Milli q reagent grade system

Recombinant HUαβ heterodimer from Escherichia coli was purified in-house as previously described [9 (link)]. Alkaline protease-treated streptavidin from Streptomyces avidinii was purchased from Promega (Madison, WI, USA). Myoglobin, ubiquitin, cytochrome c, and biotin were obtained from Sigma-Aldrich (Saint Quentin Fallavier, France). The matrices 4-hydroxy-α-cinnamic acid (HCCA), dihydroxybenzoic acid (DHB), and sinapinic acid (SA) were from Bruker Daltonics (Bremen, Germany). All other matrices, i.e., 9-aminoacridine (9AA), aniline (ANI), 2-nitrophenol (2NPHL), 6-aza-2-thiothymine (ATT), 3-aminoquinoline (3AQ), 4-aminoquinaldine (4AQA), and picolinic acid (PLA), were purchased from Sigma-Aldrich. Ammonium hydroxide was procured from Fluka (Steinheim, Germany), while diethanolamine (DEOA) and glycerol (Gly) were from Sigma. Ammonium acetate was from Merck (Darmstadt, Germany) and acetic acid from Thermo-Fisher Scientific (Illkirch, France). The organic solvents acetonitrile and methanol were purchased from Carlo Erba (Milan, Italy). All solvents and buffers were prepared using 18 MΩ water purified with Milli-Q reagent grade system from Millipore (Bedford, MA, USA), herein referred to as “ultrapure water.”