DataSheet1_Kinetics and Reaction Mechanism of Biothiols Involved in SNAr Reactions: An Experimental Study.PDF

Few kinetic parameters, or reaction rates, are known up to date in detail about 1-chloro and 1-fluoro-2,4-dinitrobenzene (ClDNB and FDNB, respectively) with a series of biothiols in aqueous media. These biological nucleophiles with thiol groups have been widely used as a reference in nucleophile reactivity assays due to their prevalence and cellular abundance. The main aim of this study was to elucidate the reaction mechanism based on Brönsted-type plots and reactivity patterns of the electrophile/nucleophile pairs. A complete kinetic study was performed in terms of the comparison of Brönsted-type slope parameters (βnuc) for the reactions and was used for assigning the mechanism and the rate-determining step associated with the reaction route. A mass spectrometry analysis demonstrated that the nucleophilic center of the biothiols is the -SH group and there is only one kinetic product. The kinetic study suggests that the reaction mechanism might be the borderline between concerted and stepwise pathways. An amine–enol equilibrium for the most reactive nucleophiles appears to be the main determining factor controlling the nucleophilic attack in the nucleophilic aromatic substitution reactions investigated, highlighting the anionic form for these nucleophiles. This amine–enol equilibrium involves a hydrogen bond which stabilizes the intermediate species in the reaction pathway. Thus, intramolecular bonds are formed and enhance the nucleophilic strength through the contribution of the solvent surrounding the electrophile/nucleophile pairs. Finally, we highlight the importance of the formation of electrophile/nucleophile adducts that could modify structures and/or functions of biological systems with potential toxic effects. Therefore, it is essential to know all these kinetic and reactivity patterns and their incidence on other studies.

截至目前，针对水介质中1-氯-2,4-二硝基苯（1-chloro-2,4-dinitrobenzene, ClDNB）与1-氟-2,4-二硝基苯（1-fluoro-2,4-dinitrobenzene, FDNB）分别与一系列生物硫醇的反应，其详细动力学参数（或反应速率）相关数据仍较为匮乏。这类带有巯基的生物亲核试剂因分布广泛且在细胞内含量丰富，被广泛用作亲核试剂活性测定的参照体系。本研究的核心目标是基于亲电试剂/亲核试剂对的布朗斯特型（Brönsted-type）作图与活性模式，阐明该反应的反应机制。本研究通过对比各反应的布朗斯特型斜率参数（βnuc）开展了完整的动力学研究，以此确定反应路径对应的反应机制与决速步。质谱分析结果表明，生物硫醇的亲核活性中心为巯基（-SH），且该反应仅生成一种动力学产物。动力学研究结果显示，该反应的机制可能处于协同路径与分步路径的临界状态。对于活性最强的亲核试剂而言，其胺-烯醇平衡似乎是调控所研究的亲核芳香取代反应中亲核进攻的核心决定因素，这凸显了这类亲核试剂的阴离子形态。该胺-烯醇平衡过程会形成氢键，稳定反应路径中的中间体物种。由此形成的分子内键可通过亲电试剂/亲核试剂对周围溶剂的作用，增强亲核试剂的亲核能力。最后，本研究强调了亲电试剂/亲核试剂加合物形成的重要性：这类加合物可能改变生物系统的结构与/或功能，并带来潜在毒性效应。因此，全面掌握这类动力学与活性模式及其对其他研究的影响，具有重要意义。