Cannon, Barbara, and Jan Nedergaard. Brown Adipose Tissue: Function and Physiological Significance. Physiol Rev 84: 277–359, 2004; 10.1152/physrev.00015.2003.—The function of brown adipose tissue i...

Brown Adipose Tissue: Function and Physiological Significance

Membranes are the most common cellular structures in both plants and animals. They are now recognized as being involved in almost all aspects of cellular activity ranging from motility and food entrapment in simple unicellular organisms, to energy transduction, immunorecognition, nerve conduction and biosynthesis in plants and higher organisms. This functional diversity is reflected in the wide variety of lipids and particularly of proteins that compose different membranes. An understanding of the physical principles that govern the molecular organization of membranes is essential for an understanding of their physiological roles since structure and function are much more interdependent in membranes than in, say, simple chemical reactions in solution. We must recognize, however, that the word ‘understanding’ means different things in different disciplines, and nowhere is this more apparent than in this multidisciplinary area where biology, chemistry and physics meet.

/pdf/physical-principles-of-membrane-organization-2enzb9mlqb.pdf

Physical principles of membrane organization.

The function of sterols in membranes.

The NMR free induction decay from a spinning sample having inhomogeneous anisotropic interactions (chemical shifts, first order quadrupole couplings) takes the form of a train of rotational spin echoes. The Fourier transform of the echo envelope is a sharp spectrum from which the effects of anisotropy have been removed. The Fourier transform of the echo shape contains information concerning the anisotropies: This information can be extracted by a moment analysis. The effects of localized homonuclear spin–spin interactions are to convert the ’’isotropic’’ spectrum into a characteristic powder pattern. Second order quadrupole coupling produces a similar effect. It is shown in this case that the usual second‐order level shifts cannot be used to calculated this pattern, which must be described by a proper average Hamiltonian theory. Finally it is shown that rotational spin echoes provide a convenient means of studying very slow random molecular rotations (τc≲1 sec).

NMR in rotating solids

Proton and carbon-13 nmr spectra of unsonicated lipid bilayers and biological membranes are generally dominated by strong proton–proton and proton–carbon dipolar interactions. As a result the spectra contain a large number of overlapping resonances and are rather difficult to analyse. Nevertheless, important information on the structure and dynamic behaviour of lipid systems has been provided by these techniques (Wennerstrom & Lindblom, 1977).

/pdf/deuterium-magnetic-resonance-theory-and-application-to-lipid-3a016gcplt.pdf

Deuterium magnetic resonance: theory and application to lipid membranes.

Electron spin resonance line shapes and saturation in the slow motional region

Spin labels in membranes. Problems in practice.

A careful study is described of the ESR lineshapes for the peroxylamine disulfonate (PADS) radical dissolved in 85% glycerol solution and in frozen water and D2O. In the frozen media, spectra characteristic of rotational correlation times τR ranging from 1.0 × 10−11sec to > 10−6sec are obtained, while the range in glycerol is from 1 × 10−10sec to >10−6sec. The very rapid rotational motion in frozen water is taken to imply that PADS is rotating in a clathrate cage. The activation energies in ice and 85% glycerol are 14.7 ± 0.1 and 11.3 ± 0.1 kcal/mole, respectively, (from motional‐narrowing data). The value for ice is very similar to that obtained for other rate processes in ice. The lineshapes for τR ≲ 10−9sec are analyzed in terms of the familiar spin‐relaxation theories valid in the motionally narrowed region. These results are well fitted by the model of axially symmetric rotational diffusion with the symmetry axis in the plane of the N, O, and S atoms and parallel to a line passing through the two S a...

An ESR Study of Anisotropic Rotational Reorientation and Slow Tumbling in Liquid and Frozen Media

Deuterium (2H) nuclear magnetic resonance (NMR) quadrupole splittings and relaxation times have been measured for a variety of specifically deuterated lipids intercalated in lamellar-multibilayer dispersions and single-bilayer vesicles of egg lecithin and lecithin-cholesterol mixtures. The deduced order parameters and relaxation times vary with position of deuteration, acyl chain length, unsaturation, and temperature. The order parameters and spinlattice relaxation times T1 indicate rapid intramolecular motions of restricted amplitude in both the choline head group and hydrocarbon chains. The ordering profile for the acyl chains is similar to that predicted by statistical-mechanical theory. The order parameters yield estimates of the bilayer thickness and linear coefficient of expansion in close agreement with the x-ray determinations. A comparison of the deuterium and electron spin resonance spinprobe order parameters demonstrates the perturbation of the bilayer by the bulky nitroxide probe. The transverse relaxation time T2 for single-bilayer vesicles is quantitatively accounted for by a simple modification of classical relaxation theory which takes into account the modulation of the static quadrupole interaction by rapid local molecular motions and the modulation of the residual quadrupole interaction by the slower overall tumbling of the vesicle. It is unambiguously demonstrated that molecular motion and order in single-bilayer vesicles are very similar to those in lamellar multibilayers. Significant differences occur only for a few segments near the terminal methyl groups of the acyl chains, where the order parameters for vesicles are 10-30% smaller than those found for lamellae. The incorporation of cholesterol in lecithin bilayers is shown to increase the degree of orientational order in vesicles and lamellae, and to increase the hydrodynamic radius of vesicles. Thus, single-bilayer vesicles and multilamellar dispersions of phospholipids are equally useful models for biological membranes. They yield equivalent information about the internal organization and mobility of lipid bilayers, when the spectral manifestations of overall vesicle motion are correctly taken into account.

Molecular motion and order in single-bilayer vesicles and multilamellar dispersions of egg lecithin and lecithin-cholesterol mixtures. A deuterium nuclear magnetic resonance study of specifically labeled lipids.

Carl F. Polnaszek

Papers

Electron spin resonance line shapes and saturation in the slow motional region

Spin labels in membranes. Problems in practice.

An ESR Study of Anisotropic Rotational Reorientation and Slow Tumbling in Liquid and Frozen Media

Molecular motion and order in single-bilayer vesicles and multilamellar dispersions of egg lecithin and lecithin-cholesterol mixtures. A deuterium nuclear magnetic resonance study of specifically labeled lipids.

Electron spin resonance studies of anisotropic ordering, spin relaxation, and slow tumbling in liquid crystalline solvents. 3. Smectics