Torrey Pines Institute for Molecular Studies, 11350 Southwest Village Parkway, Port St. Lucie, FL 34987, USA.
Org Biomol Chem. 2013 May 14;11(18):2979-87. doi: 10.1039/c3ob27464k.
The improved bioavailability, stability and selectivity of cyclic peptides over their linear counterparts make them attractive structures in the design and discovery of novel therapeutics. In our previous work, we developed an imidazole-promoted preparation of cyclic depsipeptides in which we observed that increasing the concentration of imidazole resulted in the concomitant increase in the yield of cyclic product and reduction in dimerization, but also resulted in the generation of an acyl-substituted side product. In this work, we used transition state analysis to explore the mechanism of the imidazole-catalyzed esterification of one such peptide, Ac-SAFYG-SCH2φ, and determined the acyl substitution product to be an intermediate in a competing reaction pathway involving acyl substitution of the thioester by imidazole. Our findings indicate that imidazole plays an essential role in this side-chain to C-terminal coupling, and by extension, in transesterifications in general, through a concerted mechanism wherein imidazole deprotonates the nucleophile as the nucleophile attacks the carbonyl. The system under study is identical to the histidine-serine portion of the catalytic triads in serine proteases and it is likely that these enzymes employ the same concerted mechanism in the first step of peptide cleavage. Additionally, relatively high concentrations of imidazole must be used to effectively catalyze reactions in aprotic solvents since the overall reaction involves imidazole acting both as an acid and as a base, existing in solution as an equilibrium distribution between the neutral form and its conjugate acid.
环状肽相对于其线性对应物具有更高的生物利用度、稳定性和选择性，这使得它们成为设计和发现新型治疗药物的有吸引力的结构。在我们之前的工作中，我们开发了一种咪唑促进的环二肽的制备方法，我们观察到增加咪唑的浓度会导致环状产物的产率增加，同时二聚体的减少，但也会导致酰基取代的副产物的生成。在这项工作中，我们使用过渡态分析来探索咪唑催化的这种肽，Ac-SAFYG-SCH2φ 的酯化反应机制，并确定酰基取代产物是涉及咪唑对硫酯进行酰基取代的竞争反应途径中的中间体。我们的发现表明，咪唑在这种侧链到 C 末端偶联中起着至关重要的作用，并且通过扩展，在一般的酯交换中也是如此，通过协同机制，咪唑使亲核试剂脱质子，同时亲核试剂攻击羰基。研究的体系与丝氨酸蛋白酶催化三联体中的组氨酸-丝氨酸部分相同，这些酶很可能在肽裂解的第一步中采用相同的协同机制。此外，由于整个反应涉及咪唑既作为酸又作为碱的作用，在非质子溶剂中有效催化反应必须使用相对较高浓度的咪唑，因为它在溶液中以中性形式与其共轭酸之间的平衡分布存在。
