Mechanical tightening of a synthetic molecular knot

•Single-molecule force spectroscopy tightens a synthetic molecular overhand knot •Quantum chemical calculations decipher the mechanism of tightening •The recovery of conformation after mechanical perturbation is very fast •The knot increases the mechanical stress that the strand can accommodate 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. 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.