Phospholipid surface area

The mitochondrion has an outer and an inner membrane (Figure 1). The outer membrane contains pores formed from a protein, porin, which allow exchange of molecules with molecular weights up to about 2,000 between the cytosol and the intermembrane space. The inner membrane is extensively invaginated to increase its surface area. It has a different lipid composition from the outer membrane and is rich in the acidic phospholipid cardiolipin (diphosphatidyl-glycerol) which is only found in animal cells in mitochondria. Cardiolipin confers good electrical insulating properties on the inner membrane which is impermeable... 

Ruckenstein and Li proposed a relatively simple surface pressure-area equation of state for phospholipid monolayers at a water-oil interface [39]. The equation accounted for the clustering of the surfactant molecules, and led to second-order phase transitions. The monolayer was described as a 2D regular solution with three components singly dispersed phospholipid molecules, clusters of these molecules, and sites occupied by water and oil molecules. The effect of clusterng on the theoretical surface pressure-area isotherm was found to be crucial for the prediction of phase transitions. The model calculations fitted surprisingly well to the data of Taylor et al. [19] in the whole range of surface areas and the temperatures (Fig. 3). The number of molecules in a cluster was taken to be 150 due to an excellent agreement with an isotherm of DSPC when this... 

Figure 16 shows the experimental arrangement for the measurement of the surface pressure. The trough (200 mm long, 50 mm wide and 10 mm deep) was coated with Teflon. The subphase temperature was controlled within 0.1 C by means of a jacket connected to a thermostated water circulator, and the environmental air temperature was kept at 18 °C. The surface tension was measured with a Wilhelmy plate of platinum(24.5 x 10.0 x 0.15 mm). The surface pressure monitored by an electronic balance was successively stored in a micro- computer, and then Fourier transformed to a frequency domain. The surface area was changed successively in a sinusoidal manner, between 37.5 A2/molecule and 62.5 A2/molecule. We have chosen an unsaturated phospholipid(l,2-dioleoyl-3-sn-phosphatidyI-choline DOPC) as a curious sample to measure the dynamic surface tension with this novel instrument, as the unsaturated lipids play an important role in biomembranes and, moreover, such a "fluid" lipid was expected to exhibit marked dynamic, nonlinear characteristics. The spreading solution was 0.133 mM chloroform solution of DOPC. The chloroform was purified with three consecutive distillations. 

Table I gives a compilation of the molecular composition of SFV grown in BHK-21 cells, based on the revised weight for the viral particle of 41-42 X 10 daltons (Jacrot et al, 1983). If one assumes that each phospholipid-cholesterol pair takes up a surface area of about 90-100 A (Israelachvili and Mitchell, 1975) and each glycolipid about 55 A (Pascher and Sundell, 1977), then about 80% of the surface area in the bilayer is occupied by the lipids, leaving about 20% for the spanning proteins. This is somewhat more than would be expected if 180 spike proteins span the bilayer, each having two transmembrane a helical segments.

MICELLAR SUBSTRATES. Phospholipids in micelles are frequently found to be more active substrates in lipolysis than those phospholipids residing in a lipid bilayer". Dennis first described the use of Triton X-100 to manipulate the amount of phospholipid per unit surface area of a micelle in a systematic analysis of the interfacial interactions of lipases with lipid micelles. Verger and Jain et al have presented cogent accounts of the kinetics of interfacial catalysis by phospholipases. The complexity of the problem is illustrated in the diagram shown in Fig. 2 showing how the enzyme in the aqueous phase can bind to the interface (designated by the asterisk) and then become activated. Once this is achieved, E catalyzes conversion of S to release P. ... 

In 1925, E. Gorter and F. Grendel (J. Exp. Med. 41, 439) reported measurements in which they extracted lipid from red blood cell membranes with acetone, spread the lipids as a monolayer, and measured the area of the compressed monolayer. They then estimated the surface area of an erythrocyte and calculated that the ratio of the lipids (as a monolayer) to the surface area of the red blood cell was 1.9-2.0. More modern experiments gave the following each erythrocyte membrane contains 4.5 x 10 16 mol of phospholipid and 3.1 x 10-16 mol of cholesterol. 

