Elasticity and strength of proteins influence their biological functions. Under external forces, many proteins exhibit entropic elasticity with a characteristic stiffening elastic behavior and unravel due to the rupture of interstrand H-bonds. 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 protein domains at vanishing rates [1, 2]. Our analysis predicts that individual protein domains stabilized only by hydrogen bonds cannot exhibit rupture forces larger than 100-300 pN in the asymptotic quasi-static limit.
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