PMID: 3897353;Abstract:
Recent NMR relaxation studies of lipid bilayers and biomembranes are explained and briefly discussed. The results of both 2H and 13C NMR investigations suggest that, in addition to rapid local fluctuations of the hydrocarbon chains, slower, more collective motions of the bilayer exist. When the influence of the latter is recognized and properly accounted for, the contribution from local motions can be used to estimate a value for the microviscosity of the bilayer which corresponds to that of a simple n-paraffinic liquid. In general, the dynamic behavior of lipid bilayers as studied by NMR appears quite similar to that of simpler liquid crystals. © 1985.
PMID: 12390036;Abstract:
Quasi-elastic neutron scattering (QENS) was employed to study the molecular dynamics of three structurally related sterols, namely, cholesterol, lanosterol, and ergosterol. Oriented bilayers of dipalmitoylphosphatidylcholine (DPPC) were investigated at 40 mol % sterol content and at three temperatures (20, 36, and 50 °C) for two energy resolutions. Data analysis was concentrated on a direct comparison of the out-of-plane and the in-plane high-frequency motions of the three sterols in terms of their rates and amplitudes. The (spatially restricted) diffusive motion of the three sterols in the two directions was characterized by diffusion constants in the range of (5-30) x 10-12 m2 s-1, with a significantly faster rate of diffusion along the membrane normal, resulting in a diffusional anisotropy, Da. At low temperature (20 °C), cholesterol showed the highest value (Da = 4.5), while lanosterol gave the lowest one (Da = 2.0). At high temperature (50 °C), ergosterol diffusion had the highest diffusion anisotropy (Da = 2.0) compared to lanosterol (Da = 1.8) and cholesterol (Da = 1.6). Most interestingly, cholesterol showed at all three temperatures an amplitude of its out-of-plane-motion of 1.0-1.1 nm, more than a factor of 3 higher than measured for the other two sterols. This finding suggests that the short alkyl chain of the cholesterol molecule may cross at high frequency the bilayer midplane, while the other two sterols remain confined within the geometrical limits of each monolayer leaflet. The results provide an example of how slight structural alterations of sterols can affect their molecular dynamics in bilayers, which in turn may be relevant to the membrane micromechanical properties.
Abstract:
We describe here the design and construction of a modern, state-of-the-art nuclear magnetic resonance (NMR) field-cycling instrument. Fourier transform NMR spectra of both liquid and solid samples can be measured, and spin-lattice relaxation times (T1Z) investigated over a broad range of magnetic field strengths ranging from 0 to 2 T. The instrument is based upon an existing personal computer-based NMR spectrometer [C. Job, R. M. Pearson, and M. F. Brown, Rev. Sci. Instrum. 65, 3354 (1994)] which has been expanded into a fully computer-controlled field-cycling instrument. The magnetic field cycling is accomplished electronically by utilizing fast switching thyristors and a storage capacitor based on the Redfield energy storage concept. Unique aspects of the design include the field-cycling magnet, which can reach fields as high as 2 T; the personal computer-based NMR spectrometer and associated waveform electronics; and the use of a commercially available pulse width modulation switching current amplifier, having low internal power dissipation and a fast current settling time. Using this new technology T1Z relaxation times as short as 1 ms can be readily measured. © 1996 American Institute of Physics.
PMID: 16131204;Abstract:
The human N-ras protein binds to cellular membranes by insertion of two covalently bound posttranslational lipid modifications, which is crucial for its function in signal transduction and cell proliferation. Mutations in ras may lead to unregulated cell growth and eventually cancer, making it an important therapeutic target. Here we have investigated the molecular details of the membrane binding mechanism. A heptapeptide derived from the C-terminus of the human N-ras protein was synthesized including two hexadecyl modifications. Solid-state 2H NMR was used to determine the packing and molecular dynamics of the ras lipid chains as well as the phospholipid matrix. Separately labeling the chains of the peptide and the phospholipids with 2H enabled us to obtain atomically resolved parameters relevant to their structural dynamics. While the presence of ras only marginally affected the packing of DMPC membranes, dramatically lower order parameters (SCD) were observed for the ras acyl chains indicating modified packing properties. Essentially identical projected lengths of the 16:0 ras chains and the 14:0 DMPC chains were found, implying that the polypeptide backbone is located at the lipid-water interface. Dynamical properties of both the ras and phospholipid chains were determined from spin-lattice 2H relaxation (R 1Z) measurements. Plots of R1Z rates versus the corresponding squared segmental order parameters revealed striking differences. We propose the ras peptide is confined to microdomains containing DMPC chains which are in exchange with the bulk bilayer on the 2H NMR time scale (∼10-5 s). Compared to the host DMPC matrix, the ras lipid modifications are extremely flexible and undergo relatively large amplitude motions. It is hypothesized that this flexibility is a requirement for the optimal anchoring of lipid-modified proteins to cellular membranes. © 2005 American Chemical Society.
