| Protein-like folding and thermodynamics of a homopolymer chain |
| Mark P. Taylor(1), Wolfgang Paul(2,3), and Kurt Binder(2) (1)Department of Physics, Hiram College, Hiram, OH 44234, USA (2) Institut fur Physik, Johannes-Gutenberg-Universitat, D-55099 Mainz, Germany (3) Institut fur Physik, Martin-Luther-Universitat, D-06099 Halle (Saale), Germany Many small proteins fold via a first-order "all-or-none" transition directly from an expanded coil to a compact native state. Here we report an analogous direct freezing transition from an expanded coil to a compact crystallite for a simple flexible homopolymer. Wang-Landau sampling is used to construct the 1D density of states for square-well chains up to length 256. Analysis within both the micro-canonical and canonical ensembles shows that, for a chain with sufficiently short-range interactions, the usual polymer collapse transition is preempted by a direct freezing transition. A 2D configurational probability landscape, built via subsequent multi-canonical sampling, reveals both a dominant folding pathway and an inherent configurational barrier to folding. Despite the non-unique homopolymer ground state, the thermodynamics of this direct freezing transition are identical to the thermodynamics of two-state protein folding. A free energy barrier separates a high entropy ensemble of unfolded states from a low entropy set of crystallite states and the transition proceeds via the formation of a transition-state folding nucleus. An Arrhenius analysis of the folding/unfolding free energy barrier yields a Chevron plot characteristic of proteins and the model chain satisfies the vant Hoff calorimetric criterion for two-state folding. [1} Phys. Rev. E 79, 050801(R) (2009); J. Chem. Phys. 131, 114907 (2009). |