Chain length phospholipid monolayers

The phase behavior of monolayers is determined by the molecular structure of the am-phiphile and the conditions of the subphase. Phospholipids, for example, attract each other because of van der Waals interactions between the alkyl chains. The longer the alkyl chains, the more strongly the phospholipids attract each other. Thus, the LE-LC transition pressure will decrease with increasing chain length (at constant temperature). Double bonds in the alkyl chains increase this phase transition pressure. Charges and oriented dipole moments (see Chapter 6) in the headgroups, lead to a repulsion between the phopholipids and increase the pressure at which the transition occurs. Salts in the subphase, screen this repulsion and decrease the transition pressure. 

Since the solidity or fluidity of the bilayer membrane is likely to depend on the alkyl chain interactions and consequently their length, an understanding of the relationship between chain order and chain length for tightly packed monolayers of phospholipids are important. As an example of how VSFS can be employed to study phospholipids at a liquid surface, a series of saturated symmetric chain phosphatidylcholines (PCs) were examined at the air/water and CCfr/water interfaces [49]. At the air/D20 interface, chain order within the monolayer was found to increase as the length of the chains increased (Figure 2.9a) under conditions of constant phospholipid head group area. 

Physical Methods. H.p.l.c. procedures for the separation and assay of ubiquinone and homologues278-280 and of menaquinone cis- and fra/ts-isomers, 2,3-epoxides, and chain-length homologues281 282 have been described. A XH n.m.r. study has been reported283 of the location and motion of ubiquinones in perdeuteriated phosphatidylcholine bilayers. Other aspects of the interaction of ubiquinone with phospholipid monolayers have been studied.284... 

The focus will be mainly on monolayers of fatty acids or alcohols and of phospholipids, because these are the simplest systems and have been the subject of the most recent research. These substances are similar in that their hydrophobic tails consist of long-chain hydrocarbons there are two chains per phospholipid and one per acid or alcohol, as shown in Table I. The chain lengths typically range from 13 to 20 carbons, the lower limit being... 

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. 

Free sterols are almost always found mostly associated with membranous subcellular fractions and the majority of the free sterol is widely found in the plasma membrane as documented recently in animals, plants and fungi. The thickness of the common phospholipid monolayer is about 2.1 nm. Since, as with the phospholipid Itself, one would expect the polar end of the sterol to orient toward the aqueous Interface and the nonpolar side chain to become Imbedded in the alkyl groups of the fatty acyl moieties of the phospholipid which point toward the nonpolar side of the other monolayer, it is satisfying that the overall length of natural sterols centers on the long dimension of cholesterol which is virtually the same (ca. 2.1 nm) as that of the mono-layer. Is this coincidence or cause and effect ... 

In addition to acyl chains, the phospholipid headgroups should contribute to the stability of phospholipid bilayers as reflected by the considerably higher melting points (200°C) of pure phospholipids than of fatty acids (60°C). Also, the type of headgroup is important for the properties of a phospholipid membrane. For example, monolayers of phosphatidylethanolamine are less expanded than those derived from phosphatidylcholine and the melting point of the solid, pure phospholipid is lower for the former than for the latter, irrespective of the length or unsaturation of the fatty acids (Williams and Chapman, 1970). 

The current opinion, widely held, is that all biological membranes, including mammalian plasma membranes, have as a structural framework a phospholipid bilayer of which the characteristic feature is a parallel array of hydrocarbon chains, averaging 16 carbon atoms in length. This bilayer has some of the properties of a two-dimensional fluid in which individual lipid molecules can diffuse rapidly in the plane of their own monolayer, but cannot easily pass into the other monolayer. This lipid matrix provides the basic structure of the membrane. Whereas some protein molecules cover part of the membrane, particularly its outer surface, other protein strands penetrate the lipid layer, every here and there, and some of these strands are bunched together to form water-filled tubes or pores (Wallach and Zahler, 1966). These proteins are responsible for most of the membrane s functions, e.g. receiving and transduc-... 

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