非血基質之 N3O2 五牙配位基三價錳超氧化物之熱力學、動力學與反應機制研究

Abstract

在自然界中有許多酵素透過氧氣進行活化的反應，而在本實驗室所開發的 BDPP 系統下，我們以不同取代基的配位基搭配不同的金屬來模擬這些大自然中透過氧氣活化反應之非血基質酵素，在本研究中，使用錳金屬配位氫取代 BDPP 的錯合物 MnII(BDPP) 來模擬透過氧氣活化反應之酵素，並且利用和氧氣反應後所生成的三價錳超氧化物MnIII(BDPP)(O2•) 去進行一系列反應性研究的探討。透過紫外-可見光光譜、電子順磁共振光譜、循環伏安法可追蹤到反應所生成的中間體，加入 TFA 後，可發現從原本的三價超氧化物變成四價的過氧化物，並且使用 TEA 逆滴定回來之後，可求得 pKa 值為 12.42 而三價錳超氧化物與 TEMPOH 反應則會形成三價錳氫過氧化物 MnIII(BDPP)(OOH)，測得的半電位為 0.324 V，綜合這兩個數據可透過 Bordwell 方程式得到 MnIII(BDPP)(OOH) 的 O-H 鍵能為 84.87 kcal/mol，為了進一步研究超氧化物的反應性，使用三價錳超氧化物與 2-tert-Butyl-4-X-phenols (X = OEt, OMe, Me, Et, tBu, H, Br, CF3) 反應並求得二級速率常數 (k2) 綜合所有得到的數據並做成 Hammett plot 可發現在推電子傾向的取代基有親電反應而在拉電子傾向的取代基有親核反應。並且，為了更了解反應機制，使用氘化之 2-tert-Butyl-4-X-phenols (X = OMe、Br) 進行動力學同位素效應 (KIE) 的實驗，所獲得的值為 1.21 及 0.3，代表反應機制為 PCET。透過以上的實驗，我們可以更加瞭解三價錳超氧化物的反應性及反應機制，並且在和氧氣活化的仿生酵素研究中獲得更進一步的突破。There are a lot of enzymes that activated by oxygen in nature. According to BDPP ligands developed by our laboratory, we use ligands with different substituents and different metals to simulate non-heme enzymes activated by oxygen in nature. In this study, the MnIIBDPP is going to be used to simulate these enzymes, and the MnIII(BDPP)(O2•) which was the product of reaction of MnIIBDPP and oxygen would be used to conduct a series of investigation of reactivity.Through UV/vis and EPR and CV, the intermediate can be traced. After adding TFA, we discovered that the MnIII(BDPP)(O2•) was oxidized to MnIV(BDPP)(OOH). Then, we could obtain the pKa value which was 12.42 after backtitration of TEA. Besides, the MnIII(BDPP)(OOH) formed if MnIII(BDPP)(O2•) reacted with TEMPOH. Then, we measured the redox potential E1/2 which was 0.324 V. With pKa and E1/2, we could obtain that the bond dissociation free energies of O-H of MnIII(BDPP)(OOH) that was 84.87 kcal/mol by Bordwell equation. To do further research of reactivity of MnIII(BDPP)(O2•), the experiments of MnIII(BDPP)(O2•) with 2-tert-Butyl-4-X-phenols (X = OEt, OMe, Me, Et, tBu, H, Br, CF3) were done to obtain the k2. Combining all of data, we could make a Hammett plot. Then, we discovered the electrophilic reaction when experiments of MnIII(BDPP)(O2•) with electron donating substituents phenol were done. However, the nucleophilic reaction was discovered when experiments of MnIII(BDPP)(O2•) with electron withdrawing substituents phenol were done. Also, to know more about the mechanism of the reaction of MnIII(BDPP)(O2•) with 2-tert-butyl-4-X-phenols, we chose deuterated 2-tert-Butyl-4-X-phenols (X = OMe、Br) to do the experiments of kinetic isotope effect. Then, we got the values of 1.21 and 0.3. As a result, we knew that the mechanism was PCET.Through the above experiments, we can know more about the reactivity and the mechanism of MnIII(BDPP)(O2•). Also, we can obtain further breakthrough in the research of biomimetic enzymes activated by oxygen.