With the peptide Ac-AYRGD-OH, we followed the kinetics of the reaction cycle in more detail by HPLC (Fig. ). After adding 10 mM of EDC as fuel to 3 mM of peptide, we found that the EDC was consumed within 2 hours. The consumption of EDC produced three major new peaks that we attributed to the cyclic monomer, regular dimer, and cyclodimer. Each of these peaks, as well as the smaller peaks, were transient and had completely disappeared after 6 hours. We did not find a peak for the corresponding O-acylisourea or the aspartic acid anhydrides, most likely because they are converted quickly into esters. With these kinetic profiles, we developed a kinetic model that predicts the evolution of the reaction cycle by fitting the HPLC data using a set of differential equations for each reaction. The reactions we consider are the peptide anhydride formation, and the peptide anhydride can react further inter-or intramolecularly with tyrosine to form a cyclic monomer or linear dimer. The linear dimer can also form its respective anhydride, which can again react intramolecularly to the cyclic dimer. We did not consider reactions further to the linear trimer as it was only observed in neglectable amounts or not at all. Hydrolysis can revert these reactions (see Supporting Information for details).