Packing and Viscoelasticity of Polyunsaturated ω-3 and ω-6 Lipid Bilayers as Seen by ²H NMR and X-ray Diffraction

Polyunsaturated phospholipids of the ω-3 and ω-6 classes play key roles in cellular functions, yet their mechanisms of biological action are still a matter of debate. Using deuterium (²H) NMR spectroscopy and small-angle X-ray diffraction, we show how membrane properties are modified by docosahexaenoic (DHA; 22:6) and arachidonic (AA; 20:4) acyl chains of the ω-3 and the ω-6 families, respectively. Structural and dynamical differences due to polyunsaturation are evident in both the ordered and disordered phases of mixed-chain (16:0)(22:6)PC and (16:0)(20:4)PC bilayers. Due to the lower chain melting temperature, the ω-6 AA bilayer is more disordered in the fluid (L_α) state than the ω-3 DHA bilayer; it is thinner with a larger area per lipid. The thermal hysteresis observed for the DHA bilayer may represent the influences of angle-iron conformers in the gel state and back-bended, hairpinlike conformers in the fluid state, consistent with molecular dynamics studies. Interpretation of the ²H NMR order profiles of (16:0-d₃₁)(22:6)PC and (16:0-d₃₁)(20:4)PC together with X-ray electron density profiles reveals an uneven distribution of mass; i.e., the sn-1 saturated chain is displaced toward the membrane center, whereas the sn-2 polyunsaturated chain is shifted toward the bilayer aqueous interface. Moreover, the ²H NMR relaxation rates are increased by the presence of ω-6 AA chains compared to ω-3 DHA chains. When evaluated at the same amplitude of motion, relaxation parameters give a naturally calibrated scale for comparison of fluid lipid bilayers. Within this framework, polyunsaturated bilayers are relatively soft to bending and area fluctuations on the mesoscale approaching molecular dimensions. Significant differences are evident in the viscoelastic properties of the ω-3 and ω-6 bilayers, a possibly biologically relevant feature that distinguishes between the two phospholipid classes.