Asymptotic strength limit of hydrogen bond assemblies in proteins at vanishing pulling rates

Experimental and computational studies on mechanical unfolding of proteins suggest that rupture forces approach a limiting value of a few hundred pN at vanishing pulling velocities. We develop a fracture mechanics based theoretical framework that considers the free energy competition between entropic elasticity of polypeptide chains and rupture of peptide hydrogen bonds, which we use here to provide an explanation for the intrinsic strength limit of proteins. Our analysis predicts that individual protein domains stabilized by hydrogen bonds can not exhibit rupture forces larger than ≈ 200 pN, regardless of the presence of a large number of hydrogen bonds. This result explains a wide range of experimental and computational observations.