Disorder, phospholipid alkyl chain

In aqueous systems, membrane lipids may exist in a gel-like solid state or as a two-dimensional liquid. In the case of pure phospholipids, these states interconvert at a well-defined transition temperature, Tc, that increases with alkyl chain length and decreases with introduction of alkyl chain unsaturation. In cell membranes, which have marked heterogeneity in both the polar and nonpolar domains of the bilayer, this state is described as liquid disordered . The presence of sufficient sphingolipids, with... 

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. 

Vibrational sum frequency spectroscopy in conjunction with interfacial pressure measurements provide the first direct information about the structure of phospholipid monolayers composed of DLPC, DMPC, DPPC, and DSPC, adsorbed to the interface between two immiscible liquids. Temperature controlled experiments carried out with aqueous solutions of DSPC show the lipid bilayer gel to liquid crystalline phase transition temperature to play a pivotal role in determining interfacial monolayer concentration and alkyl chain structure. Even at equivalent interfacial concentrations longer chain phosphocho-line species form more disordered monolayers with a greater number of gauche defects than shorter chain phosphocholine species, as determined from relative intensities of vibrational bands in the CH stretching region. 

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