Quantifying protein folding transition States with φ(t).

A number of reaction coordinates have been proposed for reduced-dimensionalityrepresentations of a protein's folding free energy surface. We discuss in detail the entropic reaction coordinate Φ(T) = Δ S(†)ΔS, recently introduced to quantify the conservation of mutations and the location of the folding transition state based on experimental temperature-tuning data. Numerical simulations illustrate the advantages as well as the limitations of Φ(T). Φ(T) can be determined from experiment,computation, and analytical theory; Φ(T) can also be used to investigate structurally localized perturbations of the free energy surface. However, Φ(T) is only a relative reaction cordinate; furthermore, proteins undergo cold denaturation at sufficiently low temperatures, and care must be taken ininterpreting Φ(T) near the region where ∂ΔG/∂T = 0, particularly if the heat capacity change upon folding is small.