Fig. 6. Covalent template-directed synthesis of a mixed sequence trimer. Ester base-pair chemistry was used to load two different monomer units onto the AAP template with the P–C–dP–C reaction sequence. The resulting pre-ZIP intermediate was subjected to a CuAAC reaction to give the duplex. Hydrolysis of the ester base-pairs in the cleave step regenerated the template, and capping of the terminal azide of the other product provided the complementary copy APP. The antiparallel duplex is shown as the product of the ZIP step, but the parallel duplex leading to PPA as the complementary copy is also possible. Conditions: P TBDMS-Cl, imidazole, then HCl; C EDC, DMAP; dP TBAF; ZIP Cu(CH₃CN)₄PF₆, TBTA; cleave LiOH; cap Cu(CH₃CN)₄PF₆, TBTA.