Investigation of Protonated Serine Clusters using Infrared Photodissociation Spectroscopy and Fourier Transform Ion Cyclotron Resonance Spectrometry

本論文採用振動吸收解離式紅外光譜學的分析研究方法，結合高階理論計算用於研究氣相中離子化絲胺酸團簇的結構。 在本論文中，第一章介紹質子化絲胺酸團簇，其中的絲胺酸八聚體於質譜中具有強度異常高的現象，且被認為可能與生命起源有關，因而其結構成為大家所極欲探究的主題。第二章描述本論文的研究方法、質譜儀器原理和裝置、雷射系統及所使用的理論計算參數。利用電灑游離法（Electrospray Ionization，ESI）作為離子源形成氣相質子化絲胺酸團簇（H+(Ser)n, n = 2-8），導入傅立葉轉換離子迴旋共振質譜儀（Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry，FT-ICR）中的離子迴旋共振腔體（ion cyclotron resonance cell），腔體另一方導入可調波長的紅外雷射光束，經由不同頻率的共振吸收後得到其氣相紅外線光子吸收光譜。第三章是實驗結果分析與討論的部份，本論文光譜研究的波數範圍是由2850cm-1到3750cm-1，結果主要得到兩個共振吸收峰；經由光譜指紋的比對之下，在光譜中我們觀察到第一根在3565cm-1的吸收峰來自羧基上自由氫氧根振動（C(=O)-O-H），第二根在3673cm-1的吸收峰則是脂肪族自由氫氧根伸展振動（CH2-O-H）。此兩根吸收峰的相對高度比例相當於羧基COOH與脂肪族氫氧基的官能基相對數目比；質子化絲胺酸二聚體的吸收相對高度比是1.2 :比1，意即-COOH和-CH2OH的官能基數目一樣，進而驗證二聚體的絲胺酸單位是中性的。當此團簇的絲胺酸數目漸多時，第一根吸收峰（3565 cm-1）的高度則隨著漸減，直到八聚體時這根吸收峰完全消失，證明了質子化絲胺酸八聚體在氣相狀態下的結構是以兩性離子（zwitterions）為主。第四章的結論中提到由此一系列由二至八的絲胺酸團簇紅外吸收光譜結果，我們觀察到從二聚體的中性絲胺酸單位到八聚體的兩性離子單位，中間經歷了結構轉換的過程。This thesis depicts an investigation of infrared photodissociation spectra of protonated serine clusters, H+(Ser)n, n=2-8, using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer coupled with an OPO infrared laser system. Density functional theory calculations performed at B3LYP/6-311++G* level are also presented to assist interpretation of the spectra for H+(Ser)2. This combined investigation provides information on the structure of these ionic clusters in the gas phase. The content of this thesis is divided into four chapters. Chapter one introduces the serine octamer including the formation of this magic number cluster and the strong preference for homochirality and the possibility of its importance related to the prebiotic chemistry. Chapter two depicts the methods used for both experiment and calculation. Methods and apparatus used in vibrational predissociation spectroscopy, ESI–FT-ICR mass spectrometry, optical parametric oscillator lasers and ab initio calculations are described. Chapter three illustrates results and related discussion. Two dominant peaks corresponding to the free carboxyl O-H stretching at 3565 cm-1 and the free aliphatic hydroxyl O-H stretching at 3673 cm-1, respectively, are observed over the 3500-3750 cm-1 in the infrared photodissociation spectra. For H+(Ser)2,The spectral fingerprints of the transition are the observed intensity ratio of these two bands is 1.2:1, indicating an equal number of –COOH and –CH2OH groups in this binary complex with a neutral structure. The 3565 cm-1 band, on the other hand, progressively decreases in intensity with increasing n and it disappears completely at n = 8, suggesting that the protonated serine octamer is dominated by zwitterionic structures. Chapter four gives a conclusion. It is concluded that the infrared photodissociation spectroscopy of protonated serine clusters H+(Ser)n, n = 2 – 8, provides evidence for a size-dependent transition from neutral to zwitterionic network structures.Abstract............................................vi 摘要................................................vii Chapter 1 Introduction.............................1 1.1 Serine Octamers.................................1 1.1.1 Magic Number Clusters.........................1 1.1.2 Homochirality of Serine Octamer...............3 1.2 Homochirogenesis................................6 1.3 Structures of the Serine Octamer................8 1.4 Neutral or Zwitterionic Structures..............9 Reference ..........................................12 Chapter 2 Methodology.............................14 2.1 Electrospray Ionization (ESI)..................14 2.2 FT-ICR Mass Spectrometry.......................17 2.2.1 Introduction .................................17 2.2.2 Principles of Ion Cyclotron Motion...........20 2.2.3 Ion Excitation...............................22 2.2.4 Ion Detection................................24 2.2.5 Fourier Transformation.......................25 2.3 Vibrational Predissociation Spectroscopy.......27 2.4 Optical Parametric Oscillator Lasers...........29 2.5 Sample Preparation and Experimental Conditions.30 2.6 Ab Initio Calculations.........................31 Reference ..........................................32 Chapter 3 Results and Discussion..................33 3.1. Mass Spectra of Serine Clusters...............33 3.2. Calibration of Laser Frequencies..............36 3.3. Vibrational Photodissociation.................38 3.4. Structures of the Serine Dimer................41 3.5. Structures of the Serine Octamer..............45 3.6. Progressive Stabilization of Zwitterionic Structuresin H+(Ser)2-8.......................47 Reference ..........................................49 Chapter 4 Conclusion..............................5