Fluid-state phosphatidylcholine

Effects of the Topical Application of Fluid-State Phosphatidylcholine. 303... 

EFFECTS OF THE TOPICAL APPLICATION OF FLUID-STATE PHOSPHATIDYLCHOLINE... 

Physically, the membrane may exist in two states the "solid" gel crystalline and the "liquid" fluid crystalline states. For each type of membrane, there is a specific temperature at which one changes into the other. This is the transition temperature (Tc). The Tc is relatively high for membranes containing saturated fatty acids and low for those with unsaturated fatty acids. Thus, bilayers of phosphatidylcholine with two palmitate residues have a Tc = 41°C but that with two oleic acid residues has a Tc = -20°C. The hybrid has a Tc = -5°C. Sphingomyelin bilayer, on the other hand, may have a Tc of close to body temperature. In the gel crystalline state, the hydrophobic tails of phospholipids are ordered, whereas in the fluid crystalline state they are disordered. At body temperature, all eukaryotic membranes appear to be in the liquid crystalline state, and this is caused, in part, by the presence of unsaturated fatty acids and in part by cholesterol. The latter maintains the fatty acid side chains in the disordered state, even below the normal Tc. There is thus no evidence that membranes regulate cellular metabolic activity by changing their physical status from the gel to the fluid state,... 

Hence, the interaction between lipid molecules is very similar in these foam bilayers and it can be supposed that the AF foam bilayers are in the liquid crystalline state within the temperature range studied. This assumption is in agreement with the fact that amniotic fluid contains substantial amount of unsaturated phospholipids, which as known [45], lower considerably the temperature of the chain-melting phase transition. Bearing in mind the similarity of the phase behaviour of a phosphatidylcholine aqueous dispersion and foam bilayers [38-40], it can be supposed that at the temperatures which are important for in vivo systems, the foam bilayers are in the liquid crystalline state. This assumption allows to determine the critical concentration of phosphatidylcholines in amniotic fluid, necessary for formation of a foam bilayer by extrapolation of the Arrhenius dependence of C, for AF foam bilayers to 37°C. Thus, at 37°C C, = 19.9 jxg cm 3 and d, = 1.47. This value of C, at 37°C corresponds to the lower limit (found by other methods [46,47]) of phosphatidylcholine concentration which permits to classify as mature a sample of amniotic fluid. The above value... 

Fully hydrated bilayers composed of a single phospholipid species undergo a well-defined thermotropic phase transition (see Fig. 2.1) in which the lipid chains change from an ordered, pseudocrystalline or gel state to a fluid or liquid crystalline state which is similar to that found in biological membranes. The lipid chain configuration in the high-temperature, fluid phase is that described in the final section of the previous chapter. In the low temperature, ordered phase, the chains are essentially in the parallel, all-trans state,and may be tilted relative to the bilayer normal, the state as in phosphatidylcholines, or untilted the Lq state as in phosphatidylethanolamines. Additionally, an inter-... 

Fluidity in membranes is an easily visualized phenomenon and is a term which is widely used. However, it can be misleading. For example, it is commonly assumed (and stated) that making membrane lipids more saturated or adding cholesterol makes a membrane less fluid. The assumption is that such alterations will reduce the speed of movement of lipids. However, introduction of cholesterol into phosphatidylcholine model membranes has no effect on lateral movement and may actually increase rotational diffusion rates. What cholesterol and increased saturation do is to increase the order in the hydrocarbon matrix and this is what can be measured easily by NMR or ESR order parameters. 

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