探究超分子化學及電噴灑游離法中的催化現象

Abstract

電噴灑游離法(electrospray ionization)串聯質譜分析(mass spectrometry analysis)使我們能夠研究高分子的化學計量、構形，也使我們可以研究生物高分子與小分子或脂質的作用。此外，由於質譜的高特異性、高靈敏度及掃描速度快的特性，電噴灑游離法串聯質譜分析(ESI-MS)亦是一個監測化學反應及探究反應中間體的有力工具。然而，超分子化學在質譜偵測上面臨了幾個困難。另外，雖然有些反應在電噴灑游離法下的催化現象已經被研究過，但利用氣體分子做為反應物的有機金屬催化反應在電噴灑游離法下的催化效果並未被完整探討。所以我們針對超分子化學研究建立了一個以電噴灑游離法為基礎的方法，並且研究有機金屬催化反應在電噴灑游離法下的催化效果以及其反應中間體。 在巧妙設計聚合物鏈的末端基團後，經由金屬-配體配位的超分子嵌段共聚物可以在常溫常壓下被合成。然而，這種金屬超分子嵌段共聚物的質譜分析尚存在許多困難。其中一個原因是這種聚合物的非極性使其難以溶於一般電噴灑游離法適合的溶劑裡，而另一個困難點則是金屬配位的非共價鍵在游離化的過程中容易斷裂而無法保持分子的完整。於是，我們證明了這種金屬超分子嵌段共聚物在電噴灑游離化的過程中，可以被微胞充分地保護並在水溶液中維持其完整分子的狀態。高解析質譜的證據顯示了介面活性劑分子在此類金屬超分子嵌段共聚物變成氣態分子的過程中有效地保護了非共價鍵結。有趣的是，在進入質譜的過程中，大部分的介面活性劑分子都從金屬超分子嵌段共聚物表面脫附，使得質譜分析中的訊號主要由完整的金屬超分子嵌段共聚物的訊號所構成。這樣以電噴灑游離為基礎的方法使我們可以利用離子井中的碰撞誘發解離(collision-nduced dissociation)分析金屬和配基的鍵結能量。該金屬超分子嵌段共聚物的構型和碰撞截面(collision cross-section)也可藉由後續的離子遷移譜串聯質譜儀來分析。此平台亦可觀測到該類分子所具環境敏感(environmental-dependent)的構型變化之特殊性質。 質譜儀已經被用來研究微小液珠裡的反應加速現象以及探究反應的中間體。因為在以電噴灑游離法為基礎的方法中能夠產生微小液珠並串聯質譜並做分析，所以這些方法在研究微小液珠下的反應加速現象上是很有潛力的工具。此外，質譜儀也可以用來監測化學反應以及觀測化學反應的中間體。然而，對於超分子化學及有機金屬催化反應在微小液珠下的加速反應尚未被研究透徹。而且大部分的有機金屬催化反應的中間體尚未被鑑定出來。所以，我們建立了以電噴灑游離為基底的各種方法(EESI和DESI)來研究超分子化學及兩種不同有機金屬催化反應在微小液珠下的催化現象。我們比較在大體積溶液中與微小液珠環境下的反應來追蹤反應速率的加速現象。另外，我們比較了離線反應與在線反應來找尋反應中間體。

Electrospray ionization (ESI) coupled with mass spectrometry (MS) allows us to study the macromolecules, such as the subunit stoichiometry, structural information and interaction of bio-macromolecules with small molecules/lipid. Moreover, ESI-MS has also been a powerful tool for monitoring chemical reactions and searching the intermediate because of its high specificity, sensitivity and speed. However, the supramolecular chemistry has faced some challenges. In addition, though catalytic behaviors by using electrospray in some reactions has been studied, the organometallic catalysis reactions using gas molecules as reactant have not been investigated. As a result, in this research, we develop an ESI-based method for supramolecular copolymers and study the catalytic behavior during electrospray ionization as well as the intermediate in organometallic catalysis reactions. Supramolecular diblock copolymers using metal-ligand coordination can be synthesized under ambient conditions by delicate design of the end groups of the homopolymer chains. However, mass spectrometric analysis of such metallo-supramolecular copolymers is challenging. One of the reasons is the nonpolarity of the polymer chains, making it hard to disperse the complexes in electrospray ionization (ESI)-friendly environments. The other difficulty is the noncovalent nature of such copolymers, which is easily disrupted during the ionization. Here, we demonstrate that the intact metallo-supramolecular diblock copolymers can be maintained sufficiently during the ESI process in aqueous solution within micelles. The high resolution mass spectrometric evidence revealed that the surfactant molecules effectively protect the noncovalent binding of the complexes into gaseous ions. Intriguingly, surfactant molecules were sufficiently detached away from the copolymer complexes, giving unambiguous mass spectra which were predominated by intact diblock copolymers. This ESI-based approach allowed us to investigate the relative bond strengths of metal-to-ligand complexation using collision-induced dissociation (CID) in the ion trap mass spectrometry. Conformational features and collision cross-sections of the copolymers were thus obtained using subsequent ion mobility spectrometry mass spectrometry (IMS-MS). Remarkable environment-dependent conformations of the denoted diblock copolymers were found using this mass spectrometric platform. Mass spectrometry has been used for studying acceleration of reaction in microdroplet and for searching intermediate in chemical reactions. In ESI-based methods, microdroplets can be generated and then coupled with mass spectrometry for analysis, so these methods are potential tools for studying the acceleration in microdroplet. Moreover, the reaction can be monitored and the intermediate can be observed by mass spectrometry in some chemical reactions. However, reaction acceleration of supramolecular chemistry and organometallic catalyzed reactions in microdroplet has not been well studied. Also, the intermediate of organometallic catalyzed reactions has not been identified in most cases. Hence, we set up ESI-based methods (EESI and DESI) for studying the reaction acceleration of supramolecular chemistry and two different organometallic catalyzed reactions. We compare the experiment in bulk solution and microdroplet environment to trace the enhancement of reaction rate. In addition, to search the intermediate, we compare the off-line experiment and on-line experiment.