Organic solvents

MS-grade H₂O, organic solvents, and formic acid (FA) were purchased from Biosolve Chimie SARL (Dieuze, France). ULN and ULN-d6 were provided by BioMed Valley Discoveries (Kansas City, MO, USA). The purity of the two compounds were 99% and 98.26%, respectively. The structure of ULN is provided in Figure 1A.

MS-grade water, organic solvents, and formic
acid (FA) were purchased from Biosolve Chimie SARL (Dieuze, France).
2dGO, O6-m2dGO, 2′-deoxy-N-ethyl-guanosine (2dNetGO), d3-O6-methyl-2′-deoxyguanosine
(d3-O6-m2dGO), N7-mG, O6-mG, and N2-methyl-2′-deoxyguanosine
(N2-m2dGO) were purchased from Toronto Research Chemicals (North York,
Canada) (Supporting Information, Figure S1). Nucleoside Digestion Mix NEB #M0649 was purchased from New England
Biolabs (New England Biolabs, Ipswich, MA, USA). Cell culture medium,
Dulbecco’s Modified Eagle’s Medium (DMEM), and fetal
calf serum (FCS) were purchased from PAN-Biotech (Aidenbach, Germany).
Glutamine and penicillin/streptomycin were purchased from Sigma-Aldrich
(Taufkirchen, Germany). Crystal violet was purchased from Applichem
(Darmstadt, Germany). The bromodeoxyuridine (BrdU) assay kit was purchased
from Merck (Darmstadt, Germany). The DNA extraction kit was purchased
from Qiagen (Hilden, Germany).

Materials: Chemicals were purchased from Sigma‐Aldrich Chemical Co. (St. Louis, MO), TCI Europe N.V., Thermo Fisher Scientific (Geel, Belgium) or Fluorochem Co. (UK). Organic solvents were purchased from Biosolve (Valkenswaard, The Netherlands). Oligonucleotides were purchased from Eurofins Scientific (Ebersberg, Germany). Proteins were analysed by SDS‐PAGE on precast gels (NuPAGE^TM 12 % Bis‐Tris protein gels). High performance liquid chromatography (HPLC) was performed with a Shimadzu LC‐10AT HPLC and a Shimadzu SPD‐M20A diode array detector. GC‐MS was conducted with a Shimadzu GC‐MS‐QP2010 SE. NMR spectra were recorded on a Bruker 500 MHz machine at the Drug Design laboratory of the University of Groningen. Chemical shifts (δ) are reported in parts per million (ppm). Spectrophotometric measurements were performed on a V‐650 or V‐660 spectrophotometer from Jasco (IJsselstein, The Netherlands). Spectrophotometric measurements in 96‐well format were performed on a SPECTROstar Omega plate reader (BMG LABTECH, Isogen Life Science, de Meern, The Netherlands) in 96‐well microtiter plates (UV‐star μclear, Greiner Bio‐one). DNA was purified using a PCR purification kit (QIAquick®, QIAGEN).