Dualflex

The data were collected at 100(1)K on a Synergy, Dualflex, AtlasS2 diffractometer using CuKα radiation (l = 1.54184 Å) and the CrysAlis PRO 1.171.39.27b suite. Using SHELXLE [52] and Olex2 [53] the structures were solved by dual space methods (SHELXT [54] ) and refined on F 2 using all the reflections (SHELXL-2018/3 [55] ). Generally, non-hydrogen atoms were refined using anisotropic atomic displacement parameters, hydrogen atoms bonded to carbon were inserted at calculated positions using a riding model, and hydrogen atoms bonded to nitrogen were located from difference maps and their coordinates refined. Each structure exhibited some disorder and details of the refinements, along with further figures are included in supplementary S-3.

All chemical reagents and solvents were obtained from Sigma-Aldrich (Ireland) Ltd. and unless otherwise stated were used without further purification. C8-Alkyne-dU-CEP was purchased from BaseClick GmbH. NMR spectra were recorded on Bruker AC 400 MHz or 600 MHz spectrometers (Supplementary S1). FT-IR spectra were collected on Perkin Elmer Spectrum Two spectrometer. ESI-MS analysis was performed on a Bruker HCT Mass Spectrometer. pH was monitored using a Mettler Toledo InLab Expert Pro-ISM pH probe. Crystallographic data was collected at 100(1)K on a Synergy Dualflex, AtlasS2 diffractometer (Supplementary S2). The structure was solved by dual space methods and refined on F2 using all the reflections (SHELXL-2018). Mass analysis of oligonucleotides was characterised at Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI-TOF) on a Bruker Daltonics Autoflex II instrument (Supplementary S3). Thermal melting experiments were conducted on Agilent Cary 100 UV-Vis dual beam spectrophotometer equipped with a 6 × 6 Peltier multicell system with temperature controller. DNA was quantified on a Jasco UV-Vis spectrophotometer. Polyacrylamide gels were imaged on Syngene G:Box mini 9 gel documentation system.