掌性聚乳酸分子內與分子間掌性作用力之研究

Intramolecular and Intermolecular Chiral Interactions in Chiral Polylactides

黃健維(Jian-Wei Huang)

指導教授 ： 何榮銘

國立清華大學/工學院/化學工程學系/碩士(2012年)

https://doi.org/10.6843/NTHU.2012.00197

摘要

手性或者稱掌性被認為在不同層級間分子螺旋構型的產生扮演重要的角色。掌性傳遞或掌性辨識在自然界尤其是生物體中常扮演關鍵作用。因此,我們想要藉由了解掌性傳遞的機制去解釋螺旋高分子形成的原因,進一步的探討掌性或是螺旋構型在自組裝的效應。聚乳酸是一種廣為人知的生物可降解高分子,而掌性聚乳酸具有特殊的光學活性和結晶行為都是值得我們去研究的。一系列具特定掌性中心的聚乳酸分子被合成出來,用以研究掌性訊號傳遞從分子內到分子間的機制並提出一個模型解釋。圓二色光譜儀能鑒定出掌性聚乳酸在主鍊上具有的掌性特性,而進一步誘導出二級的螺旋構型,則能經由振動圓二色光譜儀被證明。
藉由合成一系列不同分子量的掌性聚乳酸,很容易發現掌性聚乳酸在振動圓二色光譜儀的光學活性在小分子量時隨著分子量增加而成長,緊接著成長幅度逐漸趨緩,趨向一個定值。這個趨勢符合動態螺旋鍊高分子的特徵,其較低的螺旋反轉能障使得分子鍊在空間中有機會改變旋性和前進的方向,導致光學活性的成長不會無止盡傳遞下去,而能夠提供分子鍊足夠自由能的特徵長度便稱為螺旋持續距離。除了光譜的角度,中子小角度散射也是鑑定動態螺旋分子構型的方便工具,根據我們得到的結果,掌性聚乳酸完美的符合蠕蟲狀鏈的構型,證明掌性聚乳酸分子確實在局部維持柱狀的結構並且每隔一段長度就會出現一個節點改變方向,如同我們所描述的動態螺旋構型一般。
分子內的掌性或是螺旋構型能夠進一步地在分子間傳遞,藉由控制分子鍊對稱的掌性或旋性時常能得到具有對稱性的三級結構。掌性聚乳酸分子便是一個很好的例子,其掌性作用力作用在分子自組裝之間並且增強光學活性,並且有不同於二級分子構型對應在酮基吸收位置上光學活性,三級結構的光學活性主要是觀察對應醚基位置上的吸收。這是因為醚基幾乎平行於高分子鍊的長軸,因此在二級構型中形成的螺旋有序排列不明顯,但是在三級以上的螺旋結構中則能很好地表現出來。另外,我們還發現節晶行為影響到分子鍊排整的規則度和緊密程度,高結晶度對於光學活性的增強有很明顯的效果。

關鍵字

掌性 ； 聚乳酸 ； 螺旋構型 ； 紅外光圓二色光譜儀 ； 螺旋結構

並列摘要

Chirality is known as an important feature of molecules and macromolecules for the formation of helical architectures at different hierarchical levels. In nature, homochiral evolution is a key molecular process for the communication, replication and enzyme catalysis through the association of chiral molecules and macromolecules. Herein, we aim to examine the mechanisms of chiral information transfer for chiral polymers from configurational chirality to conformational chirality so as to give fundamental understanding of the self-assembly of helical polymer chains. Polylactide is a well-known biodegradable polymer, and chiral polylactides with specific optical activity are crystalline polymers due to their regular configuration. As a result, enantiomeric chiral polylactides were synthesized in this study so as to give a model system for the examination of chiral information transfer from molecular level. Opposite chiral entities (i.e., configurational chirality) with main-chain chirality as examined by circular dichroism (CD) will give rise to the PLLA and PDLA chains as helical conformations with preferential left- and right-handedness (i.e., conformational chirality), respectively, due to intramolecular interaction as evidenced by vibrational circular dichroism (VCD). Accordingly, the molecular chirality not only plays the key factor to give helical conformation but also controls its helicity.
The dynamic helical polymers, such as chiral polylactides, possess a very low helical inversion barrier so as to give the reversal of handedness. This specific distance between helical reversal points named helical persistence length. Accordingly, the helical reversal occurs once the chain length is over the helical persistence length. To examine the behavior of the helical reversal, chiral polylactides with different molecular weights were synthesized so as to clarify the molecular weight dependence of optical activity by VCD. Our results indicate that the ellipticity of chiral polylactides is indeed strongly dependent upon molecular weight at which the ellipticity linearly increases with the molecular weight until the molecular weight reaches 1000 g/mol and then levels off at a higher molecular weight. Moreover, in comparison with the spectroscopic results, small angle neutron scattering (SANS) experiments are carried out to visualize the changes of scattering profiles for various chiral polylactides. According to the SANS results, the scattering results of chiral polylactides perfectly fit the worm-like chain model, indicating that the polymer chain of chiral polylactide is rod-like and possess several turning points. Namely, the behavior of rod-like chiral polylactide is in line with the suggested chain conformation with helical persistent length.
Self-assembling plays an important role in the formation of many chiral structures and in the preparation of chiral functional materials. Therefore the control of chirality in synthetic or self-assembled systems is important either for the comprehension of recognition phenomena or to obtain materials with predictable and controllable properties. The intermolecular chiral interaction acts onto chiral polylactides during the aggregation process, so as to give the symmetric optical activity. This result is well observed from ellipticity contributed from ether group on chiral polylactides. Because the vibrational transition of ether group is almost parallel to helical conformational axis, and only supramolecular but no conformational chirality exist here. Whereas conformational chirality can be identified by the vibrational absorption of carbonyl group, and supramolecular chirality is recognized by the at ether group, a methodology is built to observe chiral information transfer from conformational chirality to supramolecular chirality by vibrational circular dichroism. Furthermore, the intensity of intermolecular chiral interaction will be enhanced by the degree of chain packing (that is, the crystallinity). The optical activities significantly amplify from amorphous states to crystalline phase.

並列關鍵字

chiral interaction ； vibrational circular dichroism ； helical ； helical conformation ； helical structure ； chiral amplification

參考文獻

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