NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) can be used to observe signals from various nuclei, but this book focuses only on the proton (1H) of water molecules (H2O) as the nucleus of interest. This is because more than 90% of protons in the body are located in water molecules, and theMRI signal is therefore dominated by water. When we perform NMR spectroscopy, we put our sample (in this case, water) in a tube and place it inside a magnet.

We then input energy into the sample (a process called excitation) and observe the signal emitting from the sample. This is similar to ringing a bell and listening to the sound. There are only three types of information in the signal: frequency (i.e., high or low pitch), intensity (i.e., loudness), and phase. To visualize this information, we use a waveform diagram

These waves are often called time-domain data because the horizontal axis is the time elapsed while we are listening to the signal. After Fourier transform, the time axis transforms to the so-called frequency domain, in which the horizontal axis is frequency

An Internalization Wave of Caveolae can be Stimulated by Virus

Studies on the entry of simian 40 (SV40) virus by Pelkmans and coworkers [52–54] have documented that caveolae can actually play a role in nonconstitutive endocytosis. Thus, after binding of SV40 virus to the cell surface via the MHC class I molecule, the virus particles move laterally in the plasma membrane to end up in caveolae. Although these caveolae are initially immobile, the virus initiates a complex signaling cascade leading to a profound disorganization of the cortical actin cytoskeleton and a transient recruitment of the GTP-binding protein dynamin known to be involved in membrane fission (see Section 4.8). Importantly, without SV40-stimulation, less than 10% of the caveolae were associated with dynamin.

These changes, in turn, resulted in a wave of incoming caveolar vesicles containing virus where reorganized actin filaments formed “tails” necessary for internalization of the SV40-containing caveolae [53]. Subsequently, the virus was delivered

to caveosomes (see Section 4.7) and transported further downstream to the endoplasmic reticulum (ER). However, a delay of several hours then occurs before caveolin returns from the caveosomes to the plasma membrane in vesicles now devoid of virus particles [52].

It is interesting to note that antibody-induced crosslinking of MHC class I moleules (the SV40 receptor) results in an accumulation of MHC class I clusters in small uncoated surface invaginations” identical to caveolae, as reported 25 years go by Huet and coworkers. No clusters were found in clathrincoated pits. From he caveolae-like invaginations the clusters were apparently internalized and delivred to lysosomes [55]. It is therefore tempting to speculate that it is the samen derlying mechanism that is responsible for caveolae-mediated uptake of SV40 virus particles after their binding to MHC class I and of MHC class I clusters

enerated by antibody crosslinking.

Source:: lipid raft and C. Fielding, J.C. Jhonwilley and Son

Evidence for Phase Separation in Model Membrane Systems

Liquid-Ordered and Liquid-Disordered Phases

Various model membrane systems have been used by physicists and chemists to study phase separation in lipid mixtures. They are either monolayers or bilayers. Monolayers are either assembled at an air-water interface with the packing density of the lipids being adjusted by applying lateral pressure, or on a supporting lipid monolayer that is fixed to a solid support. Bilayers are used in the supported version as described above, or in the form of vesicles. The most commonly used vesicles are large or giant unilamellar vesicles (LUV or GUV, respectively) composed of only a single bilayer, but also multilamellar vesicles (MLV) are used. The basic principles were first established in simple binary lipid mixtures, but recently

ternary mixtures which more closely mimic the composition of the cell plasma membrane have been used. The mixtures usually contain one lipid with a high melting temperature (Tm), one with a low Tm, and cholesterol. GUVs are probably the system closest to a cell membrane, because artifacts from a support are excluded. Still, cell membranes are asymmetric with different lipid compositions of the outer versus the inner leaflet, while the GUVs used so far were all symmetric.

Since maintaining an asymmetric lipid distribution is energy-consuming, perhaps by reconstituting lipid translocators into liposomes this drawback can be overcome in the future. Although model membrane systems produce very simplified pictures of cell membranes, there are many examples of a close correlation with experimental data obtained in living cells [14]. Ipsen et al. were the first to describe the formation of a liquid-ordered phase by cholesterol and saturated phospholipids [15,16]. This phase can coexist with other lipid phases, and its characteristics are described as follows: the translational order of lipid molecules within the liquid-ordered phase is similar to that in a fluid bilayer state, whereas the configurational order of the hydrocarbon chains compares more to that in a gel state. The formation of the liquid-ordered phase was attributed to the unique chemical nature of cholesterol (for a review, see [17]), but later it was shown that all natural sterols promote domain formation and that also

small amounts of ceramide (3%) can stabilize domains formed in vesicles [18]. Leventis and Silvius showed that the interaction of cholesterol with different lipid species is dependent on the nature of their hydrocarbon chains and, to a lesser extent, also on their headgroup. The interaction preference decreases with SM > PS > PC > PE and with increasing unsaturation of the acyl chains [19]. Whereas the kink in unsaturated hydrocarbon chains is likely to hinder tight packing with the flat sterol ring of cholesterol, the reason for the preferential interaction of cholesterol with SM is still a debated issue.

Source::

Fielding, J.Christtoper, lipid raft and caveolae . Jhown willey and Son Publishe

Physical Modeling of Lipid Membranes

For preparing next semester Biofisika blog and team launch many modules for student of college especially for jember university of student (Jurusan Fisika/ Prodi Biofisika) . including teaching department, agriculture, pharmachy, biology and many department or faculty that need this module>>

Mathematical models of deformable, fluid membranes have been available for many years and have been successfully compared with experimental results, both on artificial and biological membranes. At the most fundamental level these theories rely on the single basic principle underlying statistical mechanics:

that the probability of observing a given membrane deformation depends on the
energy change involved in making this deformation The higher the energy,
the less likely the deformation.
Statistical mechanics tells us that the probability pi of an event i is related to its energy Fi according to:

pi ~ exp [– Fi /kBTm ]

This probability compares the deformation energy Fi to some energy source in the system. This energy is written F to remind us that it is a free energy and therefore includes changes in entropy, as well as internal and chemical energies Reactions that reduce the entropy of the system are disfavored in the same way as are those that involve a spontaneous increase in the energy by, for example, disruptingchemical bonds. Strictly speaking, that equation only holds for (sub)systems that are at equilibrium but this can often be a reasonable approximation, for example for small patches of membrane that can move and relax quickly, even though it may be inappropriate for the cell as a whole. In passive systems, the only energy source comes from the thermal fluctuations of energy kBT, where kB is the Boltzmann constant and T is the temperatureBiological systems are called “active”, because chemical energy, coming from, for example, ATP hydrolysis, can be harnessed by specific enzymes (molecular motors) to perform mechanical work. The cell membrane is generally the site of many active processes, including cytoskeleton polymerization and ion pumping. One may adopt the approach that these active processes provide an effective “membrane” temperature Tm > T and it is this that appears in Equation above.