Much interest has been focussed recently on sphingomyelin as an essential component of a variety of biological membranes. Using solid-state 2H NMR spectroscopy, we investigated the micromechanical effect of varying concentrations of cholesterol in ternary mixtures composed of N-palmitoylsphingomyelin (PSM), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and cholesterol in unoriented multilamellar bilayers. The hydrocarbon chains of PSM or POPC were 2H labeled which enabled us to investigate the distribution and the order profiles of the individual lipid components in the mixtures [1]. A mean torque potential model [2] was employed to characterize the structural properties and map the existence of lipid domains in these mixtures. By calculating the average hydrocarbon thickness, area per lipid, and structural parameters such as chain extension and thermal expansion coefficients, we were able to further characterize the structural properties of these domains. We then measured R1Z relaxation rates, which in combination with order parameter profiles gave a signature square-law dependence corresponding to the mechanical properties of the respective lipid membranes on a mesoscopic length scale [3]. The slope of the square-law plots of relaxation rates and order parameter were found to decrease progressively with the mole fraction of cholesterol, due to a stiffening of the membrane. Different membrane domains thus gave distinctively different micromechanical signatures which indicated that the modes contributing to R1Z relaxation rates are on a length scale comparable to the lipid domain size.[1] Bartels, T. et al (2008) J. Am. Chem. Soc., in press.[2] Petrache, H.I. et al (2000) Biophys. J. 79, 3172-3192.[3] Brown, M.F. et al (2002) J. Am. Chem. Soc. 124, 8471-8484.