Reversible hydrogen bond network dynamics: molecular dynamics simulations of calix[4]arene-catenanes

We present detailed molecular dynamics (MD) simulations of mechanically interlocked calix[4]arene-catenanes under external force. Single-molecule force spectroscopy experiments revealed that the separation of dimers with two aliphatic loops results in reversible hydrogen bond breakage through an intermediate in a triple-well potential, while the tetra-loop species separates in a one-step manner (Janke, M.; et al. Nat. Nanotechnol. 2009, 4, 225). MD simulations show that calix[4]arenes interlocked by four loops (1) display a complete restructuring of the hydrogen bond network under mechanical force. All hydrogen bonds of the closed structure open, and new ones are formed in the extended structure. For small loading rates, we found reversible rejoining of the hydrogen bond network, while the rebinding ability diminishes with increasing pulling velocity, demonstrating the feasibility of MD simulations to capture also rebinding dynamics. Calix[4]arene dimers with two longer loops (2) under external force display more intricate physics because the elongation proceeds in a two-step transition from a compact structure to an open one, in which complete dissociation into calix[4]arene monomers is only prevented by the mechanical locking of the loops. We present a detailed analysis of hydrogen bond breakage and show that the transition from the closed to the intermediate structure is very similar to the transition from the closed to the open structure in the tetra-loop case. The stability of the intermediate is explained in terms of the closed hydrogen bonds, which are only broken when the transition to the open structure is enforced.