New view of lipid bilayer dynamics from ²H and ¹³C NMR relaxation time measurements

Natural abundance ¹³C spin-lattice (T₁) relaxation time measurements are reported for unilamellar vesicles of 1,2-dipalmitoylphosphatidylcholine (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), in the liquid crystalline phase, at magnetic field strengths of 1.40, 1.87, 2.35, 4.23, 7.05, 8.45, and 11.7 tesla (resonance frequencies of 15.0, 20.0, 25.1, 45.3, 75.5, 90.5, and 126 MHz, respectively), and the results are compared to previous ²H T₁ studies of multilamellar dispersions. For both the ¹³C and ²H T₁ studies, a dramatic frequency dependence of the relaxation was observed. At superconducting magnetic field strengths (4.23-11.7 tesla), plots of the ¹³C T₁^-1 relaxation rates as a function of acyl chain segment position clearly reveal the characteristic 'plateau' signature of the liquid crystalline phase, as found previously from ²H NMR studies. The dependence of T₁^-1 on ordering, determined previously from ²H NMR, and the T₁^-1 dependence on frequency, determined from both ¹³C and ²H NMR studies, suggest that a unified picture of the bilayer molecular dynamics can be provided by a simple relaxation law of the form T₁^-1 ≃ Aτf + BS² (C-H) ω0(- 1/2 ). In the above expression, A and B are constants, S(C-H)(=S(C-D) is the bond segmental order parameter, and ω₀ is the nuclear Larmor frequency. The first (A) term includes contributions from fast, local segmental motions characterized by the effective correlation time τf, whereas the second (B) term describes slower, collective fluctuations in the local ordering. The value of τf ≃ 10^-11 sec, obtained by extrapolating T₁^-1 to infinite frequency, suggests that the segmental microviscosity of the bilayer hydrocarbon region does not differ appreciably from that of the equivalent n-paraffinic liquids of similar chain length.