Abstract
Unexpected fragmentation reactions of tripeptides H-Pro-Hyp[CH2(R)]-Pro-OBn (R = cytosine (Boc) or uracil) to afford diketopiperazine (DKP) were investigated. Although peptide fragmentation to DKP has not been observed in N-terminal unprotected triproline esters, these tripeptides containing nucleobase underwent unexpected amide cleavage. The nucleobase at the second proline unit of the tripeptide was found to create an internal hydrogen bonding environment around the tripeptide. This increased the cis-state population and the basicity of the N-terminus proline nitrogen of the tripeptide, thus accelerating the unexpected amide cleavage.
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