In-plane and out-of-plane gigahertz sound velocities of saturated and unsaturated phospholipid bilayers from cryogenic to room temperatures

Here, we examined the gigahertz sound velocities of hydrated multibilayers of saturated (1,2-dimyristoyl-snglycero-3-phosphocholine, DMPC) and unsaturated (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) phospholipids by Brillouin spectroscopy. Out-of-plane and in-plane (lateral) phonons were studied independently of each other. Similar strong temperature dependences of the sound velocities were found for phonons of both types. The sound velocities in the low-temperature limit were two-fold higher than that at physiological temperatures; a significant part of the changes in sound velocity occurs in the solid-like gel phase. The factors that may be involved in the peculiar behavior of sound velocity include changes in the chain conformational state, relaxation susceptibility, changes in the elastic modulus at infinite frequencies, and lateral packing of molecules.

Automated summary

We used Brillouin spectroscopy to examine the gigahertz sound velocities of hydrated multibilayers of saturated and unsaturated phospholipids. Both out-of-plane and in-plane phonons were studied independently and showed similar temperature dependences. The sound velocities at low temperatures were twice as high as physiological temperatures, with a significant portion of the velocity changes occurring in the solid-like gel phase. Factors such as changes in chain conformational state, relaxation susceptibility, elastic modulus at infinite frequencies, and lateral packing of molecules may contribute to the peculiar behavior of sound velocity.