All chemical syntheses were performed in a Vacuum Atmospheres model MO-40M glovebox under an inert atmosphere of purified nitrogen unless otherwise stated. All solvents used in the glovebox were obtained anhydrous and oxygen-free by bubble degassing with argon and purification through columns of alumina and Q5 by the method of Grubbs, except for water and acetone. 1 HPLC grade water was purchased from Sigma Aldrich and used as received. Dried acetone was prepared by stirring overnight over anhydrous calcium sulfate followed by distillation under an anhydrous nitrogen atmosphere from fresh calcium sulfate. The tetrabutylammonium (TBA) salts of 5'-UMP, 5'-AMP, and 5'-ADP were prepared according to the procedure of Taylor, 3 by ion exchange on Dowex 50WX8 (H+) followed by neutralization with TBA hydroxide and subsequent lyophilization. The resulting TBA nucleotide salts were stored as either DMF or acetonitrile stock solutions (0.10 g/mL) over 4Å molecular sieves (dried under vacuum at 200 • C for 24 hours) for at least 24 hours prior to use. was prepared according to the literature procedure. All other reagents were purchased and used as received. Deuterated solvents were purchased from Cambridge Isotope Labs and used as received. NMR spectra were obtained at ambient temperature (ca. 25 • C) on a Bruker Avance 400 instrument. 1 H and 13 C NMR spectra were referenced internally to residual solvent signals. 31 P NMR spectra were referenced internally to the PPN signal or externally to 85% phosphoric acid. Electrospray ionization mass spectra (ESI-MS(-)) were acquired on a Micromass Q-TOF ESI spectrometer. Samples were prepared in acetonitrile or acetonitrile/water at an approximate concentration of 20 ng/µL, and a source temperature of 80 • C and desolvation gas temperature of 400 • C were used. Elemental analyses were performed by Midwest Microlab. HPLC was performed with Waters 515 HPLC pumps coupled to a Waters 2487 UV absorbance detector and a PRP-X100 anion exchange column. HPLC purifications were carried out with a gradient method from 100% A, 0% B to 0% A, 100% B (A = 100% water; B = 1M (NH 4 )(HCO 3 ) in water) with a 2.5 mL/min flow rate. The HPLC purifications were monitored by absorbance at 260 nm. S1 2 Synthesis and Characterization of Compounds In a glovebox, [PPN] (1.36 g, 0.72 mmol) was suspended in 10 mL of anhydrous acetone. (Note: At a concentration less than 1 g/mL, precipitation does not occur in the following step.) To this stirring suspension was added triethylamine (0.20 mL, 1.43 mmol) followed by a solution of TsCl (0.27 g, 1.43 mmol) in anhydrous acetone (2 mL). The solution was stirred vigorously for 15 min. After the addition of reagents, the mixture briefly became homogeneous before a thick precipitate formed. The mixture was then filtered through Celite® to remove the precipitate. The resulting colorless solution was left undisturbed at ambient temperature for 12 hours, resulting in precipitation of colorless crystals. The supernatant solution was decanted off the crystals, which were then rinsed three times with 1 mL of acetone. The resulting crystals were dried under vacuum, yielding the product as a white, crystalline solid (0.41 g, 0.39 mmol, 55% yield). The product was stored in the glovebox freezer at -35 In