Cyclic dipeptides, also known as 2,5-diketopiperazines (DKPs), represent the simplest peptides that were first completely characterized. DKPs can catalyze the chiral selection of reactions and are considered as peptide precursors. The origin of biochemical chirality and synthesis of peptides remains abstruse problem believed to be essential precondition to origin of life. Therefore, it is reasonable to believe that the DKPs could have played a key role in the origin of life. How the formation of the DKPs through the condensation of unprotected amino acids in simulated prebiotic conditions has been unclear. Herein, it was found that cyclo-Pro-Pro could be formed directly from unprotected proline in the aqueous solution of trimetaphosphate (P₃m) under mild condition with the yield up to 97%. Other amino acids were found to form proline-containing DKPs under the same conditions in spite of lower yield. During the formation process of these DKPs, P₃m promotes the formation of linear dipeptides in the first step of the mechanism. The above findings are helpful and significant for understanding the formation of DKPs in the process of chemical evolution of life.

中文翻译：

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在潜在的地球早期条件下，环状二肽的益生元形成。

环状二肽，也称为2，5-二酮哌嗪（DKPs），是最先被完全鉴定的最简单的肽。DKP可以催化反应的手性选择，并被认为是肽的前体。生化手性和肽合成的起源仍然是奥妙的问题，被认为是生命起源的必要前提。因此，有理由相信DKP可能在生命起源中发挥了关键作用。在模拟的益生元条件下如何通过未保护的氨基酸的缩合形成DKP尚不清楚。在这里，发现可以在三偏磷酸酯水溶液中由未保护的脯氨酸直接形成cyclo-Pro-Pro（P ₃m）在温和的条件下，收率可达97％。尽管收率较低，但发现其他氨基酸在相同条件下仍可形成含脯氨酸的DKP。在这些DKP的形成过程中，P ₃ m在该机理的第一步促进了线性二肽的形成。上述发现对理解生命化学进化过程中DKP的形成是有帮助和重要的。