EXCESS-ENTHALPIES OF MIXING IN PHOSPHOLIPID-ADDITIVE MEMBRANES

Isothermal titration calorimetry (ITC) allows the measurement of compo sition-dependent mixing heats of amphiphiles. A number of experimental protocols are now established to measure molecular transfer heats bet ween, for example, micellar and lamellar aggregates. This study deals with the principle understanding of the physical effects contributing to the ITC data. The physical state of the mixture is described in ter ms of the molar excess enthalpy as a function of its composition h(E)( X). A relation is derived between this system property and the observa ble heat per mole of titrant (q(obs)) as q(obs) = (X-syr - X)(partial derivative h(E)/partial derivative X) + h(E)(X) - h(E)(X-syr) with X a nd X,,, being the mole fractions of one chosen component within the mi xed aggregates in the sample cell and in the injection syringe, respec tively. According to this differential equation, one may derive inform ation about the second and further derivatives (i.e., the curvature) o f the excess enthalpy function. This can serve to construct the h(E)(X ) plot based on the ITC data. We emphasize that for aggregates mixing nonideally (which must be considered rather the rule than the exceptio n) one has to carefully distinguish between observed mixing heats and enthalpic state of the mixture. The formalism is presented at the exam ple of mixtures of the phospholipid POPC and detergents of the type C1 2EOn with n = 3-6. For instance, the system C12EO3/POPC was found to s how an extremely asymmetric mixing enthalpy function with an attractiv e part (i.e., h(E) < 0) for low and a repulsive one for higher deterge nt contents in the mixed membranes. Such excess enthalpy functions cou ld be modeled by a polynomial equation and discussed in terms of coope rative interactions between the molecules.