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  • 學位論文

藉非線性光學活性研究膠原蛋白組織中之旋性與各向異性

Contribution of Chirality and Anisotropy for Nonlinear Optical Activity in Collagenous Tissues

指導教授 : 朱士維
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摘要


大自然偏好左撇子,使得旋性研究一直是一個很熱門的議題。生命體內的基本組成,諸如去氧核醣核酸,蛋白質,醣類也需仰賴正確的旋性,則反應與辨識過程才有可能發生。傳統的旋性光譜由於其微弱的信號對比以及無法提供精確的旋性化學鍵結訊息,使得此技術僅限於用來量測材料表面或高純度材料的旋性。然而,研究複雜生物分子在原本微環境下旋性的迫切需要,致使如何開發突破旋性光譜限制的嶄新技術成為旋性研究的重要課題。 在這篇論文中,我們利用了兩種非線性光學活性的技術,克服了傳統線性光學旋性技術的限制。其中,倍頻旋性光譜展現了相對優越的信號對比; 而對旋性分子結構高度敏感的拉曼光學活性光譜,則彌補了旋性光譜在細微旋性結構敏感度上的不足。 旋性材料的倍頻訊號在左旋與右旋的偏振光激發下會有不同的響應,此種效應稱為:倍頻旋性光譜。此技術不只能提供較高的旋性信號對比,也展現了極好的光學切片能力,因此適用於厚生物組織樣本的量 測。然而我們發現,在實際生物組織量測中,倍頻旋性光譜的來源取決於兩個主要因素,分別為膠原蛋白分子的內秉旋性結構與膠原蛋白的非等向性。在非共振情況下,倍頻旋性信號,僅提供了膠原蛋白纖維的非等向性資訊; 至於我們真正感興趣的膠原蛋白內秉旋性結構,唯有在共振條件下才可以完全的被體現。 如果想進一步在化學鍵結的觀點下瞭解膠原蛋白分子的內秉旋性來源,拉曼光學活性光譜是一個非常好的選擇。拉曼光學活性光譜主要 是量測旋性分子的振動拉曼光譜在左旋與右旋圓偏振激發光下的微小 變化。此技術對於與旋性來源高度相關的化學鍵結有相當高的靈敏度。 我們首次將此技術應用在複雜的膠原蛋白組織分子,並且量測了其所對應的拉曼光學活性光譜特徵。我們的實驗結果,不但驗證了拉曼光學活性的信號主要源自於膠原蛋白的螺旋骨架結構,也是首次由化學分子鍵結的觀點截取出真正與旋性相關的對應結構。除了拉曼光學活性光譜特徵外,此技術強大的潛力在於我們還結合了拉曼顯微影像技術,提供了與旋性來源相對應的化學影像。 我們不但是全世界第一個將倍頻旋性光譜與拉曼光學活性光譜技術應用在非破壞性量測真實生物組織樣本的團隊,在此篇論文中,我們 也展示了此技術應用在真實複雜生物樣本未來的可行性。

並列摘要


Nature prefers chiral molecules so the chirality studies have always been a very hot topic. Natural chiral compounds such as DNA, proteins, peptides, carbohydrates, etc. require correct handedness in chemical recognition and interaction. Conventionally, to study chirality, circular dichroism (CD) is the most popular technique. However, CD spectroscopy is limited by its poor signal contrast and relatively inadequate chiral structure information, so previous studies are constrained to surfaces or purified bulk materials. It is highly desirable to study chirality in complex biological molecules under native microenvironment. Therefore, we aim to develop techniques that can overcome the limitations of the linear CD method. In this dissertation, we study two nonlinear optical activity approaches to address these limitations. The first technique is second harmonic generation circular-dichroism (SHG-CD), which detects variation of second harmonic generation signals for left- and right-circularly polarized light. The main advantages of SHG-CD is that the signal contrast reaches unity. The second one is circular intensity difference (CID) derived from Raman optical activity (ROA), which measures a small difference in the intensity of vibrational Raman scattering from chiral molecules with right- and left-circularly polarized light. The outstanding property of ROA is that the molecular bond chirality can be unraveled via vibrational modes analysis. In our study of SHG-CD, we achieve several accomplishments which will be vital in the future to study of chirality in complex tissue samples. First, we found that SHG-CD response sensitively depends on two main factors, intrinsic chirality and molecular anisotropy, the latter of which is not possible to avoid when studying chiral molecules under native microenvironment. Furthermore, we verify that chirality-induced SHG-CD is significantly enhanced at molecular resonance, while anisotropy-induced SHG-CD kept roughly the same at every wavelength. Second, SHG-CD shows much better chiral contrast than traditional chiroptical techniques in thick tissue. Third, SHG-CD provides good optical sectioning capability that is suitable for three dimensional imaging application. To further explore the molecular origins of chirality, we use CID spectroscopy to investigate chiral sensitive chemical bonds of the entire collagen molecular structure. Our main achievements are the following. First, to our knowledge we are the pioneer to report CID spectra of type I collagen within a native tendon tissue. Second, we find the largest CID signals occur at the wavenumbers of 938 cm−1 (α helix), 1246-1271 cm−1 (Amide III), and 1668cm−1 (Amide I), which are all associated with vibrational modes related to the collagen backbone. In contrast, 2944 cm−1 (amino acids side chains) exhibits very large Raman signals, but diminishing CID response. Therefore, our study verify that the chiral response of collagen is dominated by the stereo organization of the backbone. That is, CID spectra extracts the information about the main chiral sources which cannot be provided by using any other conventional measurement. Third, our CID values are one order of magnitude larger than theoretical estimation from isotropically distributed molecules. The reason is that type-I collagen molecules arrange orderly in the tissue, resulting in significant anisotropy, which in turn leads to coherent summation of Raman signals. Fourth, we find that the sign of CID values from a single collagen fiber alters from positive to negative, manifesting the influence of anisotropy and fiber orientation. Fifth, we demonstrate CID microscopic images, which can provide not only direct visualization of molecular chirality, but also molecular specificity from Raman signature in intact biological tissues. We are the first group in the world to use SHG-CD and CID microspectroscopyto identify intrinsic chirality and chiral sensitive chemical bondsof the type I collagen tissue samples in situ. Our investigation constitutes an important landmark towards a realistic chiralspectroscopic/microscopic technique and allowed us to expand our current knowledge of the standard spectral characteristic of the classical, long-range, three-dimensional structures of polypeptides and proteins in complex structures noninvasively.

參考文獻


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