Kinetics of peptide folding: computer simulations of SYPFDV and peptide variants in water.

The folding of Ser-Tyr-Pro-Phe-Asp-Val (SYPFDV), and sequence variants of this peptide (SYPYD and SYPFD) are studied computationally in an explicit water environment. An atomically detailed model of the peptide is embedded in a sphere of TIP3P water molecules and its optimal structure is computed by simulated annealing. At distances from the peptide that are beyond a few solvation shells, a continuum solvent model is employed. The simulations are performed using a mean field approach that enhances the efficiency of sampling peptide conformations. The computations predict a small number of conformations as plausible folded structures. All have a type VI turn conformation for the peptide backbone, similar to that found using NMR. However, some of the structures differ from the experimentally proposed ones in the packing of the proline ring with the aromatic residues. The second most populated structure has, in addition to a correctly folded backbone, the same hydrophobic packing as the conformation measured by NMR. Our simulations suggest a kinetic mechanism that consists of three separate stages. The time-scales associated with these stages are distinct and depend differently on temperature. Electrostatic interactions play an initial role in guiding the peptide chain to a roughly correct structure as measured by the end-to-end distance. At the same time or later the backbone torsions rearrange due to local tendency of the proline ring to form a turn: this step depends on solvation forces and is helped by loose hydrophobic interactions. In the final step, hydrophobic residues pack against each other. We also show the existence of an off the pathway intermediate, suggesting that even in the folding of a small peptide "misfolded" structures can form. The simulations clearly show that parallel folding paths are involved. Our findings suggest that the process of peptide folding shares many of the features expected for the significantly larger protein molecules.