Abstract
The area balance or imbalance between the inner and outer monolayer of biological membranes is a key parameter for driving shape changes (including exo and endocytosis) and controlling the bilayer curvature stress. The asymmetric incorporation of a drug or biological agent interferes with these processes, and the subsequent stress may lead to a membrane permeation or permeabilization. A main goal of this study is to introduce new methods to characterize such phenomena using isothermal titration calorimetry. POPC unilamellar vesicles and a series of alkyl maltosides are used as model systems; the unilamellarity was checked by NMR with the shift reagent Pr(3+). The free energy, enthalpy, and entropy associated with the asymmetry stress are estimated by comparing partitioning data of uptake versus release assays. The asymmetry stress is of enthalpic nature and somewhat reduced by entropic effects. Stimulated membrane permeation occurs at a mean maltoside-to-lipid ratio of approximately 0.2, which corresponds to an apparent area asymmetry of approximately 30% and a limiting free energy of the order of 2 kJ/mol of maltoside. Membrane solubilization to coexisting micelles proceeds at mole ratios of approximately 0.73, 0.81, and 0.88 (C(12)-, C(13)-, and C(14)-maltoside, respectively). Experiments with vesicles pre-loaded with surfactant in both monolayers provide evidence that the translocation threshold is controlled by the asymmetrically incorporated surfactant, whereas the onset of solubilization depends on the total surfactant content in the membrane. Free copies of the uptake and release fitting script including instructions are available upon request to heerklotz@gmx.net.