The kinetics of diketopiperazine formation from the peptide H-Ala-Pro-NH2 with the unprotected amino group in the form of its trifluoroacetate salt have been investigated in a large number of solvents, including aprotic and hydroxylic solvents. The first-order rate constant is considerably affected by the solvent properties, its value spanning more than three orders of magnitude. Moreover, alkylammonium carboxylate salts are efficient catalysts of the reaction. The correlation with Kamlet–Taft solvent parameters shows that the reaction rate is retarded by solvents with a high capacity to stabilise solutes that are charged or dipolar, and that are hydrogen donors and/or acceptors. Solvents with high cohesive energy density values significantly increase the reaction rate. These results are discussed in terms of a proton switch in the rate determining step and of solvation stabilisation of the initial state of the peptide and of the transition state of the rate-limiting step.
在大量溶剂(包括非质子溶剂和羟基溶剂)中,对肽H-Ala-Pro-NH2中未保护的
氨基以
三氟乙酸盐形式形成二酮
哌嗪的动力学进行了研究。一阶速率常数受溶剂性质的影响很大,其值跨越三个数量级。此外,烷基
铵羧酸盐是反应的有效催化剂。与Kamlet-Taft溶剂参数的相关性表明,反应速率受溶剂的影响,溶剂具有高容量,可稳定带电或双极性的溶质,以及氢供体和/或受体。具有高内聚能密度的溶剂可显著提高反应速率。这些结果从速率决定步骤中的质子开关以及肽初始状态和速率限制步骤过渡状态的溶剂化稳定性的角度进行了讨论。