Calorimetric and spectroscopic studies of the thermotropic phase behavior of lipid bilayer model membranes composed of a homologous series of linear saturated phosphatidylserines

The thermotropic phase behavior of lipid bilayer model membranes composed o f the even-numbered, N-saturated 1,2-diacyl phosphatidylserines was studied by differential scanning calorimetry and by Fourier-transform infrared and P-31-nuclear magnetic resonance spectroscopy. At pH 7.0, 0.1 M NaCl and in the absence of divalent cations, aqueous dispersions of these lipids, whic h have not been incubated at low temperature, exhibit a single calorimetric ally detectable phase transition that is fully reversible, highly cooperati ve, and relatively energetic, and the transition temperatures and enthalpie s increase progressively with increases in hydrocarbon chain length. Our sp ectroscopic observations confirm that this thermal event is a lamellar gel (L-beta)-to-lamellar liquid crystalline (L-alpha) phase transition. However , after low temperature incubation, the L-beta/L-alpha phase transition of dilauroyl phosphatidylserine is replaced by a higher temperature, more enth alpic, and less cooperative phase transition, and an additional lower tempe rature, less enthalpic, and less cooperative phase transition appears in th e longer chain phosphatidylserines. Our spectroscopic results indicate that this change in thermotropic phase behavior when incubated at low temperatu res results from the conversion of the L-beta phase to a highly ordered lam ellar crystalline (L-c) phase. Upon heating, the L-c phase of dilauroyl pho sphatidylserine converts directly to the L-alpha phase at a temperature sli ghtly higher than that of its original L-beta/L-alpha phase transition. Cal orimetrically, this process is manifested by a less cooperative but conside rably more energetic, higher-temperature phase transition, which replaces t he weaker L-beta/L-alpha phase transition alluded to above. However, with t he longer chain compounds, the L-c phase first converts to the L-beta phase at temperatures some 10-25 degrees C below that at which the L-beta phase converts to the L-alpha phase. Our results also suggest that shorter chain homologues form L-c phases that are structurally related to, but more order ed than, those formed by the longer chain homologues, but that these L-c ph ases are less ordered than those formed by other phospholipids. These studi es also suggest that polar/apolar interfaces of the phosphatidylserine bila yers are more hydrated than those of other glycerolipid bilayers, possibly because of interactions between the polar headgroup and carbonyl groups of the fatty acyl chains.