Water-Catalyzed Excited-State Proton-Transfer Reactions in 7‑Azaindole and Its Analogues

The mechanism of the water-catalyzed excited-state proton-transfer (ESPT) reaction for 7-azaindole (<b>7AI</b>) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a <b>7AI</b> analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in <b>7AI</b> and its analogues in an attempt to unify the mechanism. Herein, a series of <b>7AI</b> analogues and their methylated derivatives were synthesized to carry out a systematic study on p<i>K</i>_a, p<i>K</i>_a*, and the associated fluorescence spectroscopy and dynamics. The results conclude that all <b>7AI</b> derivatives undergo water-catalyzed ESPT in neutral water. However, for those derivatives with −H (<b>7AI</b>) and a electron-donating substituent at C(3), they follow water-catalyzed ESPT to form an excited N(7)–H proton-transfer tautomer, T*. T* is rapidly protonated to generate an excited cationic (TC*) species. TC* then 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 group at the C(2) or C(3) or with the 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 p<i>K</i>_a* of the cationic form, that is, 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 <b>7AI</b>, the previously missing T* emission is clearly resolved with a peak wavelength at 530 nm in the pH interval of 13.0–14.3 (<i>H</i>_– 14.2)

Dated_phylogeny_for_Chinese_angiosperms

The tree file is a dated phylogeny for Chinese angiosperms calculated in treePL. The data set includes 5,864 species representing 2,665 genera native to China (ca. 92% of angiosperm genera in China)

Probing Water Environment of Trp59 in Ribonuclease T1: Insight of the Structure–Water Network Relationship

In this study, we used the tryptophan analogue, (2,7-aza)­Trp, which exhibits water catalyzed proton transfer isomerization among N(1)-H, N(7)-H, and N(2)-H isomers, to probe the water environment of tryptophan-59 (Trp59) near the connecting loop region of ribonuclease Tl (RNase T1) by replacing the tryptophan with (2,7-aza)­Trp. The resulting (2,7-aza)­Trp59 triple emission bands and their associated relaxation dynamics, together with relevant data of 7-azatryptophan and molecular dynamics (MD) simulation, lead us to propose two Trp59 containing conformers in RNase T1, namely, the loop-close and loop-open forms. Water is rich in the loop-open form around the proximity of (2,7-aza)­Trp59, which catalyzes (2,7-aza)­Trp59 proton transfer in the excited state, giving both N(1)-H and N(7)-H isomer emissions. The existence of N(2)-H isomer in the loop-open form, supported by the MD simulation, is mainly due to the specific hydrogen bonding between N(2)-H proton and water molecule that bridges N(2)-H and the amide oxygen of Pro60, forming a strong network. The loop-close form is relatively tight in space, which squeezes water molecules out of the interface of α-helix and β2 strand, joined by the connecting loop region; accordingly, the water-scant environment leads to the sole existence of the N(1)-H isomer emission. MD simulation also points out that the Trp-water pairs appear to preferentially participate in a hydrogen bond network incorporating polar amino acid moieties on the protein surface and bulk waters, providing the structural dynamic features of the connecting loop region in RNase T1