Spectrin R16: broad energy barrier or sequential transition states?

A number of models have been proposed to account for nonlinearity in the relation between observed rate constants for folding and/or unfolding and denaturant concentration. Where curvature is seen principally in the arm of a chevron plot, three explanations are proposed: a change in the ground state at increasing concentration of urea, movement of the transition state along a broad energy barrier, and a switch between two sequential transition states separated by an on-pathway high-energy intermediate. Here we demonstrate that the latter two models in particular can be used to describe the data for the all-alpha protein spectrin R16. Further, whatever the method of analysis, the pattern of Phi-values seen is robust; thus we would draw the same conclusions from our data set independently of the method used for analysis. While this is not a novel observation, this is the first systematic study where a comparison has been made between Phi-values calculated using the broad and sequential transition state models.