  The thermodynamics of proteins indicate that folding/unfolding takes place
either through stable intermediates or through a two-state process without
intermediates. The rather short folding times of the two-state process indicate
that folding is guided. We reconcile these two seemingly contradictory
observations quantitatively in a schematic model of protein folding. We propose
a new dynamical transition temperature which is lower than the thermodynamic
one, in qualitative agreement with in vivo measurement of protein stability
using E.coli. Finally we demonstrate that our framework is easily generalized
to encompass cold unfolding, and make predictions that relate the sharpness of
the cold and hot unfolding transitions.
