Flip-flop motion

Although phospholipids diffuse laterally in the plane of the bilayer and rotate more or less freely about an axis perpendicular to this plane, movements from one side of the bilayer to the other are a different matter. Diffusion across the membrane, a transverse, or flip-flop, motion, requires getting the polar head-group of the phospholipid through the... 

Gohil, R. M. and Petermann, J. Chain conformational defects in polyvinylidene fluoride. Polymer 22, 1612 (1981) Takahashi, Y. and Tadokoro, H. Formation mechanism of kink bands in modification II of poly(vinylidene fluoride). Evidence for flip-flop motion between TGTG and TGTG conformations. Macromolecules 13, 1316 (1980) Takahashi, Y., Tadokoro,... 

At the higher kinetic energy, of course the same forces act upon the molecule. But now the molecule is too fast to become significantly reoriented before it hits the repulsive wall of the potential at t = 50 fs. When the molecule reaches the surface in the tilted configuration, it starts quickly rotating in a flip-flop motion until the other end hits the surface. The molecule is then scattered back into the gas phase rotationally excited (which can not been infered from the panel at t = 300 fs). 

Three signals at 141.7, 134.0, and 128.5 ppm as observed in the C CP+DDPh NMR spectrum of PMPhSM (Fig. 16(a)) are assigned to the Cl, C2, and C3 carbons, respectively. This shows that dipolar interactions with protons vanish due to the flip-flop motion of the phenyl ring of PMPhSM at room temperature. 

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. 

It is possible to reveal very low fluctuation motions by MAS two-site exchange analysis. The rate constant for flip-flop motions of Tyr side chain in Leu -enkephalin trihydrate, for instance, was obtained as 1.3 X 10 s at ambient temperature from NMR by means of spectral simulation utilizing the two-site exchange model [160]. The similar... 

The currently accepted structure of B. is the fluid mosaic model. Lipid molecules and membrane proteins are free to diffuse laterally and to spin within the bilayer in which they are located. However, a flip-flop motion from the inner to the outer surface, or vice versa, is energetically unfavorable, because it would require movement of hydrophilic substituents through the hydrophobic phase. Hence this type of motion is almost never displayed by proteins, and it occurs much less readily than translational motion in the case of lipids. Since there is little movement of material between the inner and outer layers of the bilayer, the two faces of the B. can have different compositions. For membrane proteins, this asymmetry is absolute, and, at least in the plasma membrane, different proportions of lipid classes exist in the two monolayers. Attached carbohydrate residues appear to be located only on the noncytosolic surface. Carbohydrate groups extending from the B. participate in cell recognition, cell adhesion, possibly in intercellular communication, and they also contribute to the distinct immunological character of the cell. 

Y. Tkkahashi and H. Ikdokoro. Formatioo mechanism of kittk bandsJn mod cation II of poly(vfaiytidcne fluoride). Evidence for flip-flop motion between TUItl and TGTG confor-matioos, tdacromokades 73 1316 (1980). 

The temperature dependence of the angular distribution of can be explained by a flip-flop motion of PE chains coupled with a longitudinal translation by half the period, i.e., c/2, but not by an oscillation or uniform rotation around the chain axis. ° ... 

The third term in Eq. 4 is the homonuclear dipolar interaction for the I spins. In the absence of dipolar decoupling, exchange interactions which induce rapid flip-flop motion of the I spins can reduce the I-S broadening from its full rigid-lattice value. In this manner, the magnitude of I-I dipolar interaction can... 

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