Fig. 1.
Energy landscapes and mobilities of phospholipids in membranes entail characteristic timescales. a Fluctuations of 13C–1H or C–2H bonds are given by ΩID Euler angles for internal segmental frame (I) with respect to the membrane director axis (D). Phospholipid molecular motions entail anisotropic reorientation of the molecule-fixed frame (M) with respect to the membrane director axis (D) as described by ΩMD Euler angles. The liquid-crystalline bilayer lends itself to propagation of thermally excited, quasi-periodic fluctuations due to motion of the local membrane normal (N) relative to the average director axis (D) characterized by ΩND Euler angles. Examples are shown respectively for glycerophospholipids, b POPC, c DOPC, and d DMPC where the lipid headgroups are highlighted in brown; e glycolipids where R denotes the rest of the lipid molecule; f cholesterol which is the sterol component of animal biomembranes; and g lanosterol which is a precursor in the sterol biosynthesis pathway