Bilayer surface

Two physically reasonable but quite different models have been used to describe the internal motions of lipid molecules observed by neutron scattering. In the first the protons are assumed to undergo diffusion in a sphere [63]. The radius of the sphere is allowed to be different for different protons. Although the results do not seem to be sensitive to the details of the variation in the sphere radii, it is necessary to have a range of sphere volumes, with the largest volume for methylene groups near the ends of the hydrocarbon chains in the middle of the bilayer and the smallest for the methylenes at the tops of the chains, closest to the bilayer surface. This is consistent with the behavior of the carbon-deuterium order parameters,. S cd, measured by deuterium NMR ... 

Fig. 4. Tentative allocation of probe binding sites within the three-dimensional structure of Ca -ATPase derived from vanadate-induced E2-type crystals. The top picture is the projection view of the Ca -ATPase down the x-axis, revealing the pear-shaped contours of ATPase molecules. The maximum length of the cytoplasmic domain to the tip of the lobe is =r65A. In the middle and bottom pictures the same structure is viewed down the x-axis, revealing the gap between the bridge and the bilayer surface and the connections between ATPase molecules in neighboring dimer chains. The proposed binding sites for lAEDANS and FITC are indicated. The bottom right picture is the same structure viewed down the y-axis. Adapted from Taylor et al. [90].

We have shown that the cationic species are almost dominant and only a small portion of the cationic species is neutralized (discharged) when they are trapped at the bilayer interface. The discharging of the small portion of the cationic species DEC H in membranes is confirmed by the pH change study [48]. The proton should be released into the aqueous phase in the discharging process of DEC H+ at the membrane surface. In fact, the pH of the solution is decreased to a small extent after the addition of the EPC SUV. This means that only a small portion of the DEC H molecules is discharged the pH value is not decreased to the value of the complete discharging. The fraction of the DEC neutralized at the bilayer surface is estimated to be smaller than 1% of the total amount. 

Although the contribution is rather small, the partial discharging of the anesthetics in membranes can be important in the mechanism of the anesthetic action. The most plausible mechanism can be summarized as only a small portion of the cationic species are neutralized (deprotonated) at the bilayer surface and the neutral species are deeply penetrated and widely distributed in the hydrophobic bilayer interior, while the cationic species still remain at the hydrophilic bilayer surface where the hydration is significant. 

The favourable properties which mark out vesicles as protocell models were confirmed by computer simulation (Pohorill and Wilson, 1995). These researchers studied the molecular dynamics of simple membrane/water boundary layers the bilayer surface fluctuated in time and space. The model membrane consisted of glycerine-1-monooleate defects were present which allowed ion transport to occur, whereby negative ions passed through the bilayer more easily than positive ions. The membrane-water boundary layer should be particularly suited to reactions which are accelerated by heterogeneous catalysis. Thus, the authors believe that these vesicles fulfil almost all the conditions required for the first protocells on earth ... 

In the CNS of mammals, two proteins are needed to accomplish the dual role of adhesion at both myelin bilayer surfaces that P0 effects in the PNS. The four-trans-membrane-domain proteolipid protein (PLP) is likely to be responsible for adhesion of the extracellular surfaces, while the very basic cytoplasmic myelin basic protein... 

The crystal structure can be considered as a structure regularly stacked with bimolecular layers along the a-axis. Within the bimolecular layer, two molecules related by inversion symmetry face each other in the tail-to-tail fashion with their molecular axes inclined by about 26° to the bilayer surface. This inclination enables the head-to-tail arrangement of azobenzene chromophores as expected from the spectroscopic study. 

Here the angular brackets mean that the ensemble average is taken, and the angle 3 can be defined and computed for each bond in the molecule. When a bond is perfectly aligned normal to the bilayer surface, the order parameter assumes a value of unity. In a random orientation, the order parameter is zero. Bonds that are perfectly aligned parallel to the surface gives a negative order parameter of S = —0.5. 

The values of these autocorrelation functions at times t = 0 and t = 00 are related to the two order parameters orientational averages of the second- and fourth-rank Legendre polynomial P2(cos/ ) and P4 (cos p). respectively, relative to the orientation p of the probe axis with respect to the normal to the local bilayer surface or with respect to the liquid crystal direction. The order parameters are defined as... 

For pure phosphatidylcholine bilayers, the orientation of the headgroup has been well characterized showing that headgroups are aligned approximately parallel to the bilayer surface. Because only one phosphorus with 100% natural abundance is contained in the phospholipid molecule, NMR has become an important tool to study the phospholipid headgroup structure and dynamics. ... 

We have encountered examples of simple lipid bilayers earlier. These bilayers are composed largely of amphipathic molecules. In water, they have their hydrophobic parts occupying the center of the bilayer and their hydrophilic parts occupying the bilayer surface. Such bilayers form a continuous and essential structural feature of virtually all biological membranes. We need to distinguish between that layer which faces out from the cell and is in contact with the external environment, the exoplasmic leaflet, and that which faces in and is in contact with the cellular contents, the cytoplasmic leaflet. As we shall see, these two aspects of the lipid bilayer are quite distinct. 

The thermodynamics of mixing upon formation of the bilayered surface aggregates (admicelles) was studied as well as that associated with mixed micelle formation for the system. Ideal solution theory was obeyed upon formation of mixed micelles, but positive deviation from ideal solution theory was found at all mixture... 

Supporting the bilayer surfaces by synthetic polymer scaffolds (Figure 4.6) enables one to enhance the vesicle stability and to control the permeability of their membranes. The polymers can be embedded within the vesicles among... 

Fig. 67. Schematic representation of three types of anionic porphyrins in a cast multibilayer film of 34. The overall bilayer organization is assumed to be the same as that of Fig. 64. For clarity, counterions are not shown and the bilayer units are separated from each other. The spacer portion is also not shown in Stereogram . Type I porphyrins (Fig. 66) are inserted into the bilayer along the molecular axis of the spacer chain. Type II porphyrins are randomly placed on the bilayer surface. Type III porphyrins lie flat on the bilayer [445]...

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