Flip-flop mechanism

Incorporation of surfactant molecules into the interface (most likely via a flip-flop mechanism). 

This is because any scalar relaxation occurs through a flip-flop mechanism (Fig. 3.8)5 ... 

Unpaired electronic density can be delocalized onto the various nuclei of the complex via through-bond scalar hyperfine interactions involving occupied orbitals containing s-character (direct interaction or polarization according to the Fermi mechanism, Wertz and Bolton (1986)). Random electron relaxation thus produces a flip-flop mechanism which affects the nuclear spin and increases nuclear relaxation processes (Bertini and Luchinat, 1996). Since these interactions are isotropic, they do not depend on molecular tumbling and re is the only relevant correlation time for non-exchanging semi-rigid complexes. Moreover, only electronic spin can be delocalized via hyperfine interactions (no orbital contribution) and the contact re-... 

Platelet participation in normal hemostasis. The hemostatic plug is the specific response to external vessel lesion and depends on the extent of vessel wall damage, the specific interaction between endothelial cells and activated platelets, release of the contents of platelets intracellular granules in response to activation, the conjoint activity of activated factor Vll and platelet agonists, and the open conditions of blood flow. After activation, platelets also produce the external ization of membrane phosphatidylserine through the flip-flop mechanism that will support the function of the prothrombinase complex ending in thrombin generation and local clot formation. 

It is now widely accepted that atherosclerosis is a chronic inflammatory arterial disease associated with risk factors, platelet, and other blood cells activities and their interactions with subendothelial cells, Activated platelets release active components from citosol and induce the externalization of phosphatidylserine through the flip-flop mechanism (23) that supports the function of the prothrombinase complex ending in thrombin generation,... 

Flip-flop mechanism Release of citosol components Interacts with leukocytes Source of inflammatory modulator. )... 

Note, however that the concepts about the lipid membrane as the isotropic, structureless medium are oversimplified. It is well known [19, 190] that the rates and character of the molecular motion in the lateral direction and across the membrane are quite different. This is true for both the molecules inserted in the lipid bilayer and the lipid molecules themselves. Thus, for example, while it still seems possible to characterize the lateral movement of the egg lecithin molecule by the diffusion coefficient D its movement across the membrane seems to be better described by the so-called flip-flop mechanism when two lipid molecules from the inner and outer membrane monolayers of the vesicle synchronously change locations with each other [19]. The value of D, = 1.8 x 10 8 cm2 s 1 [191] corresponds to the time of the lateral diffusion jump of lecithin molecule, Le. about 10 7s. The characteristic time of flip-flop under the same conditions is much longer (about 6.5 hours) [19]. The molecules without long hydrocarbon chains migrate much more rapidly. For example for pyrene D, = 1.4x 10 7 cm2 s1 [192]. 

The division of protocells is an important issue. In general it should be the result of insertion of membranogenic molecules into the membrane. This can be achieved from outside [31], or from inside. Needless to say, growth from inside is the favoured solution. Whether growth occurs from outside or inside, amphiphiles must be able to jump from one side of the bilayer to the other, otherwise the two layers of the membrane would grow out of proportion. The latter could happen transiently, but not notoriously. Amphiphiles are known to jump from one layer of the membrane to the other (flip-flop mechanism) this reaction must be sufficiently fast in a protocell to render growth and division possible. 

An interesting mechanism for the establishment of a pH gradient in growing fatty acid vesicles was recently shown by Chen and Szostak [62], Fatty acid vesicles are usually very permeable to cations, including H+. Maintenance of a gradient thus requires a non-permeant cation, such as arginine. Incorporation of protonated (neutralised) fatty acid molecules from the external medium results in acidification of the internal milieu, by a flip-flop mechan-... 

When this type of disorder occurs in long cooperative chains or in cyclic systems of hydrogen bonds, it has been described as a. flip-flop mechanism [118], because it must proceed from bond to bond like a domino effect and involves only two well-defined states. 

Sobue, K., Morimoto, K., Inui, M., Kanda, K. and Kakiuchi, S. (1982). Control of actin-myosin interaction of gjzzard smooth muscle by calmodulin- and caldesmon-linked flip-flop mechanism. Biomed. Res. 3, 188-196. 

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... 

Alternate models to the differential bindings of the multiple site enzymes are flip-flop mechanisms (Lazdunski et al 1971) and half-site reactivity (Seydoux et al, 1974) of enzymes. 

FIG U RE 2.2 H/D exchange via a Flip-Flop mechanism. The reaction is shown for a generic carboxylic acid reacting with deuterium oxide (DjO). 

Flip-Flop Mechanisms and Half-Site En-zjnnes... 

Figure 6.12. The structure of the adsorbed water in an aqueous medium as a function of electrode potential. The snapshots follow the well-known water flip-flop mechanism as one moves from a potential of zero charge to more cathodic potentials. The dark atoms refer to oxygen and the light atoms refer to hydrogen...

At the end of its journey through the extracellular space, anandamide eventually reaches the target neuronal membrane containing its receptor (e.g., the GPCR protein CBl). Due to its lipidic nature, anandamide will first penetrate the plasma membrane before interacting with this receptor. The insertion of anandamide in the membrane bilayer is a cholesterol-dependent process. Anandamide interacts specifically with cholesterol in the plasma membrane of brain cells. Following this interaction, anandamide is pulled inside the membrane through a flip-flop mechanism (Fig. 5.15). 

The initial interaction with cholesterol is mediated by the formation of a hydrogen bond linking the OH groups of both anandamide and cholesterol. Then, anandamide enters the membrane through a flip-flop mechanism. At this stage, its interaction with cholesterol is stabilized by both van der Waals forces and a new hydrogen bond between the OH group of... 

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