M.C. Phillips

Monolayer characteristics of saturated 1,2,-diacyl phosphatidylcholines (lecithins) and phosphatidylethanolamines at the air-water interface.

Abstract Surface pressure—area data have been obtained for the homologous series of saturated 1,2-diacyl phosphatidylcholines and phosphatidylethanolamines at the air—water interface. The results are compared with data already in the literature and the various physical states of the monolayers are described. The lecithins formed more expanded films than the phosphatidylethanolamines and this is interpreted in terms of differences in the size and orientation of the polar groups. The heats and entropies associated with the transition from condensed to liquid-expanded film were calculated for dipalmitoyl lecithin. The values of these themodynamic parameters were similar to those observed for the transition from gel to smectic mesophase for this lecithin. This transition occuring in the bimolecular lamellae in water corresponds to the transition from condensed to expanded monolayer.

Molecular interactions in mixed lecithin systems

Abstract The effects of mixing synthetic 1,2-diacyl lecithins having different chain lengths have been studied by differential scanning calorimetry and monolayer techniques, and their phase behaviour in excess water investigated. It is shown that, when the chain lengths of the two components are similar, co crystallisation and ideal mixing occurs. When the chain length or unsaturation of the two components is very different monotectic behaviour is observed. In the latter case the liquid crystalline transition of the higher melting component becomes broader and is at a reduced temperature. This effect is due to the highly disordered liquid hydrocarbon chains of one component causing an increase in the kinetic motions of the ordered chains of the second component. Monolayer results indicate that the molecular area occupied by the higher melting component becomes larger because of the increased configurational freedom of its chains. Such effects occur within natural lipid extracts and the significance of this is discussed, particularly with respect to the effect of temperature on lipids and membranes.

Differences between conformations of lecithin and phosphatidylethanolamine polar groups and their effects on interactions of phospholipid bilayer membranes.

Abstract The preferred orientation of the polar group in lecithin has the zwitterion extended normal to the bilayers so that the end-group contribution to the X-ray long spacing is about 11 A. This conformation contributes a repulsive dipolar energy to the interactions of lecithin molecules within and across bilayers, helping to produce a rapid uptake of water to shield the mutually repelling dipoles. In contrast, there is a net neutralisation of charge between the polar groups of phosphatidylethanolamine, which are arranged so that the zwitterions are either approximately tangential to the plane of the bilayer, or interdigitated with opposing groups in a multilamellar system (net contribution 8 A to the X-ray long spacing). The resulting cohesion makes phosphatidylethanolamines difficult to hydrate. The differences in zwitterion conformation, and hence hydration and motional freedom, result in the different stabilities of lecithin and phosphatidylethanolamine dispersions.

Mixed monolayers of phospholipids and cholesterol

1. n1. The force-area characteristics of mixed monolayers of cholesterol with synthetic lecithins and phosphatidylethanolamines are presented. n n2. n2. The nature of the fatty acid constituent of the phospholipid and the temperature are important since the action of cholesterol is related to the physical state of the pure phospholipid monolayer. n n3. n3. Generally an expanded monolayer is condensed by the addition of cholesterol and this is associated with an effect upon the hydrocarbon chain mobility. This is greatest when the phospholipid is close to the transition temperature for the change from condensed to expanded monolayer. In condensed monolayers the hydrocarbon chain fluidity is reduced and cholesterol does not have a large effect. n n4. n4. Cholesterol effectively reduces the transition temperture by disrupting the cooperative movements of the hydrocarbon chains. n n5. n5. The position of cis double bonds in unsaturated phospholipids is not important for condensation effects with cholesterol; furthermore the presence of a double bond is not a necessary condition for such effects. n n6. n6. Specific molecular structures are not necessary for a condensation with cholesterol and a complex is not formed.

Monolayer characteristics of some 1,2-diacyl, 1-alkyl-2-acyl and 1,2-dialkyl phospholipids at the air-water interface

Abstract Surface pressure and surface potential-molecular area data have been obtained for some 1,2-diacyl, 1-alkyl-2-acyl and 1,2-dialkyl phospholipids at the air-water interface. Replacement of the ester linkages in lecithins by ether links has no significant effect upon the molecular packing in fully expanded or condensed monolayers and only a small effect upon the phase transition from condensed to expanded monolayer. Analogous effects are predicted for lecithins dispersed in excess water. Ether phospholipids have surface potentials which are 30–200 mV lower than those of the corresponding ester compound. This occurs because the carbonyl dipoles play a large role in determining the surface potential of phospholipid monolayers. Estimates are derived for the orientation of the carbonyl groups within the film at various molecular areas. It is concluded that, during the crystallisation of phospholipids in the transitions from expanded to condensed monolayer and liquid crystal to gel phase, the carbonyl groups are forced further out of the plane of the monolayer or bilayer. The biological significance of these findings is discussed.

The inter- and intra-molecular mixing of hydrocarbon chains in lecithin-water systems.

