A calorimetric study of phospholipid hydration. Simultaneous monitoring of enthalpy and free energy

We present a novel method for monitoring isothermal lipid hydration using a sorption microcalorimeter. A measuring cell of the double-twin calorimeter consists of two vessels connected by a stainless steel tube. The upper vessel contains pure water, and the bottom vessel is loaded with the lipid sample. This calorimeter allows for simultaneous measurement of the partial molar enthalpy and the chemical potential (or the partial molar free energy) of the water. The versatility of the method is demonstrated by studies of the hydration of the phospholipids dipalmitoyl phosphatyl choline (DPPC), dimyristoyl phosphatidyl choline (DMPC), and dilauroyl phosphatidyle choline (DLPC) at 25 and 27 C. The measurements provide a relation between water content and water chemical potential, which, in these lamellar systems, is often recast as a force-distance relation and has been called the hydration force. Through the simultaneously monitored calorimetric values, the partial molar enthalpy of water is also obtained. The method consequently provides a rather unique combination of information on both partial molar enthalpy and partial molar free energy and thus also the partial molar entropy of the process. We find that the incorporation of the first three to four water molecules per lipid is exothermic. These water molecules presumably interact directly with oxygen atoms on the phosphate of the lipid headgroup. When the first waters have been added, the remaining ones are incorporated endothermically. This applies to the water molecules taken up both in the gel phase and in the liquid crystalline state. We also observe that the sorption process triggers a first-order phase change from a gel (L') to a liquid crystalline (L) phase. For DLPC, this occurs at 25 C at a relative humidity of 0.79 with an endothermic transition enthalpy of 42 ± 2 kJ/mol (DLPC) and, for DMPC, at 27 C at 0.93 relative humidity with H = 56 ± 5 kJ/mol (DMPC). We use a previously established model to quantitatively interpret these phase transitions. Furthermore, the observed endothermic nature of the sorption process above three to four waters per lipid is fully consistent with the suggestion that the negative free energy of the sorption (swelling) is due to increased thermal excitations and thus a positive entropy. It is more problematic to reconcile the data with models proposing structuring effects in the water as the main cause of the swelling