Both the constituent amino acids in the formyl-dipeptidyl ester affected the ester lability, despite their inability to form diketopiperazines. It has previously been shown that a Leu-tRNA ester was more stable than an Ala-tRNA ester or a Pro-tRNA ester. We observed the same trend. Furthermore, the 2'/3'-formyldipeptidyl-esters containing fAla are always found to be more stable than those containing fGly.