Membranes lifetime distributions

In some circumstances, it can be anticipated that continuous lifetime distributions would best account for the observed phenomena. Examples can be found in biological systems such as proteins, micellar systems and vesicles or membranes. If an a priori choice of the shape of the distribution (i.e. Gaussian, sum of two Gaussians, Lorentzian, sum of two Lorentzians, etc.) is made, a satisfactory fit of the experimental data will only indicate that the assumed distribution is compatible with the experimental data, but it will not demonstrate that this distribution is the only possible one, and that a sum of a few distinct exponentials should be rejected. 

The measurement of fluorescence lifetimes is an integral part of the anisotropy, energy transfer, and quenching experiment. Also, the fluorescence lifetime provides potentially useful information on the fluorophore environment and therefore provides useful information on membrane properties. An example is the investigation of lateral phase separations. Recently, interest in the fluorescence lifetime itself has increased due to the introduction of the lifetime distribution model as an alternative to the discrete multiexponential approach which has been prevalent in the past. 

B. W. Williams and C. D. Stubbs, Properties influencing fluorophore lifetime distributions in membranes, Biochemistry 27, 7994-7999 (1988). 

T. Parassassi, F. Conti, E. Gratton, and O. Sapora, Membrane modification of differentiating proerythroblasts. Variation of l,6-diphenyl-l,3,5-hexatriene lifetime distributions by multifrequency phase and modulation fluorimetry, Biochim. Biophys. Acta 898, 196-201 (1987). 

R. M. Fiorini, M. Valentino, E. Gratton, E. Bertoli, and G. Curatola, Erythrocyte membrane heterogeneity studies using l,6-diphenyl-l,3,5-hexatriene fluorescence lifetime distribution, Biochem. Biophys. Res. Commun. 147, 460-466 (1987). 

To determine the shape of the hydrophobic barrier of bilayer membranes, fatty acids and PC molecules spin labeled with nitroxides at various positions along the lipid chains were diffused into vesicles and their solvent-sensitive isotropic coupling constants were measured [54]. Results are plotted in Figure 5 in terms of distance of the probe from the bilayer center. Also shown is the profile of the dielectric constant along the membrane normal evaluated from the fluorescence lifetime distribution of fluorescence probes in PC liposomes [55]. These data correlate well with results from neutron diffraction studies that map the positional distribution of water and lipid moieties along the bilayer normal [56]. 

This section describes a methodical procedure that allows reliability issues to be approached efficiently. MEMS reveal specific reliability aspects, which differ considerably from the reliability issues of integrated circuits and macroscopic devices. A classification of typical MEMS-failure modes is given, as well as an overview of lifetime distribution models. The extraction of reliability parameters is a Tack of failures situation using accelerated aging and suitable models. In a case study, the implementation of the methodology is illustrated with a real-fife example of dynamic mechanical stress on a thin membrane in a hot-film mass-airflow sensor. 

Figure 11.15 Interaction of PDKl and PKB detected by two-photon time domain FLIM. NIH3T3 are transfected with GFP-PDKl (upper panel), co-transfected with mRFP-PKB (middle panel) and stimulated by growth factor PDGF (lower panel). The lifetime maps indicate that the GFP-PDKl lifetime changes at the plasma membrane of these cells upon stimulation. In the presence of the acceptor mRFP-PKB there is no variation of the donor lifetime (GFP-PDKl) at the plasma membrane. The lifetime distributions are indicated by the histograms (right panels). It can be clearly seen that, upon stimulation, the GFP-PDKl lifetime at the plasma membrane decreases from 2.5 to 1.9 ns and the GFP-PDKl lifetime at the cytoplasm (2.3 ns) remains the same as when the acceptor is present. The decrease in lifetime at the plasma membrane illustrates that PDKl and PKB associate upon growth factor stimulation...

