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Distinguishing Individual Lipid Headgroup Mobility and Phase Transitions in Raft-Forming Lipid Mixtures with ³¹P MAS NMR

Department of Electronic and Nanostructured Materials, Sandia National Laboratories, Albuquerque, New Mexico 87185-0886

Correspondence: Address reprint requests to Todd M. Alam, E-mail: tmalam{at}sandia.gov.

A model membrane system composed of egg sphingomyelin (SM), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and cholesterol was studied with static and magic angle spinning ³¹P NMR spectroscopy. This model membrane system is of significant biological relevance since it is known to form lipid rafts. ³¹P NMR under magic angle spinning conditions resolves the SM and DOPC headgroup resonances allowing for extraction of the ³¹P NMR parameters for the individual lipid components. The isotropic chemical shift, chemical shift anisotropy, and asymmetry parameter can be extracted from the spinning side band manifold of the individual components that form liquid-ordered and liquid-disordered domains. The magnitude of the ³¹P chemical shift anisotropy and the line width is used to determine headgroup mobility and monitor the gel-to-gel and gel-to-liquid crystalline phase transitions of SM as a function of temperature in these mixtures. Spin-spin relaxation measurements are in agreement with the line width results, reflecting mobility differences and some heterogeneities. It will be shown that the presence of DOPC and/or cholesterol greatly impacts the headgroup mobility of SM both above and below the liquid crystalline phase transition temperature, whereas DOPC displays only minor variations in these lipid mixtures.