Structural optimization of 3,4-dihydroquinolizinium ring toward practical application for chemo-selective modification of cysteine

<h2>Abstract</h2> 3,4-Dihydroquinolizinium (DQ) ring, a core structure of naturally-occurring quinocidin, reacts with cysteine (Cys) through Michael addition-type reaction in neutral aqueous media. In this study, we performed structural optimization of the DQ ring toward practical application for chemo-selective modification of Cys. Stability evaluation of the DQ–Cys adduct revealed that the adduct was gradually decomposed in neutral buffered solutions to regenerate the DQ ring through retro-Michael reaction. This observation led us to design 9-methoxy and 9-ethoxy DQ rings, which rapidly and selectively reacted with Cys and the resulting adducts showed higher stability compared with the non-substituted DQ–Cys adduct. We also demonstrated that the 9-ethoxy DQ ring is superior to the non-substituted one in regard to the reactivity toward Cys residues in peptides and the inhibitory activity against glyceraldehyde-3-phosphate dehydrogenase containing a Cys residue in its active center. These results indicate that the 9-alkoxy DQ rings could serve as practical Cys-selective electrophiles, which expand the toolbox of the methodologies available for Cys modification in biological and pharmacological research.