Reaction mechanism. (A) Schematic representation of the general reaction mechanism. (B) Detailed reaction mechanism. 5'-amino-3-'phosphate pyrimidine-purine dinucleotide (1) is activated by EDC (2) binding to phosphate group with uptake of one proton. EDC (2) in water is partially hydrolyzed with release of the proton and EDU (3) formation. Single stranded activated molecules (4) undergo pathway A, forming an inactive cyclic side product (5) and releasing EDU and proton. At high concentrations and low temperatures where duplex formation is possible, polycondensation and chain growth via pathway B takes place, leading to the product (6).