LIPID DETERGENT INTERACTION THERMODYNAMICS AS A FUNCTION OF MOLECULARSHAPE/

In dilute aqueous mixtures of the detergent C(12)EO(8) and the phospho lipid POPC the phase and partition behavior as well as the transfer en thalpies of the respective molecules between the various states (monom ers, bilayers, micelles) have been measured by isothermal titration ca lorimetry [Heerklotz et al., J. Phys. Chem. 1996, 100, 6764]. To deriv e more information about the molecular interpretation of the thermodyn amic data, we performed additional experiments for a series of deterge nts, C(12)EO(n) with 12 = 3-8 and dependent on temperature (for C(12)E O(8)). The data can be discussed in terms of a three-stage model (bila yers, coexistence, micelles) considering nonideal mixing within the ag gregates. The mixing properties are determined by packing effects cont rolling the hydration of the headgroups, the water exposure of the hyd rocarbon core, and the order of the hydrocarbon chains. Additionally, two types of systematic deviations from the simple three-stage behavio r are found for low n and low detergent contents in the bilayer. These effects could be related to special properties of detergents surround ed by lipids only and to solubilization intermediates occurring close to the lytic detergent content.