*3.2. IMS/MS Studies and Determination of Collision Cross-Sections*

All measurements were performed on a Synapt™ G2 HDMS™ from Waters (Massachusetts, USA) with equipment of a Quadrupole and Time-of-flight (Q/ToF) module for MS analysis. The IMS section is a travelling-wave IMS, which is located between the Qand ToF-sections consisting of a trap cell, an ion-mobility cell, and a transfer cell. All compounds were directly dissolved in HPLC-grade acetonitrile at a concentration of 10−<sup>5</sup> M. The solutions were filtered through a syringe filter (0.2 μm) before being injected into the spectrometer via a Harvard syringe pump at a flow rate of 5 μL·min<sup>−</sup>1. The parameters for IMS/MS measurement of all compounds have been set up with the following: ESI capillary voltage: 1.93 kV; sample cone voltage: 10 V; extraction cone voltage: 4.6 V; source temperature: 80 ◦C; desolvation temperature: 120 ◦C; cone gas flow: 15 L·h−<sup>1</sup> (N2); desolvation gas flow: 500 L·h−<sup>1</sup> (N2); source gas flow: 0 mL·min<sup>−</sup>1; trap gas flow: 2.5 mL·min<sup>−</sup>1; helium cell gas flow: 200 mL·min<sup>−</sup>1; IMS gas flow: 90.00 mL·min<sup>−</sup>1; IMS DC entrance: 25.0; helium cell DC: 35.0; helium exit: −5; IMS bias: 30; IMS DC exit: 0; IMS wave velocity: 700 m·s<sup>−</sup>1; IMS wave height: 40 V.

Collision cross-sections (CCSs) were estimated following calibration with Equine Cytochrome C and T10 olgiothymidine to determine instrument-dependent parameters A and B from published CCSs data [5], as previously described in the literature [24]. The analysis of MS spectra was carried out using Mass Lynx V4.1 Software supplied by Waters. Driftscope V2.1 was used for the analysis of IMS/MS spectra and calibration of the drift cell for the determination of CCSs. IMS experiments were performed in -ve mode. All drift times and cross-sections are quoted for the intact 2- fragment (i.e., [**X1**–**<sup>4</sup>** + 2TBA]2−) for the following reasons: (1) [**X1–4** + 2TBA]2<sup>−</sup> forms were the largest molecular fragment in ESI-MS spectra. (2) [**X1–4** + 2TBA]2<sup>−</sup> forms had relatively intact molecular structure instead of partial structural fragments. (3) As the characteristic peak, [**X1–4** + 2TBA]2<sup>−</sup> existed in all IMS/MS spectra of compounds **1**–**4**.
