Compressibility, phospholipid monolayers

As already noted, when the LE phase is expanded, bubbles of gas grow and thin lamellae of fluid are left between them, forming a two-dimensional foam structure (Fig. 10a) in which the size of the cells and the distribution of the number of sides changes with time. Dendritic structures (Fig. 21) are observed in the LE-LC transition region when the monolayer is compressed, or in temperature quenches from the LE one-phase region into the two-phase region. In the case of fatty acids,only circular islands are observed at low temperature, even with rapid compression. The temperature threshold for the appearance of dendritic patterns is quite sharp. Such structures are also found in phospholipid monolayers. ... 

Monolayers of distearoyl lecithin at hydrocarbon/water interfaces undergo temperature and fatty acid chain length dependent phase separation. In addition to these variables, it is shown here that the area and surface pressure at which phase separation begins also depend upon the structure of the hydrocarbon solvent of the hydrocarbon oil/aqueous solution interfacial system. Although the two-dimensional heats of transition for these phase separations depend little on the structure of the hydrocarbon solvent, the work of compression required to bring the monomolecular film to the state at which phase separation begins depends markedly upon the hydrocarbon solvent. Clearly any model for the behavior of phospholipid monolayers at hydrocarbon/water interfaces must account not only for the structure of the phospholipid but also for the influence of the medium in which the phospholipid hydrocarbon chains are immersed. 

By using a surface radioactivity technique, the penetration of the hydrophobic and flexible 1-14C-acetyl--casein and the rigid and globular 1-14C-acetyl-lysozyme molecules into phospholipid monolayers in different physical states was monitored. The adsorption of ff-casein to lecithin mono-layers is described by a model in which it is assumed that the protein condenses the lecithin molecules so that the degree of penetration is a function of the lateral compressibility of the phospholipid monolayer. The interaction of ff-casein with phospholipid monolayers is dominated by the hydrophobicity of the macromolecule, but lysozyme tends to accumulate mostly beneath phospholipid monolayers in this situation, electrostatic interactions between the lipid and protein are important. 

Film pressure change An vs. time for a phospholipid monolayer, 1 - compression with lOcm/min, 2 - oscillation with f = 0.05 Hz, 3 - one oscillation in higher time resolution, 4 - phase lag tick, according to Kretzschmar (1988)... 

As an example of a membrane model, phospholipid monolayers with negative charge of different density were used. It had already been found ( ) and discussed O) that the physical and biological behavior of phospholipid monolayers at air-water interfaces and of suspensions of liposomes are comparable if the monolayer is in a condensed state. Two complementary methods of surface measurements (using radioactivity and electrochemical measurements), were used to investigate the adsorption and the dynamic properties of the adsorbed prothrombin on the phospholipid monolayers. Two different interfaces, air-water and mercury-water, were examined. In this review, the behavior of prothrombin at these interfaces, in the presence of phospholipid monolayers, is presented as compared with its behavior in the absence of phospholipids. An excess of lipid of different compositions of phos-phatidylserine (PS) and phosphatidylcholine (PC) was spread over an aqueous phase so as to form a condensed monolayer, then the proteins were inject underneath the monolayer in the presence or in the absence of Ca. The adsorption occurs in situ and under static conditions. The excess of lipid ensured a fully compressed monolayer in equilibrium with the collapsed excess lipid layers. The contribution of this excess of lipid to protein adsorption was negligible and there was no effect at all on the electrode measurements. 

A consideration often overlooked in BAM studies is the possible influence of the compression rate on the domain structures. In the case of A-acylamino acid monolayers, the associations due to amide-amide hydrogen bonding are very strong and promote rapid domain growth and also make it unlikely that relaxation to an equilibrium domain shape can occur on any realistic experimental timeframe. Domain shape relaxation kinetics are noted to be dependent on the strength of intermolecular forces for example, dendritic condensed-phase domains formed in a phospholipid monolayer required 5 h to relax to equilibrium shapes and, for the phospholipid DMPE, compression rates as slow as 0.2 per molecule per minute were needed to observe equilibrium domain shapes. Examination of the variation of domain structure with time after then-formation or with compression rates are not commonly reported however, it is advisable to consider examining these variables when carrying out BAM experiments. 

S-layer-stabilized solid-supported lipid membranes have also been fabricated as follows (Figure 15) [21], After compressing a phospholipid monolayer on a Langmuir trough into the... 

The insertion of the polymers into monolayers was also studied by comparing the compression isotherms. Lipid monolayers were formed on 0. IM sodium acetate buffer (pH 7.4) subphase in the presence or absence of polymers in a rectangular Teflon cuvette of 28.5 cm x 17.5 cm. After 10 min stabilisation period, a Teflon barrier compressed the monolayer at a speed of 4.2 cm/min and surface pressure vs. area (tc vs. A) isotherms were recorded. Data are summarised in Table 11. We observed no changes in the shape of area vs. pressure curves obtained in the presence of polymers in the subphase (data not shown), but polymers induced an expansion of the monolayer. These changes were detected at various surface pressures (10, 20, 30, 40 and 50 mN/m) and are expressed as area/molecule of phospholipid values (Table 2). These values indicate significant differences in the interaction of polycationic and amphoteric/polyanionic polypeptides. Marked expansion of DPPC monolayer occurred in the presence of SAK or AK (AA=0.23-0.51), while EAK and Ac-EAK initiated only moderate changes in this parameter (AA=0.01-0.13). The effect of polylysine and OAK was negligible (AA=0.01-0.03). 

In DPPC or DPPC/PG monolayer experiments changes in surface pressure (penetration kinetics) and area/phospholipid molecule values (compression isotherms) indicated similar differences in expansion of membranes. It can be concluded that the effect of polymeric polypeptides on phospholipid monolayers depends not only on the polymer charge (positive/negative, neutral), but also on charge density. 

Unlike electron and scanning tunneling microscopy, the use of fluorescent dyes in monolayers at the air-water interface allows the use of contrast imaging to view the monolayer in situ during compression and expansion of the film. Under ideal circumstances, one may observe the changes in monolayer phase and the formation of specific aggregate domains as the film is compressed. This technique has been used to visualize phase changes in monolayers of chiral phospholipids (McConnell et al, 1984, 1986 Weis and McConnell, 1984 Keller et al., 1986 McConnell and Moy, 1988) and achiral fatty acids (Moore et al., 1986). 

Cardiolipin Monolayers. Among various phospholipids studied by monolayer techniques, only cardiolipin (41) and phosphatidylserine (36) monolayers show significant condensation of their surface pressure-area curves in the presence of divalent as compared with monovalent cations in the subsolution. The condensation of cardiolipin is explained by the decrease in molecular area caused by the attraction between a divalent cation and the two phosphate groups in the molecule. This condensation is eliminated when the ratio of monovalent to divalent cations is greater than 5 to 1. At high surface pressures, the difference in the compressibility of cardiolipin monolayers correlates with the ionic radii of the metal ions (Mg2+ < Ca2+ < Sr2+ < Ba2+). 

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. 

Vibrational modes of the phospholipid polar head groups (in particular the symmetric and anti-symmetric PO stretching vibration) reflect their ionization and hydration state. The hydration state of the head group of DPPC was found to change under monolayer compression or by addition of cations such as Ca + There are indications that the transition at Ttg (to the solid state S, see fig. 3.6) involves ordering and dehydration of the head groups. 

---