研究生: |
姜怡君 Chiang, Yi-Chun |
---|---|
論文名稱: |
立體電子效應對聚脯胺酸以及β-hairpin結構與穩定度之探討 Stereoelectronic effects on polyproline and β-hairpin conformation and stability |
指導教授: |
洪嘉呈
Horng, Jia-Cherng |
口試委員: | |
學位類別: |
碩士 Master |
系所名稱: |
理學院 - 化學系 Department of Chemistry |
論文出版年: | 2009 |
畢業學年度: | 97 |
語文別: | 中文 |
論文頁數: | 85 |
中文關鍵詞: | 聚脯胺酸 、β-hairpin 、立體電子效應 、proline-aromatic interactions 、PPII |
外文關鍵詞: | polyproline, PPII, β-hairpin, stereoelectronic effects, proline-aromatic interactions |
相關次數: | 點閱:2 下載:0 |
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聚脯胺酸可以形成PPI或是PPII結構, n→ π*作用已經被提出在穩定PPII結構具有一個關鍵性的角色,並證實立體電子效應對於聚脯胺酸的結構具有一定程度的影響力。另一方面,研究指出可以利用立體電子效應對蛋白質的穩定度與結構性進行調控,但目前尚未有利用β-hairpin為研究模型探討立體電子效應之報導,故本次研究特地選用一小段的β-hairpin胜□,HP7為模型,以探討立體電子效應對β-hairpin的影響。
本研究中,合成一系列脯胺酸衍生物 (脯胺酸的C4位置接上不同的拉電子基,OH、F、OCH3基),藉由合成一系列host-guest胜□,(Pro)5-X-(Pro)5-Gly-Tyr,當 X為脯胺酸以及脯胺酸衍生物 (脯胺酸 C4位置有不同的拉電子基),來探討立體電子效應對聚脯胺酸結構在PPI和PPII結構轉換間活化能的影響。利用圓二色光譜儀進行動力學研究,可得到聚脯胺酸轉換結構時的速率,結果顯示立體電子效應對於聚脯胺酸結構由PPII轉換到PPI時會產生明顯的影響。另一方面,在HP7胜□中置入含有拉電子基的脯胺酸衍生物來探討立體電子效應對β-hairpin的結構與穩定度的影響。結果顯示,立體電子效應造成的 preorganization並非是造成 HP7系列胜□穩定與否的主要原因,因為在此結構只有 Pro-aromatic作用力,且 Proline衍生物的支鏈與鄰近的 Trp3與 Asn4亦會產生立體阻礙而影響結構的穩定度,此外,因為 proline接上拉電子基後環上電子被部份拉離,也會與 Trp3形成 π-cation interaction而穩定結構,因此,這些作用力在探討此結構穩定性時,都是必須考慮的。
The n → π* interactions have been suggested to play a critical role in stabilizing a PPII helix, and stereoelectronic effects can have a substantial impact on polyproline conformation. Thus, understanding of the basis of polyproline structures will be one aim in this proposal. Polyproline can form either type I (PPI) or type II (PPII) helices. Besides polyproline structure, stereoelectronic effects were found to be capable of tuning protein stability. However, no study of stereoelectronic effects on β-hairpin has been reported. We choose a small β-hairpin peptide (HP7) as our model to study stereoelectronic effects on β-hairpin structure.
In this work we have prepared a series of proline derivatives with an electron-withdrawing substituent on proline C4 position, such as –OH, -F and –OCH3 group. We have prepared a series of host-guest peptide, (Pro)5-X-(Pro)5-Gly-Tyr, where X = proline, and proline derivatives to evaluate the stereoelectronic effects on the transition energy barrier of the conversion between PPI and PPII conformation. Time-dependent dichroism (CD) spectra have been used to measure the conversion rates. The results show that stereroelectronic effects have a significantly impact on the conversion rates from PPII to PPI. We have also incorporated those proline derivatives with an electron-withdrawing substituent on C4 position into the HP7 peptide to investigate stereoelectronic effects on β-hairpin. The data showed that Cγ-ring puker of proline does not significantly contribute to the β-hairpin structure stability. Other forces, such as steric repulsion, cation-π and proline-aromatic interactions, need to be considered in stabilizing the β-hairpin structure.
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