The response mechanism of QDA to GSH and Hcy was assumed as represented in
Scheme 2. According to the reported reaction mechanism of OPA and GSH [
45], we speculated that QDA could react with GSH or Hcy to form thiazino[3′, 2′: 1, 2]pyrrolo[3, 4-
b]quinoline derivatives due to amino and sulfhydryl functional group of GSH or Hcy possessing the reactivity with aldehyde, and the supposed mechanism was further confirmed by
1H NMR and MS analysis of adduct of QDA and GSH (or Hcy). As shown in Fig. S10 (Supporting information), the protons of aldehyde groups of QDA appeared at 10.67 and 10.26 ppm in
1H NMR spectrum. When GSH or Hcy was added to the DMSO‑
d6 solution of QDA, the proton signals of aldehyde groups disappeared and new proton signals appeared at 6.54–6.11 ppm, which indicated that both aldehyde groups of QDA were involved in the reaction. Moreover, MS analysis of the adduct of QDA with GSH also supported the generation of thiazino[3′, 2′: 1, 2]pyrrolo[3, 4-
b]quinoline derivative, the peak at
m/
z 457.1199 was observed (Fig. S11 in Supporting information), corresponding to [QDA-GSH + H]
+, calcd. for C
21H
21N
4O
6S: 457.1176. Similarly, a peak of
m/
z 285.0705 was observed from MS spectrum of the adduct of QDA with Hcy (Fig. S12 in Supporting information), corresponding to [QDA-Hcy + H]
+, calcd. for C
15H
13N
2O
2S: 285.0692. Therefore, the speculated mechanism was correct and supported the spectral experimental results.