Stepwise unfolding of titin under force-clamp atomic force microscopy

Here we demonstrate the implementation of a single-molecule force damp adap ted for use with an atomic farce microscope. We show that under force-clamp conditions, an engineered titin protein elongates in steps because of the unfolding of its modules and that the waiting times to unfold are exponenti ally distributed. Force-clamp measurements directly measure the force depen dence of the unfolding probability and readily captures the different mecha nical stability of the 127 and 128 modules of human cardiac titin, Force-cl amp spectroscopy promises to be a direct way to probe the mechanical stabil ity of elastic proteins such as those found in muscle, the extracellular ma trix and cell adhesion.