5 years ago

The Excited-State Triple Proton Transfer Reaction of 2,6-Diazaindoles and 2,6-Diazatryptophan in Aqueous Solution

The Excited-State Triple Proton Transfer Reaction of 2,6-Diazaindoles and 2,6-Diazatryptophan in Aqueous Solution
Pi-Tai Chou, Jiun-Yi Shen, Yi-Han Chen, Yen-Hao Hsu, Kun-You Chung, Yi-An Chen, Yi-Ting Chen, Chi-Lin Chen
3-Me-2,6-diazaindole ((2,6-aza)Ind) was strategically designed and synthesized to probe water molecule catalyzed excited-state proton transfer in aqueous solution. Upon electronic excitation (λmax ∼ 300 nm), (2,6-aza)Ind undergoes N(1)–H to N(6) long-distance proton transfer in neutral H2O, resulting in normal (340 nm) and proton-transfer tautomer (480 nm) emissions with an overall quantum yield of 0.25. The rate of the water-catalyzed proton transfer shows a prominent H/D kinetic isotope effect, which is determined to be 8.3 × 108 s–1 and 4.7 × 108 s–1 in H2O and D2O, respectively. Proton inventory experiments indicate the involvement of two water molecules and three protons, which undergo a relay type of excited-state triple proton transfer (ESTPT) in a concerted, asynchronous manner. The results demonstrate for the first time the fundamental of triple proton transfer in pure water for azaindoles as well as pave a new avenue for 2,6-diazatryptophan, an analogue of tryptophan exhibiting a similar ESTPT property with (2,6-aza)Ind, to probe biowaters in proteins.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b01672

DOI: 10.1021/jacs.7b01672

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