title: Probe-dependent and nonexponential relaxation kinetics: unreliable signatures of downhill protein folding.
abstract: The theoretical suggestion that some proteins may encounter no significant free energy barriers during their folding raises an important question: What experimental signature does this downhill folding produce? Several authors have argued that nonexponential (and especially stretched exponential) or probe-dependent kinetics represent useful experimental signatures of a downhill free energy surface. Here we examine more closely the connection between unusual kinetics and downhill energy surfaces. Simulation of diffusive relaxation dynamics on a variety of generically downhill, two-dimensional free energy surfaces shows that these surfaces do not necessarily produce either probe-dependent or significantly nonexponential kinetics. Conversely, we find that two-dimensional surfaces with significant (>3 k(B) T) energy barriers can readily give rise to nonexponential and probe-dependent kinetics. These results show that downhill folding does not constitute a necessary or sufficient condition for nonexponential and/or probe-dependent folding kinetics. One cannot easily prove or disprove that a protein folds downhill simply by studying its relaxation kinetics.
authors: Department of Physics, University of Florida, Gainesville, Florida 32611-8440, USA. sjhagen@ufl.edu
authors: Stephen J
authors: SJ
authors: Hagen
mesh_terms: D001703:Biophysics / Q000379:methods; D019295:Computational Biology / Q000379:methods*; D007700:Kinetics; D008956:Models, Chemical; D011489:Protein Denaturation; D017510:Protein Folding; D011506:Proteins / Q000737:chemistry*
chemical_list: D011506:Proteins