This work investigated PIM-1 membranes in the three states discussed above. Nickel-foil supported NaCl was used as a positron source and stacks of film samples, each about 1mm thick, were placed either side of the source. Annihilation lifetime decay curves were measured with an EG G Ortec fast-fast lifetime spectrometer. Measurements were made both in air and under an inert atmosphere (N2). However, o-Ps lifetimes in air were reduced due to quenching by oxygen, so only results obtained under N2 are discussed here. Results were analysed in terms of a four component lifetime distribution, which allowed obtaining better statistical fit. The two longest lifetimes, T3 and T4, for PIM-1 in... 

The properties of membranes commonly studied by fluorescence techniques include motional, structural, and organizational aspects. Motional aspects include the rate of motion of fatty acyl chains, the head-group region of the phospholipids, and other lipid components and membrane proteins. The structural aspects of membranes would cover the orientational aspects of the lipid components. Organizational aspects include the distribution of lipids both laterally, in the plane of the membrane (e.g., phase separations), and across the membrane bilayer (phospholipid asymmetry) and distances from the surface or depth in the bilayer. Finally, there are properties of membranes pertaining to the surface such as the surface charge and dielectric properties. Fluorescence techniques have been widely used in the studies of membranes mainly since the time scale of the fluorescence lifetime coincides with the time scale of interest for lipid motion and since there are a wide number of fluorescence probes available which can be used to yield very specific information on membrane properties. 

Figure 5.1. Representations of double-exponential and bimodal Lorentzian distribution analyses of DPH fluorescent decay lifetimes in liver microsomal membranes. Results (see Table 5.2) are normalized to the major component. The double-exponential analysis, represented by the vertical lines, recovers lifetimes near the centers of the Lorentzian distributions. The width of the distributions represents contributions from a variety of lifetimes. (From Ref. 17.)...

If a collisional quencher of the fluorophore is also incorporated into the membrane, the lifetime will be shortened. The time resolution of the fluorescence anisotropy decay is then increased,(63) providing the collisional quenching itself does not alter the anisotropy decay. If the latter condition does not hold, this will be indicated by an inability to simultaneously fit the data measured at several different quencher concentrations to a single anisotropy decay process. This method has so far been applied to the case of tryptophans in proteins(63) but could potentially be extended to lipid-bound fluorophores in membranes. If the quencher distribution in the membrane differed from that of the fluorophore, it would also be possible to extract information on selected populations of fluorophores possibly locating in different membrane environments. 

Asymmetric hollow fibers provide an interesting support for enzyme immobilization, in this case the membrane structure allows the retention of the enzyme into the sponge layer of the fibers by crossflow filtration. The amount of biocatalyst loaded, its distribution and activity through the support and its lifetime are very important parameters to properly orientate the development of such systems. The specific effect that the support has upon the enzyme, however, greatly depend upon both the support and the enzyme involved in the immobilization as well as the method of immobilization used. 

When CRs are in extra vascular volume, they occupy extra cellular space, and are designated as CRo (outside) where they directly bind with extra cellular water. 1H2O0. Since the transverse relaxation time, T is an intensive property of F O, its CR induced change depends directly on the molar ratio of CRo to H2O0. Thus measurement of T allows the direct determination of the concentration of CR (i.e.) [CR]o in the space in which CR is distributed. Further, because most of the water is intracellular ( F Oj), the change in the tissue l- O T from the entire voxel by CR allows the determination of the kinetics of water movement across the cell membrane. The kinetics is characterized by the average lifetime of a water molecule inside a cell, x. and is inversely proportional... 

Positronium lifetime spectroscopy is particularly well suited for stud)hng defects in crystals and structural fluctuations in amorphous materials and can give an estimate of free volumes in condensed matter [116]. It is a useful technique to estimate the free volume of polymeric membranes [117]. In a study on silica gels, the decay lifetime has been found (Fig. 4.16) to be proportional to the pore diameters (measured by N2 adsorption) between 30 and 100 A [118]. Information on pore size distribution and surface area may also be obtained by means of calibration curves. 

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