The Atomic Force Microscope is a simple instrument capable of imaging single macromolecules with submolecular resolution and measure the forces required to unfold a single protein domain (Fig. 1).

The AFM method of unfolding a tandem modular protein generates a characteristic sawtooth pattern in the force-extension curves (Fig. 2). The measured spaces between the peaks are thought to correspond to the increase in contour length (ΔΔLC) of the protein caused by the unfolding of one of its modules. In order to demonstrate the validity of this model and measure the sensitivity of the AFM we have designed an experiment where we have inserted a cluster of 5 Gly into the loop joining the F and G b-strands of the I27 module of the I band from human cardiac titin. This loop is clearly within the folded structure of the module and hence, if our predictions are correct, unfolding of the I27-5Gly module should reveal an expanded ΔLC. We expect to measure an increase in the contour length increment of ΔΔLC = 5 ✗ 0.38 nm =1.9 nm.