雖然芳香團胺基酸在紫外光區吸收係數很高,然而其螢光產率卻很低。因此,從電子激發能階,經由內轉換變成電子最低能階的激發振動能階,被視為最主要的非放光途徑。在這種氣相無碰撞的狀態下,分子經由內轉換到基態之後而形成熱分子。如果累積到一定程度能量,且超過鍵能,分子便會進行斷鍵。另一方面,如果這樣的機制是發生在液相的話,分子便會在斷鍵之前,就周遭溶劑分子把能量給帶走而穩定下來。然而,近年來理論計算指出,從一斥力態掉氫原子,才是芳香團胺基酸最主要走的非放光途徑。由於斥力態分解的速率太快,即使在液相也無法完全被溶劑分子把能量帶走。因此,胺基酸分子被紫外光照射後,所走的光穩定路徑很值得探討。在這實驗裡頭,我們主要針對胺基酸發色團來進行光分解探討,利用多質量離子影像儀來驗證,分子吸收紫外光之後,究竟是從斥力態分解還是從基態分解。結果發現到,在小的胺基酸發色團分子,如:三甲基蚓垛,吸收紫外光之後,走斥力態分解途徑是很重要的途徑;然而,當作到更大的胺基酸發色團分子時,如:n-acetyl tryptophan ether ester and n-acetyl phenylalanine methyl ester,吸收紫外光之後,由於是走基態斷碳碳鍵,因此,在液相環境裡頭,便有機會被溶劑分子在斷鍵之前就把能量給帶走。
Although the absorption coefficients of aromatic amino acids in the ultraviolet region are very large, it has been shown that the fluorescence quantum yields are very small. Internal conversion to the electronic ground state was assumed to be the major non-radiative process. In the gas phase under collisionless conditions, molecules become highly vibrational excited in the electronic ground state after internal conversion. If the initial photon energy is larger than the chemical bond energy, bond cleavage eventually occurs. On the other hand, in the condensed phase molecules are quenched by surrounding solvent molecules and are stabilized. However, recent theoretical calculation predicts that H atom elimination from a repulsive electronic excited state is the major non-radiative process for aromatic amino acid chromophores. The dissociation from the repulsive state is too fast to be quenched even in the condensed phase. It becomes a potential problem for the photostability of amino acids upon UV irradiation. In this experiment, we focused on the photodissociation dynamics of amino acid chromophores. Dissociation occurs on the ground electric state or on a repulsive state after UV excitation was investigated by multimass ion imaging techniques. The results show that H atom elimination from the repulsive potential energy surface is important in small amino acid chromophores like 3-methylindole, but it is completely quenched in large amino acid chromophores like n-acetyl tryptophan ether ester and n-acetyl phenylalanine methyl ester. The only dissociation channel for this large chromophore is carbon-carbon cleavage.
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