A PICONEWTON FORCE TRANSDUCER AND ITS APPLICATION TO MEASUREMENT OF THE BENDING STIFFNESS OF PHOSPHOLIPID-MEMBRANES

The bending stiffness of a phospholipid bilayer (k(c)) was measured by forming thin bilayer cylinders (tethers) from giant phospholipid vesi cles. Based on the balance of forces, the tether force was expected to be proportional to the square root of the membrane tension, with a co nstant of proportionality containing k(c). The membrane tension was co ntrolled via the aspiration pressure in a micropipette used to hold th e vesicle. The force on the tether was generated by an electromagnet a cting on a paramagnetic bead attached to the vesicle surface. The magn itude of the force was determined from measurements of the magnet curr ent, which was adjusted to maintain the position of the bead, Measurem ents were performed on vesicles composed of stearoyl-oleoyl-phosphatid ylcholine plus 5% (by mole) biotinylated phosphatidylethanolamine to m ediate adhesion to streptavidin-coated beads. From each vesicle, tethe rs were formed repeatedly at different values of the membrane tension. The expected relationship between membrane tension and tether force w as observed. The mean value of k(c) for 10 different vesicles was 1.17 x 10(-19) J (SD = 0.08 x 10(-19) J). The precision of these data demo nstrates the reliability of this approach, which avoids uncertainties of interpretation and measurement that may be associated with other me thods for determining k(c).