The Sensing Mechanism Of Novel Thiol Fluorescent Probe-quinolineα,β Unsaturated Malonic Acids

Aliphatic thiols play crucial roles in many physiological processes. The detecting of thiols draws more and more attention from chemists and biologists. The most used method to detect thiols is fluorescent probe method. Large amounts of fluorescent probe molecules were reported during the last decade. These probes could react with thiols through different reaction pathways. They detect thiols based on the optical signal change after the reaction. In our previous work, we proposed a series of novel fluorescent probes,2-(quinolin-2-ylmethylene) malonic acids derivatives(QMAs), which could react with thiols through Michael-addition reaction. A Michael addition usually is taken as a base-catalysed reaction. However, our synthesized QMAs as Michael-type thiol fluorescent probes are acid-active in their sensing reaction. In this work, based on theoretic calculation and experimental study on7-hydroxy-2-(quinolin-2-ylmethylene) malonic acid (QMA) as a representatives, we demonstrated that QMAs as Michael acceptors are acid-activatable, i.e., they work only in solutions at pH<7, and the lower pH of solutions, the reactivity of QMA is higher. In alkaline solution, the malonate QMA[-2H+]2-cannot react with both RS-and RSH. In contrast, QMEs, the esters of QMAs, reveal a contrary pH effect on their sensing reactions, that is, they can sense thiols in alkaline solutions but not in acidic solutions, like a normal base-catalysed Michael addition. The values of activation enthalpies from DFT calculation support above sensing behavior of two probes under different pH conditions. In acidic solutions, the protonated QMA is more highly reactive towards electrophilic attack over its other ionized states in neutral and alkaline solutions, and so can react with lowly reactive RSH. In contrast, there is a big energy barrier in the interaction of QME with RSH (acidic solutions), and the reaction of QME with the highly reactive nucleophile RS-is a low activation energy process (in alkaline solutions). DFT calculation reveals the sensing reaction of QMA undergoes an acid-activated1,4-addition process with neutral (RSH), and a base-activated α,β-addition pathway for the sensing reaction of QME with RS-...