In our reaction cycle, we used propionic acid (C3) as a catalyst because it forms a symmetric anhydride that does not self-assemble. We used the C3 in 200 mM MES buffer at pH 6 at 21 °C in all experiments unless stated differently. First, we studied the effect of the fuel on the formation of the N-acylurea side product. We initiated the reaction cycle by adding 15 mM EDC, CMC, or DIC to 75 mM C3