7-Azaindole及相似物之水催化激發態質子轉移反應

Abstract

水催化7-azaindole (7AI)進行激發態質子轉移的機制已經長期被研究但其結論具有爭議性。由於最近成功的使用7-azaindole (7AI)相似物2,7-diazatryptophan來偵測蛋白質周圍的水分子，因此我們再回去研究7AI及其相似物之水催化激發態質子轉移的機制，並將過去的論點及目前的結果整合整理出更合理的機制。藉由合成一系列7AI相似物及其甲基化衍生物，並具有系統性的研究pKa， pKa*以及相關的螢光光譜及動力學探討。從量測出來的結果，我們認為所有7AI衍生物在中性的水中都會經由水的催化進行激發態質子轉移。然而對於在三號位接推電子基(包括氫)的衍生物經由水催化進行激發態質子轉移形成激發態互變異構體(T*)，此互變異構體迅速發生質子化產生激發態陽離子，再經由快速的去活化過程回到標準態(N)形式的基態。相反的，對於在二號或三號位接拉電子基或二號位的碳原子被拉電子性較強的氮原子取代的衍生物(2,7-diazatryptophan)其激發態互變異構體(T*)不會進行質子化形成陽離子，而會以激發態互變異構體(T*)形式產生綠色的放光。同時藉由合成相關的N(7)-CH3異構體形式的分子來作為補充支持我們的論點，對於其陽離子形式(N(7)-CH3N(1)-H+)的pKa*值，如果是三號位接推電子基其值大於7，而接拉電子基則小於7。7AI之前於pH 7-12之間看不見互變異構體(T*)的放光,但現在於pH 13.0 - 14.5之間可以很清楚的解析出放光峰在波長530nm互變異構體(T*)的放光。

The mechanism of water-catalyzed excited-state proton transfer (ESPT) reaction for 7-azaindole (7AI) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a 7AI analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in 7AI and its analogues in an attempt to unify the mechanism. Herein, a series of 7AI analogues and their methylated derivatives were synthesized to carry out a systematic study on pKa, pKa* and the associated fluorescence spectroscopy and dynamics. The results conclude that all 7AI derivatives undergo water catalyzed ESPT in neutral water. However, for those derivatives with electron-donating substituent (including –H) at C(3), following water catalyzed ESPT to form an excited N(7)-H proton-transfer tautomer, T*, rapid protonation takes place to generate an excited cationic (TC*) species that subsequently undergoes a fast deactivation to the N(1)-H normal species in the ground state. Conversely, protonation in T* is prohibited for those derivatives with an electron-withdrawing groups at the C(2) or C(3), or C(2) atom replaced by an electron-withdrawing nitrogen atom (N(2) in e.g., 2,7-diazatryptophan), giving a prominent green T* emission. Additional support is given by the synthesis of the corresponding N(7)-CH3 tautomer species, for which pKa* of the cationic form, i.e., the N(7)-CH3N(1)-H+ species, is measured to be much greater than 7.0 for those with electron-donating C(3) substituents, whereas it is lower than 7.0 upon anchoring electron-withdrawing groups. For 7AI the previously missing T* emission is clearly resolved with peak wavelength at 530 nm in the pH interval of 13.0-14.5.