In all three examples where fGly-mixed anhydride was used in the second transfer, we identified two peaks with longer retention time and greater stability toward hydrolysis (peaks marked with * in Fig. , S8, S10). These peaks may correspond to 2'/3'tripeptidyl-esters terminating in fGly. This would require fGly-mixed anhydride to react with the 2'/3'-dipeptidyl-esters we identified at the beginning of this communication (Fig. ). Due to the lack of material, we were unable to characterise these peaks further.