Membrane lipid bilayers acyl chain packing

The betulinic acid level in the E. florida leaves increased significantly in the May, June, Jully (autumn - winter) and, September, October and November (winter) which was mainly due to the accumulation of this compound in vegetal tissue. Some authors related with the pentacyclic triterpenes, just as betulinic, acid ursolic, acid, p-amyrine and lupeol, are supposed to be toxic to insects, due to their ability to inhibit acyl chain packing in the lipid bilayers of the insect membranes [Rodriguez et al, 1997 Prades et al., 2011]. 

Another factor affecting the lifetime of a membrane fluorophore probe is its proximity to the surface. The lifetimes of the DPH, DPH-phosphatidyl-choline (DPH-PC), and trimethylammonium-DPH (TMA-DPH) probes decrease in the order DPH > DPH-PC > TMA-DPH, as the probe locates nearer to the surface of the lipid bilayer.(7) The same is found for the anthroyl-stearate probes.(8) More recently, it has been shown that with TMA-DPH, the lifetime appears to be fairly sensitive to the differences in lipid bilayer packing induced by differing degrees of unsaturation in the phospholipid fatty acyl chains.(9) This aspect of the use of TMA-DPH and possibly other probes remains to be further exploited. 

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

Inclusion of other molecules of irregular shape within membranes also lowers Tm. However, a molecule of cholesterol can pack into a bilayer with a cross-sectional area of 0.39 nm2, just equal to that of two hydrocarbon chains.49 It tends to harden membranes above Tm but increases mobility of hydrocarbon chains below Tm.97 -100 A complex of cholesterol and phosphatidylcholine may form a separate phase within the membrane.101102 The ether-linked plasmalogens may account for over 30% of the phosphoglycerides of the white matter of the brain and of heart and ether linked phospholipids are the major lipids of many anaerobic bacteria.103 Their Tm values are a few degrees higher than those of the corresponding acyl phospholipids.104... 

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