Influence of headgroup structure and hydrocarbon chain length

While the precision in the detennination of phase transition temperatures is generally excellent, so that 7 m-values obtained with different instruments by different groups agree fairly well, this is not so for the transition enthalpies A//, the other thermodynamic quantity obtainable by calorimetry. The agreement between data reported by different groups and with different instruments is usually not better than 5% for the A7/-values. There are several reasons for these discrepancies, namely different purity of samples, different preparation procedures of the liposomal suspensions, inaccuracies in sample concentrations (the concentration is usually calculated from dry weight or from phosphorous analysis), different scanning speeds and incubation times at low temperature, and finally, differences in base line subtraction procedures. 

DSC curve of DSPC showing the method of integration and separation between the transition enthalpies of the pre- and main transition. 

In general, for a series of phospholipids with identical headgroups an almost linear increase of AH with increasing chain length is observed with a value of ca. 1.92-2.1 kJ mol per CH2-group. For the transition entropies AS AH Tm, the linear increase is similar, because the changes in T , on an absolute temperature scale are small. The AS increase is 5.5 J K -mol per CH2-group [12,13,24]. 

Changes in headgroup charge also lead to changes in transition enthalpies and entropies. Doubly negatively charged PAs have significantly lower A//- and AS- 

The formation of the Ca salts of these phospholipids leads to a strong dehydration of the lipid headgroup region and the formation of liquid-crystalline La-phases occurs only at very high temperatures. In addition, in the case of PA, the binding of divalent cations can again induce the dissociation of the second proton, so that the formation at 1 1 complexes of the doubly charged fonn with a divalent cation such as occurs even at pH 7 or lower due to a shift of the apparent pKi[A -Al], 

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