Flip-flop spectroscopy

Exclusive tailored correlation spectroscopy Flip-Flop spectroscopy... 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

The lipids themselves are highly mobile. Steady state and time resolved spectroscopy (absorption, emission, ir, raman, nmr, epr) and anisotropy measurements have revealed rotational, vibration and segmental motions of the headgroups and the hydrocarbon tails of the lipids. Translocation of a lipid from one half of the bilayer to the other, ("flip-flop ) as well as intermembrane... 

Serious complications arise, however, if the spins are subject to strong nuclear electric quadrupolar interactions, which tend to modify the echo amplitudes measured. In such cases, it is still possible to extract dipole-dipole coupling information from spin echo decay spectroscopy, if the 71-pulses are applied entirely selectively to the central l/2>o -l/2> transition [6]. If the resonance frequencies between the coupled nuclei are sufficiently similar to allow for spin-exchange via the flip-flop mechanism, Eq. (5) turns into... 

Phospholipids have one polar head group and two hydrophobic hydrocarbon chains ranging from 14 to 24 carbon atoms. In most cases, one of the chains has one or more double bonds (Figure 9.2). Using spin resonance spectroscopy, it can be demonstrated that a distinct lipid molecule changes its position with an adjacent molecule between 10 and 10 times/s. In contrast, a change with a molecule from the opposite membrane leaflet (flip-flop mechanisms) occurs only once per 1-2 weeks. Cholesterol has a significant impact on membrane fluidity hydroxyl... 

FLOPSY Flip-flop mixing sequence (for total correlation spectroscopy) 9.2... 

At Stanford, Harden M. McConnell developed a new technique, called spin labelling, based upon EPR spectroscopy. While carbon-centered free radicals are extremely reactive and short-lived, radical oxides of nitrogen, such as NO and NO2, are moderately stable. McConnell noted that nitroxyl radicals (RR N-O) are extremely stable if R and R are tertiary and can be chemically attached to biological molecules of interest. In 1965, he published the concept of spin labeling and, in 1966, demonstrated that a spin-labelled substrate added to a-chymotrypsin forms a covalent enzyme-substrate complex. The EPR signal was quite broad suggesting restricted motion consistent with Koshland s induced-fit model. In 1971, McConnell published a smdy in which spin labelling indicated flip-flop motions of lipids in cell membranes. This was the start of dynamic smdies of cell membranes. 

Getting stractural information on the molecular supports of electropermeabilization was not easy. A key property of biological membranes is their dynamics. At a substructural level, 3IP NMR spectroscopy showed that a tilt of the orientation of the phospholipid polar head region was present in the electropermeabilized state of the membrane [45, 46]. The consequence of the interfacial water organization was proposed to be associated with a decrease of the hydration forces and the observed fusogenic state of electropermeabilized surfaces. At a more collective level, phospholipid flip-flop between the two faces of the plasma membrane was observed in the case of electropermeabilized erythrocytes [47]. 

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