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