PMID: 19917223;PMCID: PMC2776299;Abstract:
Binary mixtures of C20BAS and POPC membranes were studied by solid-state 2H NMR spectroscopy and small angle x-ray scattering (SAXS) over a wide range of concentrations and at different temperatures. Three specifically deuterated C20BAS derivatives-[1′,1′, 20′,20′-2H4]C20BAS, [2′,2′,19′,19′-2H4]C 20BAS, and [10′,11′-2H2]C 20BAS - combined with protiated 1-palmitoyl-2-oleoyl-sn-glycero-3- phosphocholine (POPC), as well as membranes containing POPC-d31 and fully protiated bolalipid, were used in NMR experiments to obtain structural information for the mixtures. The 2H NMR spectra of [10′,11′-2H2]C20BAS/POPC membrane dispersions reveal that the bolalipid is predominantly in the transmembrane conformation at high bolalipid concentrations (100, 90, and 70 mol %). At ≤50 mol % C20BAS, smaller quadrupolar couplings appear in the spectra, indicating the presence of U-shaped conformers. The proportion of U-shaped bolalipids increases as the amount of POPC in the membrane increases; however, the transmembrane component remains the dominant bolalipid conformation in the membrane even at 45°C and 10 mol % C20BAS, where it accounts for ∼50% of the bolalipid population. The large fraction of C20BAS transmembrane conformers, regardless of the C20BAS/POPC ratio, together with the findings from molecular mean-field theory calculations, suggests the coexistence of phase-separated bolalipid-rich domains and POPC-rich domains. A single lamellar repeat distance was observed in SAXS experiments corresponding to the average repeat spacing expected for C20BAS-and POPC-rich domains. These observations are consistent with the presence of microphase-separated domains in the mixed membrane samples that arise from POPC-C20BAS hydrophobic mismatch. © 2009 by the Biophysical Society.
Photoisomerization of the retinylidene chromophore of rhodopsin is the starting point in the vision cascade. A counterion switch mechanism that stabilizes the retinal protonated Schiff base (PSB) has been proposed to be an essential step in rhodopsin activation. On the basis of vibrational and UV-visible spectroscopy, two counterion switch models have emerged. In the first model, the PSB is stabilized by Glu181 in the meta I state, while in the most recent proposal, it is stabilized by Glu113 as well as Glu181. We assess these models by conducting a pair of microsecond scale, all-atom molecular dynamics simulations of rhodopsin embedded in a 99-lipid bilayer of SDPC, SDPE, and cholesterol (2:2:1 ratio) varying the starting protonation state of Glu181. Theoretical simulations gave different orientations of retinal for the two counterion switch mechanisms, which were used to simulate experimental 2H NMR spectra for the C5, C9, and C13 methyl groups. Comparison of the simulated 2H NMR spectra with experimental data supports the complex-counterion mechanism. Hence, our results indicate that Glu113 and Glu181 stabilize the retinal PSB in the meta I state prior to activation of rhodopsin.
PMID: 6576340;PMCID: PMC384030;Abstract:
Natural abundance 13C spin-lattice (T1) relaxation time measurements are reported for unilamellar vesicles of 1,2-dipalmitoylphosphatidylcholine (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), in the liquid crystalline phase, at magnetic field strengths of 1.40, 1.87, 2.35, 4.23, 7.05, 8.45, and 11.7 tesla (resonance frequencies of 15.0, 20.0, 25.1, 45.3, 75.5, 90.5, and 126 MHz, respectively), and the results are compared to previous 2H T1 studies of multilamellar dispersions. For both the 13C and 2H T1 studies, a dramatic frequency dependence of the relaxation was observed. At superconducting magnetic field strengths (4.23-11.7 tesla), plots of the 13C T1-1 relaxation rates as a function of acyl chain segment position clearly reveal the characteristic 'plateau' signature of the liquid crystalline phase, as found previously from 2H NMR studies. The dependence of T1-1 on ordering, determined previously from 2H NMR, and the T1-1 dependence on frequency, determined from both 13C and 2H NMR studies, suggest that a unified picture of the bilayer molecular dynamics can be provided by a simple relaxation law of the form T1-1 ≃ Aτf + BS2 (C-H) ω0(- 1/2 ). In the above expression, A and B are constants, S(C-H)(=S(C-D) is the bond segmental order parameter, and ω0 is the nuclear Larmor frequency. The first (A) term includes contributions from fast, local segmental motions characterized by the effective correlation time τf, whereas the second (B) term describes slower, collective fluctuations in the local ordering. The value of τf ≃ 10-11 sec, obtained by extrapolating T1-1 to infinite frequency, suggests that the segmental microviscosity of the bilayer hydrocarbon region does not differ appreciably from that of the equivalent n-paraffinic liquids of similar chain length.
PMID: 12105929;Abstract:
In deuterium (2H) NMR spectroscopy of fluid lipid bilayers, the average structure is manifested in the segmental order parameters (SCD) of the flexible molecules. The corresponding spin-lattice relaxation rates (R1Z) depend on both the amplitudes and the rates of the segmental fluctuations, and indicate the types of lipid motions. By combining 2H NMR order parameter measurements with relaxation studies, we have obtained a more comprehensive picture of lipids in the liquid-crystalline (Lα) state than formerly possible. Our data suggest that a lipid bilayer constitutes an ordered fluid, in which the phospholipids are grafted to the aqueous interface via their polar headgroups, whereas the fatty acyl chains are in effect liquid hydrocarbon. Studies of 2H-labeled saturated lipids indicate their R1Z rates and SCD order parameters are correlated by a model-free, square-law functional dependence, signifying the presence of relatively slow bilayer fluctuations. A new composite membrane deformation model explains simultaneously the frequency (magnetic field) dependence and the angular anisotropy of the relaxation. The results imply the R1Z rates are due to a broad spectrum of 3-D collective bilayer excitations, together with effective axial rotations of the lipids. For the first time, NMR relaxation studies show that the viscoelastic properties of membrane lipids at megahertz frequencies are modulated by the lipid acyl length (bilayer thickness), polar headgroups (bilayer interfacial area), inclusion of a nonionic detergent (C12E8), and the presence of cholesterol, leading to a range of bilayer softness. Our findings imply the concept of elastic deformation is relevant on lengths approaching the bilayer thickness and less (the mesoscopic scale), and suggest that application of combined R12 and SCD studies of phospholipids can be used as a simple membrane elastometer. Heuristic estimates of the bilayer bending rigidity κ and the area elastic modulus Ka enable comparison to other biophysical studies, involving macroscopic deformation of thin membrane lipid films. Finally, the bilayer softness may be correlated with the lipid diversity of biomembranes, for example, with regard to membrane curvature, repulsive interactions between bilayers, and lipid-protein interactions.
