Lipids abundance, dynamics

In the previous two sections we discussed the electrodeformation and electroporation of vesicles made of single-component membranes in water. In this section, we consider the effect of salt present in the solutions. The membrane response discussed above was based on data accumulated for vesicles made of phosphatidylcholines (PCs), the most abundant fraction of lipids in mammahan cells. PC membranes are neutral and predominantly located in the outer leaflet of the plasma membrane. The inner leaflet, as well as the bilayer of bacterial membranes, is rich in charged lipids. This raises the question as to whether the presence of such charged lipids would influence the vesicle behavior in electric fields. Cholesterol is also present at a large fraction in mammalian cell membranes. It is extensively involved in the dynamics and stability of raft-hke domains in membranes [120]. In this section, apart from considering the response of vesicles in salt solutions, we describe aspects of the vesicle behavior of fluid-phase vesicles when two types of membrane inclusions are introduced, namely cholesterol and charged lipids. 

Note that this shotgun lipidomics approach is based on tandem MS analysis. As discussed earlier, due to the differential kinetics and/or thermodynamics of different species of a class during CID, two or more internal standards of a class spiked during lipid extraction are necessary to minimize any effects of differential fragmentation patterns on quantification. This point is particularly important to those species containing polyunsaturated fatty acyl substituents [20]. Moreover, identification and quantification of low-abundant isomeric/isobaric species overlapped with an abundant species may be missed due to the limitation of a relatively narrow dynamic range. 

Actually, a similar phenomenon to this steady-state ion suppression in shotgun lipidomics is also present in any method developed with LC-MS for quantitative analysis of lipid mixtures. For example, if it is intended to quantify a species of a minor lipid class in the presence of other abundant species [24], the amount of total lipids that can be loaded onto a column are capped by the upper limit of the linear dynamic range of the most abundant species in the mixture under the experimental conditions. The loaded amount of total lipids to expand the linear dynamic range of the minor component in the method cannot be increased greatly if there is a need for quantification of major components as well. Of course, the minor species can be analyzed separately with a pre-isolated fraction or with a saturated concentration of the abundant species to increase the dynamic range for quantification of the minor components. 

H 1 000 High resolution spectra Chemical shift, 7- T High sensitivity Natural abundance Reasonable spectra with small vesicles, micelles, high speed MAS or MAS of oriented bilayers Several relaxation mechanisms Overlapping resonances Dynamic properties Lipid diffusion... 

H g Powder spectra Quadrupole splitting T,J2 Direct determination of order parameters and bond vectors Measurable in cells and dispersed lipids 7 dominated by fast (ns) motions T2 dominated by slow (ms- ms) motions Low natural abundance Need for selective deutera tion Low sensitivity Ordering properties of phospholipids Dynamic properties of phospholipids... 

C 16 High resolution spectra Chemical shift T, Dipoiar couplings Natural abundance 7 dominated by one mechanism Need MAS NMR to resolve spectra Without selective enrichment, overlapping resonances Dynamic properties of phospholipids Lipid asymmetry Ligand-protein interactions Distance measurements... 

Whole-body MRS is potentially the most powerful tool available for the study of human metabolism in vivo. The nuclide s low natural abundance (1.1%) allows the study of important biological molecules such as glycogen and lipids, while at the same time allowing the use of C-enriched compounds in dynamic measurements of intermediate metabolism. However, several technical issues need to be addressed before its implementation in vivo, including a second transmit channel, optimum localization and proton decoupling. The latter has been one of the major stumbling blocks for the application of in vivo... 

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