Flop rate

Lipophilicity is intuitively felt to be a key parameter in predicting and interpreting permeability and thus the number of types of lipophilicity systems under study has grown enormously over the years to increase the chances of finding good mimics of biomembrane models. However, the relationship between lipophilicity descriptors and the membrane permeation process is not clear. Membrane permeation is due to two main components the partition rate constant between the lipid leaflet and the aqueous environment and the flip-flop rate constant between the two lipid leaflets in the bilayer [13]. Since the flip-flop is supposed to be rate limiting in the permeation process, permeation is determined by the partition coefficient between the lipid and the aqueous phase (which can easily be determined by log D) and the flip-flop rate constant, which may or may not depend on lipophilicity and if it does so depend, on which lipophilicity scale should it be based ... 

Vesicle size, bilayer fluidity, membrane permeability, microviscosity, ability to bind small molecules, suseeptibility to pore formation, flip-flop rates, extent of water penetration, lateral amphiphile diffusion, vesiele fusion, and kinetic medium effeets (some of which will be discussed briefly below) all depend on the paeking of... 

Within the same context, it is worth mentioning that Menger et al. [65], Ladika et al. [66] and Moss et al. [67] have recently demonstrated that macrocyclic lipids decrease the mobility of the membrane by raising the phase transition temperature and decreasing the permeation of ionic species and the flip-flop rate. 

Eq. 9 leads to spin diffusion with the flip-flop rate limiting the observed T. Below 10 s, the relaxation for the A spin is limited by the relaxation sink, which corresponds to the bonded methylene carbon. The effect of the siqall deviation In the coefficients from 1.0 and 0.0 near t s 10 s is non-exponential recovery, with the negative coefficient leading to a curve which is concave downward. For the M spin, the decay Jumps from theX curve for correlation times <10 " s to the Xg curve for correlation times >10 s. 

The values for these machine-specific parameters can be somewhat dependent on the application. For example, the flop rate can vary significantly depending on the type of operations performed. The accuracy of a performance model may be improved by using values for the machine-specific parameters that are obtained for the type of application in question, and the use of such empirical data can also simplify performance modeling. Thus, if specific, well-defined types of operations are to be performed in a parallel program (for instance, certain collective communication operations or specific computational tasks), simple test programs using these types of operations can be written to provide the appropriate values for the pertinent performance parameters. We will show examples of the determination of application specific values for a, and y in section 5.3.2. 

TYPE OF CBAMIEL NO. OF. HOLES caoss- SEcnoH (sq in.) FLOP rate (epa) AT 1 ft/sec... 

Other ESR studies of membrane structure include the determination of phospholipid flip-flop rates (Kornberg and McConnell, 1971), studies of membrane permeability, measurements of vesicle internal volumes and transmembrane potentials (Marsh et aL, 1976 McNamee and McConnell, 1973). 

The use of analytical models for spin dephasing in the solid state [39, 40] is not appropriate for the present case because they do not account for the specific distribution of nuclear spins around the electron spin. In order to consider the real matrix structure close to the chromophore we simulated the HE decay curves numerically by using the known crystal structure [9] and the nuclear flip-flop rate IE as a parameter. The echo decay is calculated as... 

The cylinder stretching protocol appears to work very well for simple solvent-free membrane models [111, 113, 114], but with more refined models this method suffers from two drawbacks, both related to the equifibration of a chemical potential. First, the cylinder separates the simulation volume into an inside and an outside. If solvent is present, its chemical potential must be the same in these two regions, but for more highly resolved models the solvent permeability through the bilayer is usually too low to ensure automatic relaxation. Second, the chemical potential of lipids also has to be the same in the two bilayer leaflets, and again for more refined models the lipid flip-flop rate tends to be too low for this to happen spontaneously. 

Danger and Sekwen (20) investigated the effect of free fatty acids on the permeability of the 1,2-dimyristoyl-5 >72-glycero-3-phosphotidylcholine (DMPC) bilayer at the main phase transition. When vesicles were formed between DMPC and oleic acid, the phase transition temperature was reduced drastically, but at the phase transition from the gel to liquid crystalline phase, membrane permeability reaches a maximum. Saturated fatty acids did not have the same effect on the bilayer as unsaturated fatty acids, in that there was a reduced effect on the phase transition temperature. They also concluded that -bilayer flip-flop is at a maximum at the phase transition temperature of DMPC being in the millisecond range for oleic acid. DMPC was found to have a flip-flop rate of 4 h at the phase transition and even longer rates at other temperatures. 

A cell membrane is illustrated in Fig. 6.1. It is built from a bilayer of lipids, usually phospholipids, associated with which are membrane proteins and polysaccharides. The antiparallel orientation of lipid layers in the bilayer is maintained due to the extremely slow flip-flop rate, i.e. the rate of diffusion transverse to the bilayer. The lipid bilayer is the structural foundation and the proteins and polysaccharides provide chemical functionality. The protein to lipid ratio shows a large variation depending on the cell, but proteins make up at least half of most cell membranes. A prominent exception is mammalian nerve cells which contain only 18 % protein (here also the lipids are sphingomyelins rather than phospholipids). Here, the primary requirement is that the cell membrane should be effective as an electrical... 

The fact that mutually different cross sectional patterns were observed in the SC-2D NMR spectrum where 7 was chosen to be 0 ms indicates that both inter- and intramolecular cross relaxation rates and spin flip-flop rates between interacting pairs of protons are relatively slow. This can be understood if one considers that dipolar interactions are partially averaged out by fast translational and rotational molecular motions in the liquid crystalline phase in contrast to the solid phase. 

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