Phospholipids pressure-area isotherms

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... 

Neutron reflectometry studies on mixed DPPC/oleic acid monolayers have been conducted using the CRISP reflectometer at RAL. First, the stmcture of DPPC monolayers was determined by measuring reflectivity profiles from three different isotopic forms of the DPPC monolayer system. This was achieved using hydrogenated (h-DPPC) and chain perdeuterated (d-DPPC) phospholipids and two different subphases of D2O and ACMW. The monolayers were studied at three surface coverages of approximately 50, 60, and 70 A /molecule. Examination of the surface pressure-area isotherm reveals that the main LE/LC phase transition for DPPC monolayers occurs over this range of molecular area (Lewis and Hadgraft, 1990). 

Figure 2. Surface pressure-area isotherm for fatty acids and phospholipids. The inset shows the behavior at large molecular areas. Four one-phase regions are identified gaseous, liquid expanded, liquid condensed, and solid. This isotherm is typical of pentadecanoic acid at room temperature. For a higher-molecular weight acid, such as octadecanoic acid, there is no LE region at room temperature the isotherm does not rise steeply until an area of about 25 A. ...

The stability and miscibility of lipid-heme with phospholipid could also be confirmed from surface pressure-surface area isotherms of the lipid monolayer on a water surface. For the heme 1, the curve shifted to right hand of the curve of the phospholipid mono-layer itself, which reveals no good packing of the lipid molecules in the monolayer film. Against this, the curve for the lipid-heme-embedded monolayer film coincided with that for the lipid film itself. 

This chapter is basically divided in two parts, namely, the study of surface pressure-molecular area (jr — A) isotherms of phospholipids at ITIES and their effect on ion transfer. In the first part, the emphasis is put on topics which have been left out from Ref [5], i.e., Langmuir film techniques and theoretical modeling of jr — isotherms, as well as on the latest progress in the field, especially on experiments that combine Langmuir techni-... 

Figure 3.67. AFM image of a dipalmitoylphosphatidylcholine (DPPC) monolayer transferred onto a quartz plate at a surface pressure of 30 mN m. On this hydrophilic substrate the phospholipids have their head groups on the surface. Therefore, the bright spots should correspond to the end-methyl groups of the DPPC hydrocarbon chains. This is corroborated by the finding that the area per bright spot (averaged over many images) corresponds to half of the value for the area per phospholipid molecule as found from the r(A) isotherm at 30 mN m. (Courtesy of X. Zhai and J.M. Kleijn" )...

Isotherms (interface pressure n [mN/m] area per molecule A [A ]) are usually measured by continuously compressing the film, which has been prepared on top of an electrolytic subphase. In this experiment, the interface pressure k A) = 7o - y A) [i.e. the difference of surface tensions without (y ) and with an interface film] is determined by monitoring the surface tension y A). For the phospholipid L-a-dimyristoyl phosphatidic acid (DMPA) a set of data at pH = 6 and various temperatures of the subphase was measured by Albrecht et al.9 and is reproduced in Fig.l. 

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. 

---