Thermodynamic and kinetic aspects of lyotropic solvation-induced transitions in phosphatidylcholine and phosphatidylethanolamine assemblies revealed by humidity titration calorimetry

Two selected unsaturated lipids, dioctadecadienoylphosphatidylcholine (DODPC) and dioleoylphosphatidylethanolamine (DOPE), undergo "solvation-induced" transitions at room temperature upon progressive hydration, varied via ambient relative humidity (RH), as demonstrated in the preceding first part of this study. These transitions are induced by the uptake of about one water molecule per lipid molecule in each case and change the lipid headgroups from a frozen, quasi-crystalline to a "melted" state. The partial molar enthalpy and partial molar entropy of water have been determined without significant interference of chain melting by a new adaptation of adsorption calorimetry, called humidity-titration calorimetry, coupled with gravimetry. The potency to form direct hydrogen bonds between the lipid headgroups decides whether the solvation-induced transition is exothermic (as in DODPC, no lipid-lipid H bonds) or endothermic (as in DOPE, lipid-lipid H bonds present). Consequently, the solvation-induced transition in DOPE is entropy-driven, and that in DODPC is enthalpy-driven. The time response of the calorimeter after a stepwise change of RH allows to study the kinetics of hydration with high resolution. In most cases, the response possesses exponential character. Despite several hypotheses that consider the adsorption process or diffusive transport of water, there is no straightforward interpretation of hydration kinetics so far. Hydration/dehydration hystereses were discussed in terms of metastability effects that accompany the formation of a complex structure in the headgroup region of the lipid aggregates.