Search

Phase-modulation fluorescence lifetime measurements were used to study the single Trp residue of the Ca(2+)-binding protein S-100a both in the absence and in the presence of Ca2+ and/or Mg2+. Trp fluorescence decay for the protein was satisfactorily described by Lorentzian lifetime distributions centered around two components (approximately 4 ns and 0.5 ns). Lifetime values were unchanged by 2 mM Ca2+, but the fractional intensity associated with longer lifetime increased up to 75%. In the presence of Mg2+, the Ca2+ induced increase of the fractional intensity associated with longer lifetime was only 57%. For the protein in buffer, about the 85% of the recovered anisotropy was associated to a rotational correlation time of 6.7 ns. After the addition of Ca2+, this value was increased to 16.08 ns. In the presence of Mg2+, Ca+2 increased the rotational correlation time to 33.75 ns. Similar studies were performed with S-100a interacting with egg phosphatidylcholine vesicles (SUV). Our data suggest that the conformation of the protein may be influenced by structural features of the lipidic membrane. Moreover, data obtained in the presence of Mg2+ indicate some interaction between lipids and S-100, likely mediated by this ion.

The changes in plasma membrane polarity of polymorphonuclear leukocytes (PMN) during the activation of the respiratory burst were investigated by measuring the steady-state fluorescence emission spectra of 2-dimethylamino(6-lauroyl) naphthalene (Laurdan), which is known to be incorporated at the hydrophobic-hydrophilic interface of the bilayer, displaying spectral sensitivity to the polarity of its surroundings. Laurdan shows a marked steady-state emission blue shift in nonpolar solvents, with respect to polar solvents. Our results show a blue shift of the fluorescence emission spectra of Laurdan during activation of PMN with phorbol myristate acetate or N-formyl-methionyl-leucyl-phenylalanine. These results suggest that the activation of the respiratory burst of PMN is accompanied by a decrease in polarity in the hydrophobic-hydrophilic interface of the plasma membrane.

The fluorescence decay of 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) was used to study micro-heterogeneity of 1,2-dimyristoyl-3-sn-phosphatidylcholine (DMPC) liposomes and to characterize the effect of phosphatidic acid on the correlation between fluorescence microheterogeneity and membrane permeability. The fluorescence decay, measured using multifrequency phase fluorometry, has been analyzed either by using a model of discrete exponential components or a model of continuous distribution of lifetime values. Both analyses have shown that TMA-DPH decay is characterized by two components: a long one of about 9 ns and a short one of about 5 ns. In the gel phase, at variance with previous DPH studies, the short component was associated with a large fractional intensity. The distributional analysis showed changes of lifetime values and width in correspondence to the calorimetric transitions. The presence of egg phosphatidic acid increased both long lifetime values and distributional width. The use of TMA-DPH as a probe to evaluate membrane heterogeneity using the distributional width is discussed. The effect of phosphatidic acid on the membrane surface and in the hydrophobic core has been related to its structural properties and to its role in water penetration.

The fluorescence decay of 1,6-diphenyl-1,3,5-hexatriene has been used to characterize the structural organization of erythrocyte membranes. At 37 degrees C a large fraction of the decay (0.96) is associated with a lifetime value of 11.31 ns, while a minor fraction has a short lifetime of 2.63 ns. The distribution analysis approach has shown that the 11 ns component can be described using a Lorentzian distribution function having a full width at half maximum of 0.27 ns. The width of this component is associated with the membrane structural organization since liposomes from erythrocyte total lipid extract exhibit a narrower width. Moreover the distribution width is sensitive to different treatments of erythrocyte membrane.

The fluorescence decay of 1,6 diphenyl-1,3,5-hexatriene (DPH) has been used to characterize aspects of the erythrocyte membrane structure related to the microheterogeneity of the lipid bilayer. The DPH decay has been studied using frequency domain fluorometry and the data analyzed either by a model of discrete exponential components or a model that assumes a continuous distribution of lifetime values. The main intensity fraction was associated with a lifetime value centered at about 11 ns in the erythrocyte membrane, but a short component of very low fractional intensity had to be considered to obtain a good fit to the data. The lifetime value of the long component was insensitive to temperature, while the width of the distribution decreased with increasing temperature. In multilamellar liposomes prepared from phospholipids extracted from the erythrocytes, the long lifetime component showed a temperature dependence. The depletion of 27% of the cholesterol in the erythrocyte membrane induced a broadening of the distribution, suggesting a homogenizing effect of cholesterol. This effect has also been detected in egg phosphatidylcholine at a very low cholesterol/phospholipid molar ratio. The role of cholesterol on membrane heterogeneity is discussed in relation to the effect of cholesterol on water penetration.

The effect of cholesterol on microheterogeneity of liposomes obtained from saturated and unsaturated phospholipids was studied by measuring the fluorescence decay of 1,6-diphenyl-1,3,5-hexatriene (DPH). Data obtained by frequency domain fluorometry have been analyzed either by discrete exponential or continuous lifetime distribution approaches. In egg phosphatidylcholine liposomes, the addition of cholesterol increases the lifetime value or the centre of the lifetime distribution. At high cholesterol concentration, good fits are obtained using a monomodal distribution analysis or single exponential component. At low cholesterol concentration an additional short component of low fractional intensity must be included to obtain a good fit. In dipalmitoylphosphatidylcholine, the addition of cholesterol decreases the long lifetime component centre value both in the gel and in the liquid-crystalline state. The DPH lifetime value is sensitive to the dielectric constant of the probe microenvironment, and cholesterol has been shown to modify water penetration in the bilayer. Using this information our data indicate that cholesterol affects the polarity of the microenvironment in liposomes of unsaturated phosphatidylcholine and saturated phosphatidylcholine in different ways. Although the major conclusions of this paper are obtained using changes of the distribution centre upon cholesterol addition, there are also preliminary indications that the lifetime distribution width decreases as cholesterol is added. We have interpreted this observation as being due to the homogenizing effect of cholesterol.

TMA-DPH fluorescence decay in human PMN before and after stimulation with FMLP was studied using frequency domain fluorometry. Membrane heterogeneity was assessed by the width of the continuous distributions of lifetime values of Lorentzian shape used to describe the fluorescence decay. In non-stimulated granulocytes TMA-DPH fluorescence decay is characterized by two distributions of lifetime values centered at 6.5 and 1.0 ns and full width at half maximum of 0.3 and 1.2 ns, respectively. Within 15 min after stimulation, the center values of the two distribution components were 5.1 and 0.8 ns and the distribution width was 0.8 and 0.6 ns, respectively. These results indicate changes of membrane domain organization which can be ascribed to compositional changes and redistribution of membrane components.

We present two-dimensional simulations of different types of strained-silicon quantum wires obtained by selective etching on silicon germanium heterostructures. Such structures are promising both for emerging ballistic devices in silicon compatible technology and for innovative nanoscale field-effect transistors. Numerical modeling has been performed with a procedure designed to solve the Poisson–Schrödinger equation for electrons and holes, that takes into account the effect of strain on the band structure, conduction band anisotropy, and the effect of states at the exposed surfaces. We show that the simulations provide insights into the capability to control the wire via an external gate voltage, and into the dependence of wire properties on geometry and surface states. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]