1. Center for Molecular Modeling, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6202, USA
2. DuPont Central Research and Development, Expt. Station, Wilmington, Delaware 19880-0328, USA

Correspondence to: Michael L. Klein¹ Correspondence and requests for material should be addressed to M.L.K. (e-mail: Email: klein|[commat]|lrsm.upenn.edu).

Many experiments have been done to determine the relative strengths of different knots, and these show that the break in a knotted rope almost invariably occurs at the point just outside the |[lsquo]|entrance|[rsquo]| to the knot¹. The influence of knots on the properties of polymers has become of great interest, in part because of their effect on mechanical properties². Knot theory³,⁴ applied to the topology of macromolecules^{5, 6, 7, 8} indicates that the simple trefoil or |[lsquo]|overhand|[rsquo]| knot is likely to be present in any long polymer strand^{9, 10, 11, 12}. Fragments of DNA have been observed to contain such knots in experiments¹³,¹⁴ and computer simulations¹⁵. Here we use ab initio computational methods¹⁶ to investigate the effect of a trefoil knot on the breaking strength of a polymer strand. We find that the knot weakens the strand significantly, and that, like a knotted rope, it breaks under tension at the entrance to the knot.