Using experimental and computational approaches to study the effects of proline derivatives on the structure of polyproline and β-hairpin

• Main Authors: Chang, Chiao-Hsin, 張巧欣
• Other Authors: Horng, Jia-Cherng
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/2p592e

碩士 === 國立清華大學 === 化學系 === 101 === Polyproline can form either type I (PPI) or type II (PPII) helices. The backbone n→π* interactions have been suggested to play a critical role in stabilizing a PPII helix. In our previous study, we prepared a series of polyproline peptides with proline derivatives incorporated to evaluate stereoelectronic effects on the transition energy barrier of the conversion between PPI and PPII conformation. We also incorporated proline derivatives into the HP7 peptide to investigate stereoelectronic effects on β- hairpin. In this work we prepared two polyproline peptides in which (2S)-4-ketoproline (Kep) and (2S)-4,4-difluoroproline (Dfp) were incorporated. Circular dichroism (CD) spectroscopy was used to characterize their conformation. We also used a computational approach to study the effects of Kep, Dfp and thiaproline (Thp) on polyproline conformation. For each model compound and system, hybrid density function theory (DFT) calculations, as implement in Gaussian 09, were carried out on the conformation, and the dihedral angels φ, ψ and χ were measured for polyproline. We used Ac-(Pro)2X(Pro)2-OMe as a model system for the computation. Typical 4- substituted Pro derivatives display either a Cγ -endo or a Cγ -exo ring pucker. Such conformations are not readily accessible to Ac-Kep-OMe, Ac-Dfp-OMe and Ac-Thp- OMe. When Ac-Kep-OMe, Ac-Dfp-OMe and Ac-Thp-OMe adopt a Cγ -endo pucker, their pyrrolidine ring is significantly distorted and distinctly different from other 4- substituted proline derivative. This may explain why Kep, Dfp, and Thp destabilize PPI conformation as observed in experiments. Moreover, the HP7 peptide could not form β-hairpin upon incorporating Thp into the peptide. We chose WN-X-ATGKW sequence as a model system for computation as it can form H-bonds required for a β-turn in HP7. ONIOM calculation was carried out on the structure. The calculated result reveals that Thp substitution on HP7 makes the H-bond between C=O of Asn4 and N-H of Lys9 broken as the distance between the residues increases. Thp substitution also makes the distance between Pro5 and Trp3 decrease, which may increase the steric repulsion between the residues. The computational results may in part explain why Thp dramatically destabilize HP7.