In vivo dynamics and phase state of natural lipid droplets

All organisms store lipids as energy resource for metabolism. Such lipids are accumulated in the form of intracellular droplets. The lipid droplets (LDs) contain un- and saturated triglycerides. Uncovering biophysics of LDs is crucial for metabolism manipulation or reducing lipid storage. The type and organization of lipids affect their phase state and dynamics in the droplet. We studied LDs in fresh human and porcine subcutaneous fat tissues, as well as yeast cells using quasi-elastic neutron scattering, probing molecular motions in a time scale of 6-400 ps and a length scale of 3-20 A. The detected two-component dynamics in the droplet is associated with lipid unrestricted diffusion (D~0.006 A²/ps) and motions of its hydrocarbon chains (D~0.2 A²/ps) in a restricted volume (5-12 A). The dynamics of lipids is reduced below 305 K and 266 K in porcine and human tissues, respectively, due to a fluid-gel phase transition of lipids. However, LDs in the yeast cells remain in a fluid-like state within range of 280-310 K. We believe, phase behavior of LDs is different in the tissues and microorganism due to lipid composition. Lipid packing in the droplet is tight in the tissues, having saturated lipids, and loose in the yeast droplet, having equal proportions of un- and saturated lipids.