Quantification of Arsenic Species

All reagents used were of analytical
grade or better. Argon (Ar) purged double deionized water (DDIW) (18
MΩ, Barnstead Nanopure Diamond) was used throughout the experiments.
Acetonitrile and pepsin were purchased from Thermo Fisher Scientific,
USA. l-Glutathione reduced (98–100%, GSH) was purchased
from Sigma-Aldrich, USA. Sodium chloride (NaCl), sodium phosphate,
dibasic (Na₂HPO₄·7H₂O), potassium
chloride (KCl), potassium dihydrogen phosphate (KH₂PO4),
used to prepare phosphate buffered saline (PBS) and phosphate buffer,
ammonium hydroxide (NH₄OH), potassium hydroxide (KOH),
and nitric acid (HNO₃), used for pH adjustments, were also
purchased from Thermo Fisher Scientific, USA. Formic acid and acetic
acid used to prepare formate and acetate buffers were purchased from
Acros Organics and Thermo Fisher Scientific, USA, respectively. DMA^III was synthesized in our laboratory following procedures
previously reported.^{30 (link)} DMA^III(GS) was obtained from Ziopharm Oncology, Boston, MA.