Unified picture for spin-lattice relaxation of lipid bilayers and biomembranes

The present study compares and interprets the ¹H, ²H, and ¹³C spin-lattice (T₁) relaxation times of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), in the liquid crystalline phase, in terms of models for the molecular dynamics of lipid bilayers. The ¹H T₁ times of the DPPC bilayer hydrocarbon region at two frequencies and ¹³C T₁ data at seven frequencies, for which the relaxation is dipolar in origin, as well as the ²H T ₁ data at three frequencies, due to the quadrupolar interaction, can be unified and interpreted in terms of a collective model for order fluctuations. In normalizing the ¹³C T₁ data to the ¹H and ²H T₁ values, a vibrationally corrected ¹³C-¹H distance parameter of r_CH ⁰=1.14 Å has been assumed, rather than the equilibrium bond length of 1.09 Å. The analysis suggests that the behavior of the individual acyl chain segments of lipid bilayers, in the liquid crystalline phase, is similar to that of molecules in nematic fluids.