DIFFERENTIAL SCANNING CALORIMETRY OF THERMOTROPIC PHASE-TRANSITIONS IN VITAMINYLATED LIPIDS - AQUEOUS DISPERSIONS OF N-BIOTINYL PHOSPHATIDYLETHANOLAMINES

The thermotropic phase behavior of a homologous series of saturated di acyl phosphatidylethanolamines in which the headgroup is N-derivatized with biotin has been investigated by differential scanning calorimetr y. In 1 M NaCl, derivatives with acyl chainlengths from C(12:0) to C(2 0:0) all exhibit sharp chain-melting phase transitions, which are reve rsible with a hysteresis of 1.5 degrees or less, except for the C(12:0 ) lipid which has a transition temperature below 0 degrees C. The tran sition enthalpy and the transition entropy depend approximately linear ly on the lipid chainlength, with incremental values per CH, group tha t are very similar to those obtained for the corresponding underivatiz ed phosphatidylethanolamines in aqueous dispersion. The chainlength-in dependent contribution to the transition enthalpy is significantly sma ller than that for the underivatized phosphatidylethanolamines, and th at for the transition entropy is much smaller, the latter suggesting t hat the N-biotinylated phosphatidylethanolamine headgroups are differe ntly hydrated from those of the underivatized lipids. The gel-to-fluid phase transition temperatures of the N-biotinylated lipids are lower than those of the parent phosphatidylethanolamines, and their chainlen gth dependence conforms well with that predicted by assuming that the transition enthalpy and entropy are linearly dependent on chainlength. Although the chain-melting phase behavior is generally similar to tha t of the parent phosphatidylethanolamines, the gel phases (and the flu id phases in the case of chainlengths C(12:0) to C(16:0)) have a diffe rent lyotropic structure in the two cases, and this is reflected in th e chainlength-independent contributions to the thermodynamic parameter s. In the absence of salt, the thermotropic phase behavior of aqueous dispersions of the N-biotinyl phosphatidylethanolamines is considerabl y more complex. The transition temperatures are consistently lower tha n those in 1 M NaCl, but the transitions are broader, contain multiple peaks and exhibit a much larger hysteresis between heating and coolin g scans. Additionally, the lipids with shorter chainlengths exhibit me tastability in the absence of salt, converting from a micellar solutio n to a lamellar gel phase only after incubation at low temperature wit h freeze-thaw cycling.