The overall free energy change for unfolding is not different from that of refolding. Rather, the hysteresis comes about because unfolding proceeds via an intermediate state that kinetically traps the native state and is not destabilized relative to the native state until much higher denaturant conditions than required for refolding to occur. The authors have a nice diagram of the proposed energetics in a paper they published the following year (see Fig 1): http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2527903/
If I understand you correct than the protein has to "reach" a intermediate state (the Niso) before it can unfold.
the energy to fold (same as refold I pressume?) is the same to unfold, but in order to unfold you first have to reach this intermediate state (meaning: first you need to invest some extra energy in the folded state to reach this intermediate state?
A few extra questions:
first question: the horizontal bars in that figure (second part) they do represent energy levels right? THe higher the bar, the more energu?
A second question: this indermediate state (Niso) is this still an active fluorophore? I assume that in this state the protein is not working anymore as a fluorophore?
A last question: what are those red dots in that figure (second part figure 1)?