Materials and characterization. Uracil (U) and 2,6-diaminopurine (D) were purchased from Carbosynth. Grinding experiments were conducted in 14 mL poly(methyl methacrylate) (PMMA) containers using InSolido Technologies vibratory ball mill. Two 1.4 g stainless steel balls (7 mm in diameter) were used as milling media and the reactions were conducted at a 30 Hz frequency. In grinding reactions with H 2 O, 20 µL of H 2 O was added to the overall 200 mg of the reaction mixture (1.5:1 ratio of H 2 O to D and U). Grinding under low-temperature conditions was performed using liquid nitrogen. PMMA jars containing the reaction mixture were immersed in liquid nitrogen before mounting on the mill and liquid nitrogen was poured continuously over the jars during 30 min of ball milling. Slurry reactions were performed by stirring an aqueous suspension of D and U (equimolar amounts of D and U, overall mass of the reaction mixture 100 mg) in 3 mL of distilled H 2 O at room temperature in a 10 mL round bottom flask. Aging reactions were conducted by placing a plastic tube containing a solid sample in a glass bottle whose bottom was filled with distilled H 2 O. The aging setup was a closed system and the glass bottle was closed with a cap.