Knotted Fishing Line, Covalent Bonds, and Breaking Points

Abstract

Every angler knows that a fishing line breaks easily at the place of a tight knot. In their report (1) “How strong is a covalent bond?” Michel Grandbois et al. present a “fly fishing method” for measuring the force needed to break long polysaccharide chains. In these experiments, a single molecule was attached with covalent bonds (Si–C, Si–O) to the tip of an atomic force microscope (AFM) at one end and to a substrate at the other. The polymers they used were very long (thousands of sugar rings) and were relaxed in a solution before attachment to the surfaces. Persistence length of such polymers roughly corresponds to the dimension of the composing monomers, as demonstrated in a report by S. B. Smithet al. (2), in which single-stranded DNA was used. Chains of this size (a thousandfold longer that their persistence length) are usually knotted and frequently will have more than one knot (3). This knotty property of long polymers was not discussed by Grandbois et al. (1), and it casts doubt on the interpretations of the data and the conclusions in the report.