PubMed · 2054899
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1991. Phospholipid phase transitions.. https://pubmed.ncbi.nlm.nih.gov/2054899/
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Gas chromatography (GC), apart from the qualitative and quantitative analysis of gaseous mixtures, offers many possibilities for physicochemical measurements, among which the most important is the determination of diffusion coefficients of gases in gases and liquids and on solids. The gas chromatographic techniques used for the measurement of diffusion coefficients, namely the methods based on the broadening of the chromatographic elution peaks, and those based on the perturbation of the carrier gas flow-rate, are reviewed from the GC viewpoint, considering their running though the history, the experimental arrangement and procedure, the appropriate mathematical analysis and the main results with brief discussions. The experimental data on diffusion coefficients, determined by the various gas chromatographic techniques, are compared with those quoted in the literature or estimated by the known empirical equations predicting diffusion coefficients. This comparison permits the calculation of the precision and accuracy of the techniques applied to the measurement of diffusion coefficients.
Modulating the relative humidity (RH) of the ambient gas phase of a phospholipid/water sample for modifying the activity of phospholipid-sorbed water [humidity-controlled osmotic stress methods, J. Chem. Phys. 92 (1990) 4519 and J. Phys. Chem. 96 (1992) 446] has opened a new field of research of paramount importance. New types of phase transitions, occurring at specific values of this activity, have been then disclosed. Hence, it is become recognized that this activity, like the temperature T, is an intensive parameter of the thermodynamical state of these samples. This state can be therefore changed (phase transition) either, by modulating T at a given water activity (a given hydration level), or, by modulating the water activity, at a given T. The underlying mechanisms of these two types of transition differ, especially when they appear as disorderings of fatty chains. In lyotropic transitions, this disordering follows from two thermodynamical laws. First, acting on the activity (the chemical potential) of water external to a phospholipid/water sample, a transbilayer gradient of water chemical potential is created, leading to a transbilayer flux of water (Fick's law). Second, water molecules present within the hydrocarbon region of this phospholipid bilayer interact with phospholipid molecules through their chemical potential (Gibbs-Duhem relation): the conformational state of fatty chains (the thermodynamical state of the phospholipid molecules) changes. This process is slow, as revealed by osmotic stress time-resolved experiments. In thermal chain-melting transitions, the first rapid step is the disordering of fatty chains of a fraction of phospholipid molecules. It occurs a few degrees before the main transition temperature, T(m), during the pretransition and the sub-main transition. The second step, less rapid, is the redistribution of water molecules between the different parts of the sample, as revealed by T-jump time-resolved experiments. Finally, in lyotropic and thermal transitions, hydration and conformation are linked but the order of anteriority of their change, in each case, is probably not the same. In this review, first, the interactions of phospholipid submolecular fragments and water molecules, in the interfacial and hydrocarbon regions of phospholipid/water multibilayer stacks, will be described. Second, the coupling of the conformational states of phospholipid and water molecules, during thermal and lyotropic transitions, will be demonstrated through examples.
An important goal in single molecule fluorescence correlation spectroscopy is the theoretical simulation of the fluorescence signal stemming from individual molecules and its autocorrelation function. The simulation approaches developed up to now are based exclusively on continuous-wave (cw) illumination and consequently on cw-excitation. However, this approximation is no longer valid in the case of two-photon excitation, for which pulsed illumination is usually employed. We present a novel theoretical model for the simulation of the fluorescence signal of single molecules and its autocorrelation function with consideration of the time dependence of the excitation flux and thus of all illumination-dependent photoprocesses: two-photon excitation, induced emission and photobleaching. Further important characteristics of our approach are the consideration of the dependence of the photobleaching rate on illumination and the low intersystem-crossing rates of the studied coumarins. Moreover, using our approach, we can predict quantitatively the effect of the laser pulse width on the fluorescence signal of a molecule, that is, the contributions of the photobleaching and saturation effects, and thus we can calculate the optimal laser pulse width. The theoretical autocorrelation functions were fitted to the experimental data, and we could ascertain a good agreement between the resulting and the expected parameters. The most important parameter is the photobleaching constant sigma, the cross section of the transition Sn<--S1, which characterises the photostability of the molecules independent of the experimental conditions. Its value is 1.7 x 10(-23) cm2 for coumarin 153 and 5 x 10(-23) cm2 for coumarin 314.