Separated Local Field 13C NMR Spectroscopy Reveals Lipid Order Fluctuations

Abstract

Solid-state NMR and small angle X-ray scattering (SAXS) are currently among the most widely used experimental methods for investigating structural and dynamic properties of phospholipid bilayers [1]. However, SAXS measurements provide information about membrane dynamics only at very high-resolution, whereas 2H NMR has a prerequisite of isotopic enrichment of the phospholipid. An alternative is magic-angle spinning (MAS) solid-state 13C NMR spectroscopy, where direct magnetic-dipolar couplings and nuclear spin-lattice relaxation times are measured at natural isotopic abundance [2]. This method provides structure and dynamics for the headgroup, glycerol backbone, and acyl chains in one experiment. As proof of principle, we measured the magnetic-dipolar couplings and nuclear spin-lattice relaxation rates in the liquid-crystalline state for the homologous series of phosphatidylcholine lipids (DLPC, DMPC, DPPC). The quasi-static dipolar lineshapes exhibit segmental order parameters that can be compared with previously measured 2H couplings. In terms of dynamics, correlation of the effective segmental order parameters with the nuclear spin-lattice relaxation rates shows that the square-law scaling for the 13C and 2H systems is universal. Moreover, 13C measurements provide valuable information about the highly dynamic headgroup region that is not commonly observed in 2H NMR experiments. Combination of solid-state 2H and 13C NMR methodologies provides greater insight into the structure and dynamics of membranes than would be determined from either technique alone [3]. This study establishes further precedent for the use of solid-state 13C NMR for research into complex biological membranes and biomaterials at natural isotopic abundance. [1] H.I. Petrache and M.F. Brown (2007) Methods in Membrane Lipids, Humana Press, 339-351. [2] J.D. Gross et al. (1997) JACS119, 796-802. [3] M.F. Brown and S.I. Chan, Encyclopedia of Nuclear Magnetic Resonance, Wiley, New York 1996, 871-885.