Slowing down in lipid bilayers: domain structure fluctuations and axial diffusion

Abstract

Ultrasonic spectra (2×10⁵Hz to 2×10⁹ Hz) and complex dielectric spectra (10⁴ Hz to 4×10¹⁰ Hz) are reported for aqueous solutions of vesicles from lipid mixtures near their main phase transition temperature T_m. The results are compared to such for 1,2-dimyristoyl-L-3-phosphatidylcholine systems. Various relaxation terms are identified. One term results from the domain structure fluctuations of the membranes. It features critical point characteristics. Another term is due to the axial diffusion of the lipid molecules within the membrane. It also reveals substantial effects of slowing near T_m. Relaxation times reflecting the zwitterionic head group reorientational motions and the alkyl chain structural isomerisation display step like changes at the phase transition. An additional relaxation term exists close to T_m only. It is assumed to indicate small-area deformations of the vesicles.