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

Abstract

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 .**GRAPHIC**. 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 .**GRAPHIC**. on ordering, determined previously from 2H NMR, and the .**GRAPHIC**. 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 .**GRAPHIC**. .apprxeq. A .tau.f + .**GRAPHIC**. .**GRAPHIC**. In the expression, A and B are constants, SC-H (= SC-D) is the bond segmental order parameter, and .omega.0 is the nuclear Larmor frequency. The 1st (A) term includes contributions from fast, local segmental motions characterized by the effective correlation time .tau.f, whereas the 2nd (B) term describes slower, collective fluctuations in the local ordering. The value of .tau.f .apprxeq. 10-11 s, obtained by extrapolating .**GRAPHIC**. 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.