Mechanical tightening of a synthetic molecular knot

Little is known regarding the effects of knotting on the mechanical properties of individual molecules. Here, we report on the force response of discrete synthetic small-molecule trefoil knots upon tightening. By combining single-molecule force spectroscopy with quantum chemical calculations, we provide evidence for the mechanism of tightening. It is associated with a higher resisting force than for larger protein knots and is modulated by the chemical environment. The central metal coordination plays a crucial role in the tightening process, as well as in the reverse process that recovers the initial knotted conformation. As a result of the compact structure, the recovery of conformation after mechanical perturbation is very fast. The tightening also plays an important role in accommodating mechanical stress. It provides a reserve of extensibility; the extra energy that the knotted strand can absorb in comparison with an unknotted strand is ∼13 kcal mol−1.