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

Two selected unsaturated lipids, dioctadecadienoylphosphatidylcholine (DODP C) and dioleoylphosphatidylethanolamine (DOPE), undergo "solvation-induced" transitions at room temperature upon progressive hydration, varied via amb ient 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 f rom a frozen, quasi-crystalline to a "melted" state. The partial molar enth alpy and partial molar entropy of water have been determined without signif icant interference of chain melting by a new adaptation of adsorption calor imetry, 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 li pid-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 wi th high resolution. In most cases, the response possesses exponential chara cter. Despite several hypotheses that consider the adsorption process or di ffusive transport of water, there is no straightforward interpretation of h ydration kinetics so far. Hydration/dehydration hystereses were discussed i n terms of metastability effects that accompany the formation of a complex structure in the headgroup region of the lipid aggregates.