Deuterium relaxation and molecular dynamics in lipid bilayers

The quadrupolar relaxation of deuterium-labeled lipid bilayers has been analyzed using standard Redfield theory and is discussed with regard to the problem of chain segmental motion and order in membranes. Considering the segmental reorientation as a stochastic process, the T₁ and T₂ relaxation rates are interpreted in terms of the rate of motion, characterized by one or more correlation times τ_2M, and statistical amplitude, characterized by the segmental order parameter S_CD. For the case of phospholipid bilayers with |S_CD| ≲ 0.2, the relaxation rates are predominantly determined by the rate of motion, rather than the ordering. Recently obtained T, relaxation data for selectively deuterated and perdeuterated multilamellar dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine are analyzed and compared to the results of previous carbon-13 T₁ relaxation studies. The available experimental results suggest that the fast segmental motions affecting T₁ in these systems can be treated to a reasonable degree of approximation in terms of a single effective correlation time.