Spectral Anisotropy Research Articles

Scintillation at two optical frequencies

Stellar scintillation data were obtained on a single night at a variety of zenith distances and azimuths, using a photon-counting photometer recording at 100 Hz simultaneously at wavelengths of 0.475 microm and 0.870 microm. Orientable apertures of 42-cm diam separated by 1 m were used to establish the average upper atmosphere wind direction and velocity. Dispersion in the earth's atmosphere separate the average optical paths at the two wavelengths, permitting a reconstruction of the spatial cross-correlation function for scintillations, independent of assumptions about differential fluid motions. Although there is clear evidence of a complicated velocity field, scintillation power was predominantly produced by levels at pressures of 130 +/- 30 mbar. The data are not grossly inconsistent with layers of isotropic Kolmogorov turbulence, but there is some evidence for deviation from the Kolmogorov spectral index and/or anisotropy.

Electron spin resonance: spin labels.

In general biological membranes possess no intrinsic paramagnetism and hence in the unlabeled state do not give rise to an electron spin resonance (ESR) spectrum. The introduction of a stable free radical (“spin label”) thus enables one to use ESR spectroscopy to study specific environments within the membrane. The spin label which is invariably used is the nitroxide radical, which has a three-line nitrogen hyperfine structure whose splitting varies with the orientation of the magnetic field relative to the nitroxide axes. It is this spectral anisotropy which has made spin label ESR such a powerful tool in the study of the molecular motions which are the characteristic feature of the highly dynamic structure of biological membranes. The nitroxide hyperfine splittings are partially averaged by the anisotropic motion, which gives a measure of the motional amplitude, and the linewidths are differentially broadened by an extent which depends on the rate of molecular motion. Other important features of the spin label spectra are the broadening by intermolecular label-label interactions and the ability to detect compartmentation of the label by quantitating spectral lineheights after selectively removing accessible spin-label signals by chemical reducing agents. Label-label interactions arise from two sources: the Heisenberg exchange interaction which is essentially a contact interaction and is therefore capable of measuring translational diffusion; and the dipole-dipole interaction which depends on the distance apart of the labels and is therefore capable of measuring intermolecular separations. Quantitation after treatment with reducing agents is chiefly concerned with the measurement of transport properties: of spin-label substrates or reducing agents, or the translocation of labelled lipid molecules.

Ultra-high-energy cosmic rays from clusters of galaxies

It is proposed that the highest-energy cosmic rays are produced in galaxies in the central regions of clusters. The particles diffuse outwards under the influence of randomly directed intergalactic magnetic fields. Many of the particles detected at the Earth come from the Virgo cluster but if the diffusion coefficient is high enough other, more distant clusters, will also contribute significantly. The model gives a natural explanation of the measured spectral shape and anisotropy above about 1018 eV; its requirements for cosmic-ray energy in cluster sources are not excessive if, as seems likely, the production spectrum is comparatively flat (j(E) varies as tildeE- gamma with gamma in the region of 2.1 to 2.2).

Lipid‐protein interactions in membranes: Effect of lipid composition on mobility of spin‐labeled cysteine residues in yeast plasma membrane

In order to gain direct evidence for lipid-dependent protein conformation in membrane, effects of modification of lipid composition on mobility of spin-labeled cysteine residues were investigated in the plasma membrane of the yeast Saccharomyces cerevisiae. Conversion of the bulk of phospholipids to diglycerides by treatment of the membrane with phospholipase C substantially enhanced spectral anisotropy. However, alterations of the viscosity of the lipid-bilayer by enriching the membrane with palmitelaidic or oleic acid had no effect on mobility of spin-labeled cysteine residues. These observations indicate that while the spin-labeled residues are not in direct contact with the lipid core of the membrane, there are lipid-protein interactions to the extent that removal of the polar portion of the bulk of phospholipids induces conformational changes in proteins, which in turn restrict mobility of these residues. It is concluded that conformation of membrane proteins on lipid structure and that phospholipids have a role in preserving the native conformation of proteins.

Bilayer structure in phospholipid-cytochromec model membranes

Lipid protein interactions in biological membranes differ markedly depending on whether the protein is intrinsic or extrinsic. These interactions are studied using lipid spin labels diffused into model systems consisting of phospholipid bilayers and a specific protein. Recently, an intrinsic protein complex, cytochrome oxidase, was examined and the data suggest there is a boundary layer of immobilized lipid between the hydrophobic protein surfaces and adjacent fluid bilayer regions. In the present study, a typical extrinsic protein, cytochrome c, was complexed with a cardiolipin/lecithin (1:4 by weight) mixture. The phospholipids in the presence and absence of cytochrome c exhibit typical bilayer behavior as jedged by four spin-labeling criteria: fluidity gradient, spectral anisotropy of oriented bilayers, response to hydration and the polarity profile. Any effects of cytochrome c on the ESR spectra of lipid spin labels are small, in contrast to the effects of intrinsic proteins. These data are consistent with electrostatic binding of cytochrome c to the charged groups of the phospholipids, and indicate that the presence of extrinsic proteins will not interfere with measurements of boundary lipid in intact biological membranes.

The influence of cholesterol on molecular motion in egg lecithin bilayers—A variable-frequency electron spin resonance study of a cholestane spin probe

Electron spin resonance spectra at 9.5, 24. and 35 GHz were obtained for a cholestane spin probe in oriented multibilayers of egg lecithin of varying cholesterol content. In agreement with earlier studies, cholesterol induced a higher degree of spectral anisotropy in the multibilayers—the variation of the hyperfine separations with cholesterol content was in agreement with the model of Lapper et al. ( Can. J. Biochem. 50, 969 (1972)) where the amplitude of anisotropic probe motion decreased with increasing cholesterol content. Analysis of the electron spin resonance line shapes was done using the relatively simple modified Bloch equation approach, and correlation times for anisotropic probe motion were extracted from the spectra at three frequencies. The data demonstrate that increasing cholesterol content results in a decreased rate of anisotropic motion of the probe, providing further insight into the molecular mechanism of the condensing effect of cholesterol.

The Zeeman effect in the spectrum of excitons bound to isoelectronic bismuth in indium phosphide

The Zeeman spectrum has been observed, in the high-field limit, of the photoluminescence emission line due to the recombination of excitons bound to isoelectronic bismuth centres in indium phosphide. It is found that the zero-field exchange splitting is less than 0.2 meV. Measurements of spectral anisotropy confirm the point defect model for this centre. Relevant splitting factors are K=0.72+or-0.05, L=-0.066+or-0.01 (hole), and geff=1.15+or-0.05 (electron).

ESR Study of NO2 and NO3 in Irradiated Lead Nitrate

Lead nitrate crystals have been irradiated with electrons at room temperature and their electron spin resonance spectra examined at 77°K to reveal three paramagnetic species. One species shows no hyperfine structure and has an isotropic g tensor with a g value at the free spin value: little can be said of its nature. The others are located, by their spectral anisotropy, at nitrate sites in the crystal with the axes of their axially symmetric g tensors parallel to the nitrate triad axes. One of these species is interpreted as NO3: it shows no nitrogen hyperfine structure but a slight interaction with 207Pb. The other is identified as NO2 rotating in the plane of the nitrate site. Theoretical predictions are in accord with these assignments. Various conclusions are drawn for the radiation chemistry. Other species than the NO2— and O2, revealed by chemical analysis, are significant products. The particular lattice determines the nature of these products, their stability and, in the case of NO2, its mechanism of formation. The annealing kinetics of NO2 and NO3 differ from that of the NO2—: Mechanisms previously postulated for the latter are almost certainly too simple.