Phosphatidic acid monolayers

Phosphatidic Acid Monolayers. Phosphatidic acid, prepared from egg lecithin by the action of phospholipase D, forms considerably more expanded monolayers than egg lecithin, presumably because of ionic repulsion between the phosphate groups in the phosphatidic acid mono-layers (42). Phosphatidic acid monolayers showed about four times more increase in surface potential when CaCl2 is substituted for NaCl in the subsolution than did egg lecithin monolayers (43). This again supports the conclusion that the trimethylammonium group competes with Ca2+ for the anionic phosphate group in egg lecithin monolayers (Figure 1A). 

The SUM was covered by a polymer film with an orifice of approximately 0.3 mm in diameter on each side, and subsequently a folded BLM was generated from a DPhPC/l,2-dipalmitoyl-in-glycero-3-phosphatidic acid (DPPA) monolayer on the side facing the SUM (Fig. 19). Interestingly, no pretreating of the orifice with any alkane or lipid was required, as is imperative for all other BLM techniques. Thus, an accumulation of such compounds could be excluded, and the physicochemical properties of the membrane and... 

Quinn, P.J., and Dawson, R.M.C., 1969, Interactions of cytochrome c and [ C] carboxylated cytochrome c with monolayers of phosphatidylcholine, phosphatidic acid and cardiohpin, Biochem. J. 115 65-75. 

It can be seen from Figure 1 that the choline-containing phospholipids, phosphatidylcholine and sphingomyelin are localized predominantly in the outer monolayer of the plasma membrane. The aminophospholipids, conprising phosphatidylethanolamine and phosphatidylserine, by contrast, are enriched in the cytoplasmic leaflet of the membrane (Bretcher, 1972b Rothman and Lenard, 1977 Op den Kamp, 1979). The transmembrane distribution of the minor membrane lipid components has been determined by reaction with lipid-specific antibodies (Gascard et al, 1991) and lipid hydrolases (Biitikofer et al, 1990). Such studies have shown that phosphatidic acid, phosphatidylinositol and phosphatidylinositol-4,5-fc -phosphate all resemble phosphatidylethanolamine in that about 80% of the phospholipids are localized in the cytoplasmic leaflet of the membrane. 

Monolayers of dicetyl phosphate-cholesterol follow the additivity rule for average area per molecule, whereas lecithin—cholesterol mono-layers deviate from it. The reverse is true for the additivity rule of average potential per molecule. Thus, the surface potential indicates that there is no interaction (or complex formation) between lecithin and cholesterol, but there is ion-dipole interaction between dicetyl phosphate and cholesterol as well as between phosphatidic acid and cholesterol. 

Figure 8.6. Fluorescence micrographs of a monolayer of L-a-dimyristoyl phosphatidic acid containing 2.5 mM of an NBD dye on increasing the surface pressure from the point of inflection on the isotherm, a, to about half way along the two-phase region. These results were obtained at a temperature of 10.5°C at pH 11.3 over a subphase containing 1 mM NaOH, 10 pM Na2HP04, 100 mM NaCl, 2 pM CaCI2 and 1 pm EDTA. (Reproduced from Losche, M., Duwe, H.-P. and Mohwald, H. 1988 J. Colloid Interface Sci. 126 432-44 by kind permission of the publishers and authors.)...

The first concerns the temperature effect for a monolayer of the phospholipid DMPA (I -dimyristoyl phosphatidic acid) at an air/water interface (see fig. 3.8). As the temperature is lowered the pressure at the onset of the LE-LC transition,, decreases and the corresponding area per molecule, increases. Hence, the... 

Abrahamson et at (34) observed a similar departure from the latter sequence of calcium and magnesium in their reaction with a phosphatidic acid dispersion at pH 7. Shah and Schulman (35) also found a higher tendency of Ca2+ than Mg2+ to interact with a monolayer of dicetyl phosphate at pH 5.6 as judged by surface pressure measurements although surface potential measurements indicated little difference between the two ions as did potential measurements on phosphatidyl serine mem-... 

Many monolayer studies have been made with phospholipids and sterols which have helped to clarify the details of the molecular properties of both types of molecule required to bring about the so-called condensation effect [21-23,27]. After various speculations and suggestions, it was shown that a cis double bond at the 9,10 position of the acyl chain was not necessary for condensation, and that trans isomers and even fully saturated phospholipid could bring about this condensation effect [24]. The condensation effects are not restricted to particular phospholipid classes but have been demonstrated to occur with phosphatidylcholines and phosphatidylethanolamines as well as with phosphatidic acid [25], sphingomyelin, phosphatidylglycerol and phospholipid derivatives [26]. 

Monolayer phase diagrams of lecithin/phos-phatidic acid and lecithin/cholesterol and cholesterol/ phosphatidic acid were also reported by Albrecht et al. (1981). An interesting feature in these binary monolayer systems is the indication of a lecithin/ phosphatidic acid 1 1 complex with crystalline chains. 

Fig. 2. Fluorescence micrographs of a monolayer of L-cx-dimyristoyl-phosphatidic acid (DMPA) on the water surface at surface pressures between Kq and TCs.

This approach was further extended to include monolayers of lipids, 1-octadecanol, and an investigation of the current response during the ultimate deformation of the liquid-air interface by an approaching mercury hemisphere UME probe [39], Figure 13.12 shows approach curves for O2 reduction with increasing surface pressure for (a) 1-octadecanol (C,gOH) and (b) DPPA monolayers, where DPPA is L-a-dipalmitayl phosphatidic acid. It can clearly be seen that by increasing the surface pressure of the monolayers there is a decrease in O2 flux across the interface, manifested as a decrease in current for O2 reduction at any normalized tip/interface distance, dia. 

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