beta-hairpin folding and stability: molecular dynamics simulations of designed peptides in aqueous solution.

The structural properties of a 10-residue and a 15-residue peptide in aqueous solution were investigated by molecular dynamics simulation. The two designed peptides, SYINSDGTWT and SESYINSDGTWTVTE, had been studied previously by NMR at 278 K and the resulting model structures were classified as 3:5 beta-hairpins with a type I + G1 beta-bulge turn. In simulations at 278 K, starting from the NMR model structure, the 3:5 beta-hairpin conformers proved to be stable over the time period evaluated (30 ns). Starting from an extended conformation, simulations of the decapeptide at 278 K, 323 K and 353 K were also performed to study folding. Over the relatively short time scales explored (30 ns at 278 K and 323 K, 56 ns at 353 K), folding to the 3:5 beta-hairpin could only be observed at 353 K. At this temperature, the collapse to beta-hairpin-like conformations is very fast. The conformational space accessible to the peptide is entirely dominated by loop structures with different degrees of beta-hairpin character. The transitions between different types of ordered loops and beta-hairpins occur through two unstructured loop conformations stabilized by a single side-chain interaction between Tyr2 and Trp9, which facilitates the changes of the hydrogen-bond register. In agreement with previous experimental results, beta-hairpin formation is initially driven by the bending propensity of the turn segment. Nevertheless, the fine organization of the turn region appears to be a late event in the folding process.