In unhomogenized dairy cream the natural phospholipids contribute to the whipping properties of the cream. However, after homogenization the particle size of the fat globules decreases, and the total fat surface area increases. This means that the interfacial concentration of polar lipids decreases because milk serum proteins adsorb at the newly formed interfaces, and the whipping properties are lost. Consequently, additional polar lipids or emulsifiers are needed to obtain good whipping properties in most industrially manufactured products. 

Intermediate-sized unilamellarvesicles (lUVs) have diameters of the order of magnitude of 100 nm, and are called large unilamellarvesicles (LUVs) if the size is more than 100 nm and they consist of a single bilayer. For unilamellarvesicles, the phospholipid content is related to the surface area of the vesicles, which is proportional to the square of the radius, while the entrapped volume varies with the cube of the radius. In addition, because of the Lnite thickness of the membrane (ca. 4 nm), as thf vesicles become smaller, their aqueous volume is further reduced since the phospholipids occupy more of the internal space. Consequently, for a given quantity of lipid, large unilamellar liposomes... 

A theoretical model predicting the shape-structure relationship between the monomeric units and their aggregates was developed by Israelachvili and was based on statistical mechanics of phospholipids.23 This model predicts the type of the aggregate formed on the basis of the packing parameter (P), which relates the volume of the molecule (V) to its length (1) and to the mean cross-sectional (effective) head group surface area (a) ... 

Similar to the definition of free volume, the mean free surface area per lipid molecule is related to the area occupied by one phospholipid molecule, A. A0(= 40.8 A) is the area of a phospholipid molecule in the crystal and o (= A0/A) is the reduced surface density. For the surface density data see ref. 5. 

The superiority of NPyT over NPAT simulations has also been claimed in a more recent 120-ps simulation of monolayers of different phospholipids, based again on the agreement of calculated order parameters with experimental data [40]. However, it was also observed that the surface area of the phospholipids did not deviate much... 

The dependence of the calculated surface pressure on other system variables has been investigated too. In simulations applying NPAT conditions, it was demonstrated that the calculated surface tension was somewhat insensitive to changes in the surface area of up to 8 A2 [35]. And Feller and Pastor [41] found that in MD simulations with fixed surface area the calculated surface tension was dependent on the number of phospholipids. The surface tension increased with decreasing system size with values of 33.5, 39.2, and 57.2 dyn/cm for systems of 72, 32, and 18 lipids respectively. 

In the simulation of Tu et al. [72] with 12.5 mol% cholesterol, the starting configuration was obtained from a simulation of a pure DPPC bilayer composed of 64 DPPC molecules. Four DPPC molecules per leaflet were replaced by cholesterol to generate the DPPC-cholesterol system. The system was simulated for 1.4 ns the area per phosholipid decreased during the first 700 ps of the simulation but then remained stable for the rest of the simulation. As pointed out by the authors, the decrease in the surface area was due to the replacement of the phospholipids by the thinner cholesterol and not to an ordering effect of cholesterol. Accordingly, little effect of cholesterol on the order of the alkyl chains was observed in this simulation, in contradiction to the results of a cholesterol-DPPC simulation performed by Smondyrev and Berkowitz that is described below. 

To determine the basis for this regular variation in rate of proton leakage, several characteristics of the mitochondria were measured, including inner membrane surface area per unit of matrix volume and fatty acid composition of mitochondrial membrane phospholipids. The largest share (about 70%) of the variation in proton flux rate appears to be due to dififer-... 

The above equation provides reasonable limiting values. When A = A0, S — 1, and when A = 3.40 (=120 A2 for the phospholipid surfactants), which is a very large molecular surface area. S = 0, indicating complete disorder. In the LE monolayer and the LE/ LC transition region, A is between Ao and JAq, and consequently, S varies between 0 and 1. 

In a recent research, effect of hydrophobic surfactant proteins SP-B and SP-C on binary phospholipid monolayers was studied by IRRAS [65], The phospholipids examined were DPPC plus either DPPG or 1,2-dioleoyl-5 -glycero-3-phosphoglycerol (DOPG). IRRAS obtained at the air-water interface for a monolayer film of 7 1 DPPC-d62 DPPG plus 5 wt.% SP-B/C are shown in Fig. 5. Both C-H and C-D vibrational bands grow in intensity as the surface pressure increases and the surface density of the lipid molecules increases. As the average surface area per molecule is reduced, hydrophobic... 

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