Biophys J, July 2001, p. 184-195, Vol. 81, No. 1

Membrane Stress and Permeabilization Induced by Asymmetric Incorporation of Compounds

Department of Biophysical Chemistry, Biocenter of the University of Basel, CH-4056 Basel, Switzerland

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 Pr³⁺. 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 ~0.2, which corresponds to an apparent area asymmetry of ~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 ~0.73, 0.81, and 0.88 (C₁₂-, C₁₃-, and C₁₄-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{at}gmx.net.

Biophys J, July 2001, p. 184-195, Vol. 81, No. 1
© 2001 by the Biophysical Society   0006-3495/01/07/184/12  $2.00

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