This "central" state is flanked by two lateral minima nearly identical, and just slightly higher, in free-energy (∼ +0.15 kcal mol -1 ), corresponding to lateral configurations where the macrocycle approaches the terminal stoppers. The kinetics of the transitions between these various states is found very fast in this system, with a transition time of the order of ∼ 10 -1 ns (Fig. and).