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Spectrin R16: Broad energy barrier or sequential transition states?

Medical Research Council (MRC) Centre for Protein Engineering, University of Cambridge Chemical Laboratory, Cambridge, CB2 1EW, United Kingdom

(RECEIVED January 26, 2005; FINAL REVISION March 21, 2005; ACCEPTED March 21, 2005)

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- protein spectrin R16. Further, whatever the method of analysis, the pattern of -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 -values calculated using the broad and sequential transition state models.

Keywords: protein folding; -value; spectrin; curvature; chevron plot; transition states

Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.051377105.

Reprint requests to: Jane Clarke, MRC Centre for Protein Engineering, University of Cambridge Chemical Laboratory, Lensfield Road, Cambridge, CB2 1EW, United Kingdom; e-mail: jc162{at}cam.ac.uk; fax: +44-(0)-1223336362.

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