Where Ion Mobility and Molecular Dynamics Meet to Unravel the (Un)Folding Mechanisms of an Oligorotaxane Molecular Switch

Abstract

At the interface between foldamers and mechanically interlocked molecules, oligorotaxanes exhibit a spring-like folded secondary structure with remarkable mechanical and physicochemical properties. Among these properties, the ability of oligorotaxanes to act as molecular switches through controlled modulations of their spatial extension over (un)folding dynamics is of particular interest. The present study aims to assess and further characterize this remarkable feature in the gas phase using mass spectrometry tools. In this context, we focused on the [4]5NPR+12 oligorotaxane molecule complexed with PF6- counterion and probed its co-conformational states as a function of the in-source-generated charge states. Data were interpreted in light of electronic secondary structure computations at the PM6 and DFT levels. Our results highlight two major co-conformational groups associated either with folded compact structures, notably stabilized by intramolecular π-π interactions and predominant for low charge states or with fully stretched structures resulting from significant Coulombic repulsions at high charge states. Between, the oligorotaxane adopts intermediate folded co-conformations, suggesting a stepwise unfolding pathway under increasing repulsive Coulombic constraints. The reversibility of this superstructural transition was next interrogated under electron-driven (nondissociative electron transfer) and heat-driven (collision-induced unfolding) activation stimuli. The outcomes support the feasibility to either unfold or (partially) refold the oligorotaxane foldamer on purpose in the gas phase. Our results show that the balance between the stabilizing π-π interactions and the versatile Coulomb interactions dictates the elongation state of the foldamer in the gas phase and emphasizes the adequacy of mass spectrometry tools for the superstructural characterization of desolvated prototypical artificial molecular machines.