Force–extension curves of dimerized polyglutamic acid

Measurement of the force–extension curve for the mechanical unfolding process of a single protein molecule is expected to provide a value for the force necessary to deform the molecule. Integration of the observed curves can then yield an accurate estimate of the intra-molecular cohesive energy of the protein. To understand the details of such force–extension curves, it is necessary to begin by understanding the mechanical properties of simple structural elements such as the α-helix and β-sheet. In a series of experiments designed to obtain force–extension curves of helical and randomly coiled polyglutamic acid, we found an interesting phenomenon in linearly cross-linked dimer samples of the polymer. The remarkable observation indicated the possibility of a lateral interaction of helical rods within molecules. Mechanical distinction of such interactions will be useful for application of atomic force microscopy in studying the nanomechanics of tertiary interactions among peptide segments in protein molecules.