ii) known estimates of frequencies and lifetimes of longitudinal acoustic phonons in free hydrated lipid bilayer and in water, which were obtained in the experiments on non-elastic X-ray scattering and the molecular dynamics simulations. We show that the membrane phonon-like excitations are underdamped for the typical values of the system parameters, and the contribution of aqueous medium to the membrane mode damping is small compared to the contribution of the lipid bilayer. The obtained results suggest the possibility of realization of thermally induced longitudinal membrane phonons in physiological conditions, as well as the possibility of resonance amplification of the impact of subnanosecond electric impulses and impulses of subterahertz electromagnetic radiation on membrane dynamics.