Abstract When pure 1,2 diacyl lecithins containing a different type of hydrocarbon chain in each position of the molecule (intra-molecular chain mixing) are dispersed in excess water, the gel to liquid crystal transition is sharp, as with pure lecithins containing a single type of hydrocarbon chain. The total heat of transition is lower than that observed when the same chain composition is distributed in a mixture of two different lecithins each containing only one type of hydrocarbon chain (inter-molecular mixing). In the latter systems broad transitions are observed, and whenever the experimental temperature is not above that of the transition range clustering of molecules occurs. Bilayers of the isolated lipids of some biological membranes exhibit such clustering at the environmental temperature of the membrane.

Differences in the interaction of inorganic and organic (hydrophobic) cations with phosphatidylserine membranes

The interaction of phosphatidylserine dispersions with hydrophobic, organic cations (acetylcholine, tetraethylammonium ion) is compared with that of simple inorganic cations (Na+, Ca2+); differences in the hydration properties of the two classes of ions exist in the bulk phase as evident from spin-lattice relaxation time T1 measurements. It is shown that the reaction products (cation-phospholipid) differ markedly in their physicochemical behaviour. With increasing concentration both classes of ions reduce the zota-potential of phosphatidylserine surfaces, the monovalent inorganic cations being only slightly more effective than the hydrophobic cations. Inorganic cations cause precipitation of the lipid once the surface charge of the bilayer is reduced to a certain threshold value. This is not the case with the organic cations. The difference is probably associated with the different hydration properties of the resulting complexes. Thus binding of Ca2+ causes displacement of water of hydration and formation of an anhydrous, hydrophobic calcium-phosphatidylserine complex which is insoluble in water, whereas the product of binding of the organic cations is hydrated, hydrophilic and water soluble. The above findings are consistent with NMR results which show that the phosphodiester group is involved in the binding of both classes of cations as well as being the site of the primary hydration shell. Besides affecting interbilayer membrane interactions such as those involved in cell adhesion and membrane fusion, the binding of both classes of cation can affect the molecular packing within a bilayer.

Monolayer characteristic of some glycolipids at the air-water interface

Abstract Surface pressure and surface potential versus molecular area data have been obtained for some galactosyldiglycerides and some galactosphingolipids at the air-water interface. The physical states of galactolipid monolayers (and bilayers) parallel those of the phospholipids. The molecular packing of the monolayers is determined primarily by the interactions between the hydrocarbon chains and chain melting causes the transition from condensed to expanded monolayer. Thus the long saturated chain cerebrosides from myelin have high chain-melting temperatures and form condensed monolayers with the chains in a quasi-crystalline array. The galactosyldiglycerides from chloroplast membranes contain polyunsaturated chains and from liquid-expanded monolayers. The surface potentials of monolayers of neutral galactosyldiglycerides are similar to those equivalent lecithins; the contributions of the hydrated galactose and phosphorylcholine moeities to the surface potential are approximately equal. The various galactosphingolipid monolayers studied have quite different surface potentials; this indicates that relatively small variations in molecular structure which do not lead to appreciable changes in the average packing density can cause large changes in surface potential.

Factors affecting molecular packing in mixed lipid monolayers and bilayers

Abstract The nature of the molecular interactions and the factors determining molecular packing in mixed phospholipid/glyceride monolayers and bilayers were investigated by monolayer and nuclear magnetic resonance (NMR) techniques. Force-area curves were obtained at various temperatures for monolayers, at the air-water interface, of synthetic lecithins and a phosphatidyl ethanolamine mixed with di- and triglycerides in different molar ratios. The linewidths of peaks in the high resolution NMR spectra of lecithin/glyceride co-dispersions in excess water at different temperatures were used to obtain information about molecular mobilities. It was found that the molecular packing in mixed lipid monolayers and bilayers is determined by the following factors: (1) Whether lipid chains are above or below their melting point ( T C ). (2) The difference between experimental temperature and T C : the larger the difference, the smaller the effect of one component on the other. (3) The degree of similarity of the chains of the components; this influences the degree of cooperativity of chain motions and the degree of mixing of the components. (4) The nature, orientation, mutual interaction and degree of hydration of the polar groups. It is shown that mean molecular area does not always reflect the state of chain motions in lipid films, because of heterogeneity of motion and structure along the molecules. Cooperativity of motion may reduce steric requirements; other effects which are of particular importance for lecithins are interactions of zwitterions, and the influence of polar group hydration.

Structural investigations of lipid, polypeptide and protein multilayers

Abstract Langmuir-Blodgett multilayers of lipids, polypeptides and proteins have been examined by X-ray diffraction and infrared spectroscopic methods. The complex polymorphism exhibited by multilayers of glycerides and various phospholipids of different chain length mirror those shown in other three-dimensional structures and suggest that multilayers of lipids can be considered as oriented “crystals”. Both the α and β types of hdyrocarbon chain packing are adopted by different classes of lipids in multilayers. Stable multilayers of the synthetic polypeptide poly-γ-benzyl- l -glutamate consist of α-helical rods stacked in an hexagonal array with a rod axis separation of 14.2 A. Poly-γ-methyl- l -glutamate behaves similarly but little structural information could be derived from potentially non-helical or sheet-like structures formed by other homopolypeptides. The observation of a single, invariant diffraction line at 9.3 A for multilayers of a number of water-soluble proteins is consistent with the occurrence of extensive structural reorganization (uncoiling, denaturation) at the air-water interface.