Phosphatidyl choline group

This lamellar phase is formed of alternate sheets of lipid and water. The lipidic sheets containing the lecithin and the cholesterol are made of two superposed layers of oriented molecules. Each of these two monolayers is mixed and consists of lecithin and cholesterol molecules arranged side by side with their paraffinic ends turned toward the inside of the sheet and their polar groups (phosphatidyl choline group for lecithin and hydroxyl group for the cholesterol) outward—i.e., toward the adjacent sheet of water. This constitution of each of the two mono-layers forming the lipidic sheet is in conformity with the conclusion arising from the study of mixed monolayers of cholesterol and lecithin spread on the free surface of water (1). 

La water-lecithin is a lamellar structure in which the polar heads (the phosphatidyl choline group of lecithin) constitute two-dimensional disordered arrays in contact with water, whereas the chains are in the molten state in between water layers in disordered moieties (see Figure 1). (For a review of x-ray studies of lecithin-water phases, see Ref. 11.)... 

Phospholipid (Section 26 4) Adiacylglycerol beanng a choline phosphate head group Also known as phosphatidyl choline... 

In l,2-diacylglycerol-3-phosphate the phosphate group was esterified with choline by means of CDI to give the phosphatidyl choline ... 

The nomenclature for associating individual fatty acid groups with particular phosphodig-lyceride derivatives is straightforward. For instance, a phosphatidic acid (PA) derivative which contains two myristic acid chains is commonly called dimyristoyl phosphatidic acid (DMPA). Likewise, a PC derivative containing two palmitate chains is called dipalmitoyl phosphatidyl choline (DPPC). Other phosphodiglyceride derivatives are similarly named. 

The different phosphoglycerides are often named by placing the constituent attached to the phosphate group after phosphatidyl , e.g. phosphatidyl choline (3-in-phosphatidylcholine or l,2-diacyl-sn-glycero-3-phosphoryl-choline). There are many phosphoglycerides because of the possible variation in the fatty acid chains, and when the full chemical structure is known, it should be used (e.g. l-palmitoyl-2-oleoyl-phosphatidylcholine). Nomenclature that entails the use of the DL system should be avoided. 

Let us consider first lipid-lipid interaction. Urry et al, showed the existence of a positive CD band at 218 m/x and a negative CD band at about 192 m/z in phosphatidyl choline and phosphatidyl ethanolamine dissolved in trifluoroethanol (86). The 192-m/z band was not characterized in detail, but the 218-m/z band is of such position and shape that the addition of lipid and protein CD bands could produce a composite CD band, and hence an ORD Cotton effect, which is red shifted. As noted by Urry, the 218-m/z CD extremum of lecithin must arise from n — 7T transitions in the fatty acid ester groups. Although the optical activities of solutions of deproteinized membrane phospholipids determined at the same concentration as in the intact membrane are negligibly small, in membranes an ordered array of lipids could greatly enhance rotation. Such an effect could yield information on the nature of lipid-lipid association. This can be tested experimentally. Halobacterium cutirubrum offers a unique system since Kates has shown that the lipids in this extreme halophile contain ether bonds rather than ester bonds (43, 44), Hence, the n — tt transition essential to the CD band at 218 m/z in phospholipids does not exist. Nevertheless, we found that the ORD... 

Relation to Other Studies. Interaction of Ca++ with phosphatidyl choline (PC) mono- and bimolecular membranes has been investigated carefully by Hauser and Phillips (15) and McLaughlin (17). Using a variety of approaches such as AV, surface radioactivity, zeta potential measurements, and NMR studies, both groups of investigators concluded that Ca++ interacts with the phosphate residues of lecithin the extent of the interaction, however, is very small since it involves less than 5% of the phosphate residues (17). 

Another example is the perturbing effect of eight calcium channel blockers on membranes prepared from two different lipids [68]. The authors used total lipids from rat brain and synaptosomal membranes. The spin probe was l-palmitoyl-2-stearoyl-phosphatidyl-choline labeled at the doxyl group at the carbon-16 position (16-PC). The apparent order parameter, S, is calculated from the apparent outer (Amax) and inner (Amin) splittings which were directly taken from the ESR spectra. It is used to describe the relative efficiency of the dmgs in perturbing the lipid membrane. 

In the case of phosphatides, we have made the assumption that one phosphatidyl-choline is equivalent to 2 hydroxyl groups. The following Table XX gives the results of... 

Lipid bilayers have been studied in vesicles of about 500 A diameter. The bilayers can be made of many lipids. The most common lipid is lecithin, phosphatidyl choline (PC). Packing the lipids in the vesicle results in two-thirds of the lipid head groups on the external face about one-third on the internal face. The head group of the lipid PC is phosphocholine -O-POJ-O-CH2-CH2N (013)3 and the head group is studied by P, H, D or C NMR and the long fatty chains by C, H, or D NMR. Conformational studies of the molecules by conventional de-coupling, nuclear Overhauser effects and by Ln(III) probes... 

Before looking at the reconstituted membranes in more detail, we shall first discuss a simpler question concerning the distribution of ions in the aqueous phase of bilayer structures. Is the ion distribution in the water phase of typically 15 A width between the lipid surfaces homogenous, or is there a preferential binding to the polar head groups of the lipids These studies have been started in collaboration with G. Biildt and some preliminary results from dipalmitoyl phosphatidyl cholin (DPPC) membranes will be reported here. ... 

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