Membrane stress and permeabilization induced by asymmetric incorporation of compounds

The area balance or imbalance between the inner and outer monolayer of biol ogical membranes is a key parameter for driving shape changes (including ex o and endocytosis) and controlling the bilayer curvature stress. The asymme tric incorporation of a drug or biological agent interferes with these proc esses, and the subsequent stress may lead to a membrane permeation or perme abilization. A main goal of this study is to introduce new methods to chara cterize such phenomena using isothermal titration calorimetry. POPC unilame llar vesicles and a series of alkyl maltosides are used as model systems; t he unilamellarity was checked by NMR with the shift reagent Pr3+. The free energy, enthalpy, and entropy associated with the asymmetry stress are esti mated by comparing partitioning data of uptake versus release assays. The a symmetry stress is of enthalpic nature and somewhat reduced by entropic eff ects. Stimulated membrane permeation occurs at a mean maltoside-to-lipid ra tio of similar to0.2, which corresponds to an apparent area asymmetry of si milar to 30% and a limiting free energy of the order of 2 kJ/mol of maltosi de. Membrane solubilization to coexisting micelles proceeds at mole ratios of similar to0.73, 0.81, and 0.88 (C-12-, C-13-, and C-14-maltoside, respec tively). Experiments with vesicles pre-loaded with surfactant in both monol ayers provide evidence that the translocation threshold is controlled by th e asymmetrically incorporated surfactant, whereas the onset of solubilizati on 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.