Mechanical unfolding of alpha2-macroglobulin molecules with atomic force microscope.

alpha2-Macroglobulin was derivatized with a sulfhydryl cross-linker and sandwiched between a mica substrate and a silicon nitride tip, both coated with gold, of an atomic force microscope and force curve measurement was carried out. An extensive downward deflection of the cantilever was observed in the retracting realm of the curve, when and only when the substrate was covered with the derivatized protein. The result was interpreted in terms of the mechanical stretching and unfolding of a single or a few protein molecules.