Single-molecule atomic force spectroscopy probes elastic properties of titin,ubiquitin and other relevant proteins. We explain bioprotein folding dynamicsunder both length- and force-clamp by modeling polyprotein modules as particlesin a bistable potential, weakly connected by harmonic spring linkers.Multistability of equilibrium extensions provides the characteristic sawtoothforce-extension curve. We show that abrupt or stepwise unfolding and refoldingunder force-clamp conditions involve transitions through virtual states (whichare quasi-stationary domain configurations) modified by thermal noise. Thesepredictions agree with experimental observations.
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