Structural dynamics in phospholipid bilayers from deuterium spin–lattice relaxation time measurements

Abstract

The quadrupolar spin–lattice (T₁) relaxation of deuterium labeled phospholipid bilayers has been investigated at a resonance frequency of 54.4 MHz. T₁ measurements are reported for multilamellar dispersions, single bilayer vesicles, and chloroform/methanol solutions of 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC), selectively deuterated at ten different positions in each of the fatty acyl chains and at the sn‐3 carbon of the glycerol backbone. At all segment positions investigated, the T₁ relaxation times of the multilamellar and vesicle samples of DPPC were found to be similar. The profiles of the spin–lattice relaxation rate (1/T₁) as a function of the deuterated chain segment position resemble the previously determined order profiles [A. Seelig and J. Seelig, Biochem. 13, 4839 (1974)]. In particular, the relaxation rates are approximately constant over the first part of the fatty acyl chains (carbon segments C3–C9), then decreasing in the central region of the bilayer. In chloroform/methanol solution, by contrast, the relaxation rates decrease continuously from the glycerol backbone region to the chain terminal methyl groups. The contributions from molecular order and motion to the T₁ relaxation rates have been evaluated and correlation time profiles derived as a function of chain position. The results suggest that the motions of the various methylene segments are correlated in the first part of the fatty acyl chains (C3–C9), occurring at frequencies up to 1/τ_c?10¹⁰ Hz. Beyond C9, the rate and amplitude of the chain segmental motions increase, approaching that of simple paraffinic liquids in the central region of the bilayer (1/τ_c?10¹¹ Hz). The T₁ relaxation rates of multilamellar dispersions of 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine (DOPC) deuterated at the 9, 10 double bond of the sn‐2 chain were also determined and found to be significantly faster than those of the CD₂ chain segments of DPPC bilayers. This is most likely due to the larger size and correspondingly slower motion of the chain segment containing the double bond. At segments close to the lipid–water interface the rate of motion is considerably less than in the hydrocarbon region of the bilayer.