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Abstract
Single-molecule force spectroscopy and molecular simulations are well-established techniques to study the mechanical unfolding of supramolecular complexes in various fields of biomolecular physics. In the present study, we investigate the details of the force-dependent folding transition of a well-studied model system, a calix[4]arene catenane dimer, using atomistic force quench simulations. This protocol allows us to reach a range of much smaller forces than possible with the more common force ramp simulations where the force is changed with a constant velocity. We find that the folding pathway changes its character as a function of external force. For small forces (on the order of 50 pN), the folding transition occurs via the transition to a metastable intermediate structure in about 30% of the simulations. We characterise the structure of this intermediate and demonstrate its relevance by considering the averaged potential of mean force of the system as a function of a well-defined reaction coordinate. When the force increases, the importance of the intermediate diminishes and for high external forces (500 pN), our results can be interpreted in terms of a simple two-state model, that has also been used in earlier simulations on the same system.
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Acknowledgements
We dedicate this paper to Jürgen Gauss on the occasion of his 60th birthday. We thank him for his continuing support and advice. The authors gratefully acknowledge the computing time granted on the supercomputer MOGON at Johannes Gutenberg-University Mainz (hpc.uni-mainz.de).
Disclosure statement
No potential conflict of interest was reported by the authors.