Abstract
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 (P(3)m) 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, P(3)m 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.