All strands were purchased from Eurogentec (Seraing, Belgium, with RP cartridge -Gold™ purification), dissolved in nuclease-free water from Ambion (Applied Biosystems, Lennik, Belgium), and used without further purification. The stock concentrations were determined using the Beer-Lambert law. The absorbance was recorded at 260 nm on a Uvikon XS, and molar extinction coefficients calculated using the IDT website by applying the Cavaluzzi-Borer Correction. The 34-mer oligodeoxynucleotides used for the native IMS-MS study are listed in Table . dT6 and dTG4T sequences were used to verify the instrument each day for the determination of collision cross section (CCS) values ( DT CCSHe = 306 Å² for dT6 2-(see supporting information of ref. ), and 788 Å² for [(dTG4T)4(NH4)3] 5-). The [(dTG4T)4(NH4)3] G-quadruplex was formed in 150 mM ammonium acetate from 200 mΜ single strand, incubated overnight at 4°C. Final solution used in IMS-MS analysis contained 150 mM NH4OAc, 2.5 μM dT6 or 5 μM [(dTG4T)4(NH4)3]. ESI-MS samples were prepared at 10 or 20 μM single strand concentration in 15-, 100-and 150-mM ammonium acetate at pH = 5.6 or 7.1 and let fold overnight at 4 °C. Acetic acid or ammonium hydroxide were used to adjust the pH. The pH was measured using a Multi9420 digital pH meter (WTW, Weilheim, Germany) calibrated daily. Ion mobility spectrometry-mass spectrometry