  A model which treats the denatured and the native conformers as being
confined to harmonic Gibbs energy wells has been used to analyse the
non-Arrhenius behaviour of spontaneously folding fixed two-state systems. The
results demonstrate that when pressure and solvent are constant: (i) a
two-state system is physically defined only for a finite temperature range;
(ii)irrespective of the primary sequence, the 3-dimensional structure of the
native conformer, the residual structure in the denatured state, and the
magnitude of the folding and unfolding rate constants, the equilibrium
stability of a two-state system is a maximum when its denatured conformers bury
the least amount of solvent accessible surface area (SASA) to reach the
activated state; (iii) the Gibbs barriers to folding and unfolding are not
always due to the incomplete compensation of the activation enthalpies and
entropies; (iv) the difference in heat capacity between the reaction-states is
due to both the size of the solvent-shell and the noncovalent interactions; (v)
the position of the transition state ensemble along the reaction coordinate
(RC) depends on the choice of the RC; and (vi) the atomic structure of the
transiently populated reaction-states cannot be inferred from
perturbation-induced changes in their energetics.
