Water-Catalyzed Excited-State Proton Transfer Reactions in 7-Azaindole and Its Analogues
Date Issued
2014
Date
2014
Author(s)
Wu, Yu-Sin
Abstract
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.
Subjects
7-azaindole
pKa
pKa＊
激發態質子轉移(ESPT)
互變異構體
Type
thesis
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