Coherent collective dynamics in phospholipid membranes: where are we now?

We review the main achievements accomplished in the study of the coherent collective dynamics of phospholipid membranes, i.e., the collective vibrational excitations active on the sub-picoseconds to picoseconds time scale and terahertz frequencies, over a length scale comparable to the angstrom length scale. The first descriptions of the terahertz collective spectrum, mainly assigned to a single longitudinal acoustic-like mode similarly to simple liquids, have been progressively replaced by a complex scenario, with longitudinal and transverse acoustic-like and optic-like inelastic excitations. Complex phenomena (avoided crossing mechanisms between modes, a phonon gap introduced by the disorder and a slow- to fast-sound transition) have been highlighted in the dispersion curves of lipid bilayers. Systematic and comprehensive studies along the lines of the investigations reviewed here contribute to understand the role of the collective (coherent) chain dynamics in the passive transport across bilayers, in the formation of water wires inside dynamic defects in lipid bilayers and in other biological functions of biomembranes.