Properties of Phosphatidyl Choline Lipids

It is worth noting that lipids in membranes are not covalently bound and are moving individually, diffusing laterally inside their layer (about one time per 100 ns) and, even, undergoing a flip from one leaflet to the other, although much more rarely. These data show that individual lipids behave as separate entities, even if they are subject to a collective physics. Moreover, one can expect that intramolecular bonding prevails over intermolecular interactions, at least, for neutral lipids. 

Most of the reported structural transformations of PC lipid bilayers with respect to environmental conditions such as temperature, pressure, pH, etc., are associated with isomerizations of the constituent lipid molecules (mostly 14—24 carbon acyl chains). For instance, the transition from the ordered gel to fluid liquid crystalline phase, called the main phase transition, has been related to the melting of the hydrocarbon chains in the gel phase, phospholipids with all trans alkyl chains are present, whereas in the disordered liquid crystalline phase the most populated conformational states correspond to gauche forms in the alkyl chains. 

In the last decade, accurate microcalorimetric and atomic force microscopy experiments have provided detailed information on the lipid melting processes [113-118]. Interestingly, a linear relationship between the isobaric heat capacity and the volume expansion with temperature has been observed for a variety of lipids [115-118] leading to the interpretation that proportional enthalpy and volume changes at the melting transition would be driven by intrinsic structural changes within the lipid molecules, whereas the changes of free volumes and intermolecular interactions could be considered as perturbations [116]. 

The thermograms of DMPC and longer chain lipid bilayers show a small peak at temperatures well below that of the sharp peak of the main phase transition. The general interpretation is that, when the bUayers are fully hydrated and sufficiently incubated at low temperature, this small endothermic peak (about 1 kcal mol ) is related to a pretransition between a Lc gel state and a rippled gel state (Pp ) both characterized by all trans chain conformers. This pretransition state is thus not related to molecular structural changes, but to a different packing arrangement of 

MD simulations of lipid bilayers as a function of temperature have not been extensive. The reliability of force fields is dependent on the accuracy of describing the torsional potential energies of alkanes which prompted improvements of some of the most commonly applied force fields for biomolecules [125] using recent ab initio computations of torsional potential in various trans/gauche n-alkanes (up to -decane) [126]. A few recent MD simulations have analyzed the temperature effects on models of PC lipid bUayers, mainly for long chains (16-18 C), with a reasonably successful prediction of the phase transition temperamre (error of 12-50 °C) [106, 127-130]. These studies relate the melting temperamre to the fast increase of gauche conformers in the alkyl chains. 

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