主族低價金屬離子協助甲基咪唑與醋酸根離子形成低能障氫鍵系統之理論研究

The effect of main group metal cations on the low barrier hydrogen bond of 4,5-dimethyl-1H-imidazole and acetate

汪昱成(Yu-Cheng Wang)

指導教授 ： 游靜惠

國立清華大學/理學院/化學系/碩士(2008年)

摘要

在酵素催化系統裡可能擁有低能障氫鍵的性質在最近幾年已經引起了非常多的關注。一開始提出酵素的活化中心存在著特殊的短且強的氫鍵是基於實驗上發現有不尋常的物化性質。低能障氫鍵系統的定義基於位能面，如果氫原子在一個雙位能井的位能面，且氫原子的零點振動能量相同或高於位能面的能障，則可以稱其為低能障氫鍵。
當金屬離子與氫鍵系統同時存在時，它們之間的相互作用，會對系統產生直接或間接的影響。
在本論文係以電子密度泛函理論的B3LYP、還有RMP2的計算層級對加入低價金屬離子後對於4,5-dimethyl-1H-imidazole (DMim)與醋酸根離子(Ac)的氫鍵系統所產生的影響來做研究。
錯合物[DMim-H-Ac]1□為普通氫鍵系統，但加入Li+、Ca2+、Sr2+、Ba2+這四個金屬離子後，可以得到低能障氫鍵的性質。而我們透過NBO電荷分析得知低價金屬離子在此氫鍵系統的電荷轉移不大。從金屬離子與[DMim-H-Ac]1□的錯合物與拿掉金屬離子後的[DMim-H-Ac]1□系統的相對位能面比較上，可以看到Li+、Na+、K+、Rb+、Cs+、Ca2+、Sr2+、Ba2+這八個金屬離子與[DMim-H-Ac]1□的錯合物系統都可以造成長距離的穩定效果，且顯示長距離穩定的能力是依照金屬離子半徑大小的順序來排：鹼金屬族金屬離子系統穩定能力為Li+系統＞Na+系統＞K+系統＞Rb+系統＞Cs+系統；鹼土金屬族金屬離子系統穩定能力為Ca2+系統＞Sr2+系統＞Ba2+系統。最後由金屬離子與系統的鍵結能來分析，進一步得知金屬離子與DMim和Ac部分最重要的作用機制為庫侖作用力。

關鍵字

金屬離子 ； 低能障氫鍵 ； 電子密度泛函理論

並列摘要

The possible involvement of low-barrier hydrogen bonds (LBHBs) in enzyme catalysis has attracted wide attention in recent years. The initial proposal of the existence of special short and strong hydrogen bonds (SSHBs) in enzymatic active sites was based on experimental findings of unique physicochemical properties. The character of a LBHB is based on the shape of the potential energy surface that is the hydrogen atom lies in a double minimum potential well with zero-point vibrational energies the same as or above the barrier height, it is a LBHB. When metal ions exist in hydrogen-bonding systems, their interaction will affect the systems directly or indirectly.
This thesis reports the theoretical investigation on the effect of main group metal cations on the LBHB of 4,5-dimethyl-1H-imidazole (DMim) and acetate (Ac), and all calculations were done at B3LYP or RMP2 level.
The complex [DMim-H-Ac]1□ is a normal hydrogen-bonding system, and the character of the hydrogen bond will become a LBHB while Li+、Ca2+、Sr2+ or Ba2+ is added to [DMim-H-Ac]1□ complex. We find the charge transfer from [DMim-H-Ac]1□ complex to the metal ion is very small by NBO charge analysis. The analysis of the two relative energy surfaces between [M-DMim-H-Ac]Z□ (M=Li+、Na+、K+、Rb+、Cs+、Ca2+、Sr2+、Ba2+) complex and [DMim-H-Ac]1□ complex constructed by removing metal ions of [M-DMim-H-Ac]Z+ shows that metal ions affect the hydrogen-bonding system by long-distance stabilization. The ability of long-distance stabilization is according to metal ionic size : in the aspect of alkali metal ionic systems : Li+＞Na+＞K+＞Rb+＞Cs+ system； in the aspect of alkaline metal ionic systems : Ca2+＞Sr2+＞Ba2+ system. The most important interaction between the metal ion and [DMim-H-Ac]1□ complex is coulombic interaction by analyzing the binding energy between the metal ions and [DMim-H-Ac]1□ complex.

並列關鍵字

metal cation ； low barrier hydrogen bond ； DFT

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