Mechanical and Structural Tuning of Reversible Hydrogen Bonding in Interlocked Calixarene Nanocapsules

Abstract

We present force probe molecular dynamics simulations of dimers of interlocked calixarene nanocapsules and study the impact of structural details and solvent properties on the mechanical unfolding pathways. The system consists of two calixarene “cups” that form a catenane structure via interlocked aliphatic loops of tunable length. The dimer shows reversible rebinding, and the kinetics of the system can be understood in terms of a two-state model for shorter loops (≤14 CH2 units) and a three-state model for longer loops (≥15 CH2 units). The various conformational states of the dimer are stabilized by networks of hydrogen bonds, the mechanical susceptibility of which can be altered by changing the polarity and proticity of the solvent. The variation of the loop length and the solvent properties in combination with changes in the pulling protocol allows to tune the reversibility of the conformational transitions.

Force probe simulations using an adaptive resolution scheme

“…6. These forces behave somewhat different from the maximum rupture force and the minimum rejoin force but capture all important features [36,37]. Furthermore, and more important in the present context, for reversibly binding systems like the calix [4]arene dimer investigated here, F rupt and F rejoin will converge to the equilibrium value, F eq , for slow pulling [46].…”

“…All production runs were performed in the canonical ensemble using this box size. We mention that this box size is larger than the box size we used in earlier investigations ((5.8 nm) 3 ) [36,37]. However, the larger box size used here is also used for the AdResS simulations and therefore a direct comparison is possible.…”

“…As mentioned above, we used a box of size of (7.49 nm) 3 for all AA simulations and the AdResS simulations although we have found in earlier studies of the calix [4]arene catenane dimer system that a box length of 5.8 nm is sufficient for all FPMD simulations performed so far [36,37]. A box of this size appears to be a good compromise between the two extreme scenarios that have to be considered in the case of FPMD simulations.…”

Force probe simulations using an adaptive resolution scheme

“…All production runs were performed in the canonical ensemble using this box size. We mention that this box size is larger than the box size we used in earlier investigations ((5.8 nm) 3 ) [37,38]. However, the larger box size used here is also used for the AdResS simulations and therefore a direct comparison is possible.…”

“…As mentioned above, we used a box of size of (7.49 nm) 3 for all AA simulations and the AdResS simulations although we have found in earlier studies of the calix [4]arene catenane dimer system that a box length of 5.8 nm is sufficient for all FPMD simulations performed so far [37,38]. A box of this size appears to be a good compromise between the two extreme scenarios that have to be considered in the case of FPMD simulations.…”