Lipids viscosity

Membrane enzymes often show a transition in the Arrhenius plot of the enzyme activity, which is usually attributed to a change in the lipid configuration. (K" " -h H" ")-ATPase preparations show such a transition at 27-28°C [71,86]. However, a transition in the polarisation signal of a lipid viscosity probe (diphenylhexatriene), was not observed in this region [71], indicating that the transition in the ATPase activity is either due to a change in the rate-limiting step of the reaction or to the presence of an annulus of specific lipids with physical properties different from those of the bulk lipids. 

We have previously suggested that in Rps. viridis. the two kinetic states described above could be related to two "conformational states of the reaction centers probably existing in the dark. We show in this work that in addition to pH, salt concentration and herbicides effects, the relative distribution of the two populations is sensitive to physical changes such as lipid viscosity or low temperature freezing. This could be used to further study the meaning of these states, especially by looking at the close environnement of primary quinone. 

Theoretical models of the film viscosity lead to values about 10 times smaller than those often observed [113, 114]. It may be that the experimental phenomenology is not that supposed in derivations such as those of Eqs. rV-20 and IV-22. Alternatively, it may be that virtually all of the measured surface viscosity is developed in the substrate through its interactions with the film (note Fig. IV-3). Recent hydrodynamic calculations of shape transitions in lipid domains by Stone and McConnell indicate that the transition rate depends only on the subphase viscosity [115]. Brownian motion of lipid monolayer domains also follow a fluid mechanical model wherein the mobility is independent of film viscosity but depends on the viscosity of the subphase [116]. This contrasts with the supposition that there is little coupling between the monolayer and the subphase [117] complete explanation of the film viscosity remains unresolved. 

Various mechanisms have been proposed to explain the hypocholesterolemic effect of GA (Annison et al., 1995 Tiss et al., 2001). Some studies have suggested that the viscosity of fermentable dietary fiber contributes substantially to the reduction of lipids in animals and humans (Gallaher et al., 1993 Moundras et al., 1994). However, other studies suggested that this property is not related to plasma lipids (Evans et al., 1992). The mechanism involved is clearly linked to increased bile acid excretion and fecal neutral sterol or a modification of digestion and absorption of lipids (Moundras et al., 1994). 

The influence of the lipophilic external phase on the production of xylan-based microparticles by interfacial cross-linking polymerization has been investigated (Nagashima et al., 2008). Three different external phases were investigated a 1 4 (v/v) chloroform cyclohexane mixture, soybean oil, and a medium chain triglyceride, with viscosities below 1, 24, and 52 cP, respectively. It was observed that the use of these different lipid phases results in different macroscopic and microscopic aspects of the system (Figure 10). 

Supercritical fluid extraction (SFE) is a technique in which a supercritical fluid [formed when the critical temperature Tf) and critical pressure Pf) for the fluid are exceeded simultaneously] is used as an extraction solvent instead of an organic solvent. By far the most common choice of a supercritical fluid is carbon dioxide (CO2) because CO2 has a low critical temperature (re = 31.1 °C), is inexpensive, and is safe." SFE has the advantage of lower viscosity and improved diffusion coefficients relative to traditional organic solvents. Also, if supercritical CO2 is used as the extraction solvent, the solvent (CO2) can easily be removed by bringing the extract to atmospheric pressure. Supercritical CO2 itself is a very nonpolar solvent that may not have broad applicability as an extraction solvent. To overcome this problem, modifiers such as methanol can be used to increase the polarity of the SFE extraction solvent. Another problem associated with SFE using CO2 is the co-extraction of lipids and other nonpolar interferents. To overcome this problem, a combination of SFE with SPE can be used. Stolker et al." provided a review of several SFE/SPE methods described in the literature. 

Sinensky, M. (1974) Homeoviscous adaptation - a homeostatic process thatregulates the viscosity of membrane lipids in Escherichia coli. Proceedings of the National Academy of Sciences of the, 71 (2), 522-525. 

When anisotropy increases due to increase of local viscosity producing higher friction on rotating unit. This can happen, for instance, in micelles or lipid... 

Nipper ME, Majd S, Mayer M, Lee JC, Theodorakis EA, Haidekker MA (2008) Characterization of changes in the viscosity of lipid membranes with the molecular rotor FCVJ. Biochim Biophys Acta 1778(4) 1148—1153... 

We should first emphasize that viscosity is a macroscopic parameter which loses its physical meaning on a molecular scale. Therefore, the term microviscosity should be used with caution, and the term fluidity can be alternatively used to characterize, in a very general way, the effects of viscous drag and cohesion of the probed microenvironment (polymers, micelles, gels, lipid bilayers of vesicles or biological membranes, etc.). 

Whatever the technique used, it is important to note that (i) only an equivalent viscosity can be determined, (ii) the response of a probe may be different in solvents of the same viscosity but of different chemical nature and structure, (iii) the measured equivalent viscosity often depends on the probe and on the fluorescence technique. Nevertheless, the relative variations of the diffusion coefficient resulting from an external perturbation are generally much less dependent on the technique and on the nature of the probe. Therefore, the fluorescence techniques are very valuable in monitoring changes in fluidity upon an external perturbation such as temperature, pressure and addition of compounds (e.g. cholesterol added to lipid vesicles alcohols and oil added to micellar systems). 

Considering that LEH is still in a developmental phase, it is imperative that each batch of LEH is fully characterized for physicochemical and biological properties. Typically, lipid content, particle diameter, oxygen affinity, hemoglobin, methemoglobin, carbonyl-hemoglobin, oncotic pressure, viscosity, endotoxin, and osmolality are determined by conventional methods. Several issues that need specific attention in the case of LEH are as follows ... 

In the mucosal environment, effects of salt, pH, temperature, and lipids need to be taken into consideration for possible effects on viscosity and solubility. A pH range of 4-7 and a relatively constant temperature of 37°C can generally be expected. Observed solution properties as a function of salt and polymer concentration can be referred to as saline compatibility. Polyelectrolyte solution behavior [27] is generally dominated by ionic interactions, such as with other materials of like charge (repulsive), opposite charge (attractive), solvent ionic character (dielectric), and dissolved ions (i.e., salt). In general, at a constant polymer concentration, an increase in the salt concentration decreases the viscosity, due to decreasing the hydrodynamic volume of the polymer at a critical salt concentration precipitation may occur. 

The generally low lipid content and the poor viscosity of lipid nanodispersions make these preparations, as they are, less suitable for dermal drug application. The handling of the preparation by the patient is improved by SLN incorporation into ointments, creams, and gels. Alternatively, ready-to-use preparations may be obtained by one-step production, increasing the lipid phase to at least 30%. However, increasing the lipid frequently results in an unwanted increase in particle size. Surprisingly, it has been found that very concentrated (30 to 40%) semisolid cetyl palmitate formulations preserve the colloidal particle size [10]. 

It should also be recalled that the diffusion coefficient of a molecule will decrease with increasing viscosity of the solvent. Thus, as might be expected with cytosols or lipid bilayers, a viscous medium will slow down the rate of encounters. Since viscosity is itself temperature dependent, such encounters in solution will have their own activation energy. 

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