Abstract:
Deuterium NMR spectroscopy is widely applicable to studies of the structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids. The properties of soft nanomaterials are also accessible on the mesoscopic length scale intermediate between the molecular and bulk dimensions. For membrane lipids in the liquid-crystalline state, rapid axial averaging occurs about the director axis (the membrane normal). One can then relate the profiles of the order parameters |(CD)| of the individual C-2H labeled segments to average bilayer properties. These include the mean area per molecule and projected acyl chain length, the area compressibility modulus, and the radius of curvature for reverse hexagonal (H(II)) phase nanotubes. In addition, measurements of the relaxation rates for Zeeman order, R(1Z), and quadrupolar order, R(1Q), enable one to investigate the mean-squared amplitudes and time-scales of the fluctuations that underlie the thermodynamic properties. A unified interpretation is provided by a composite membrane deformation model, which fits simultaneously the frequency dependence and the angular anisotropy of the R(1Z) and R(1Q) relaxation rates. The results suggest the bilayer dynamics in the MHz regime can be modeled in terms of nematic-like deformations of the membrane hydrocarbon interior, together with axial rotations of the lipid acyl chains. A small contribution from internal segmental motions is found, which implies the bilayer microviscosity is comparable to that of a liquid hydrocarbon. Finally, the 2H-NMR relaxation rates of lipid bilayers containing cholesterol in the liquid-ordered phase suggest a dynamically more rigid bilayer, involving fast axial lipid rotations together with a reduction in collective bilayer deformations. Possible future applications include studies of liquid crystals and thin films of membrane lipids and surfactants, as well as lipid-protein systems.
PMID: 1775058;Abstract:
NMR images of subintimal lipid deposits within the vessel walls of atherosclerotic human aortas were obtained at 37 and 27°C at 4.7 T. A combination of a stimulated-echo and pulsed-field gradients was used for suppressing the mobile tissue water relative to the less mobile tissue lipids. At 27°C there was also a substantial reduction of the subintimal lipid signal intensity, which is consistent with the characteristic phase transition of cholesteryl esters in human atheroma. These results represent the first direct detection of lipid deposits in nonprotruding atherosclerotic lesions with NMR imaging.
PMID: 8804611;PMCID: PMC1233479;Abstract:
Surface plasmon resonance (SPR) spectroscopy can provide useful information regarding average structural properties of membrane films supported on planar solid substrates. Here we have used SPR spectroscopy for the first time to monitor the binding and activation of G-protein (transducin or G(t)) by bovine rhodopsin incorporated into an egg phosphatidylcholine bilayer deposited on a silver film. Rhodopsin incorporation into the membrane, performed by dilution of a detergent solution of the protein, proceeds in a saturable manner. Before photolysis, the SPR data show that G(t) binds tightly (K(eq) ≃ 60 nM) and with positive cooperativity to rhodopsin in the lipid layer to form a closely packed film. A simple multilayer model yields a calculated average thickness of about 57 .Å, in good agreement with the structure of G(t). The data also demonstrate that G(t) binding saturates at a G(t)/rhodopsin ratio of approximately 0.6. Moreover, upon visible light irradiation, characteristic changes occur in the SPR spectrum, which can be modeled by a 6 Å increase in the average thickness of the lipid/protein film caused by formation of metarhodopsin II (MII). Upon subsequent addition of GTP, further SPR spectral changes are induced. These are interpreted as resulting from dissociation of the α- subunit of G(t), formation of new MII-G(t) complexes, and possible conformational changes of G(t) as a consequence of complex formation. The above results clearly demonstrate the ability of SPR spectroscopy to monitor interactions among the proteins associated with signal transduction in membrane-bound systems.
