Hydration and lyotropic melting of amphiphilic molecules: A thermodynamic study using humidity titration calorimetry

The hydration of the lipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) a nd of the cationic detergent dodecyltrimethylammonium bromide (DTAB) has be en studied by means of isothermal titration calorimetry (ITC), gravimetry, and infrared (IR) spectroscopy. During the experiments films of the amphiph iles are perfused by an inert gas of variable relative humidity. The measur ement of adsorption heats using ITC represents a new adaptation of adsorpti on calorimetry which has been called the humidity titration technique. This method yields the partial molar enthalpy of water upon adsorption. It is f ound to be endothermic with respect to the molar enthalpy of water on conde nsation for the water molecules which interact directly with the headgroups of POPC and DTAB. Consequently, the spontaneous hydration of the amphiphil es is entropy driven in an aqueous environment. IR spectroscopy shows that hydration is accompanied by the increase in the conformational and/or motio nal freedom of the amphiphilic molecules upon water binding, In particular, a lyotropic chain melting transition is induced at a certain characteristi c relative humidity. This event is paralleled by the adsorption of water, T he corresponding exothermic adsorption heat is consumed completely (POPC) o r partially (DTAB) by the hydrocarbon chains upon melting. Differential sca nning calorimetry was used as an independent method to determine transition enthalpies of the amphiphiles at a definite hydration degree. Water bindin g onto the headgroups is discussed in terms of hydrogen bonding and polar i nteractions. The adsorption isotherms yield a number of similar to 2.6 tigh tly bound water molecules per POPC and DTAB molecule. (C) 1999 Academic Pre ss.