were prepared with analytical reagent grade chemicals. Acetic acid
(HAc, glacial), formic acid (HFor 98–100%), sodium hydroxide,
sodium citrate tribasic, and ammonia (25%) were provided by Merck
(Darmstadt, Germany). DL-Dithiothreitol (DTT, ≥99%), iodoacetamide
(IAA, ≥98%), ammonium acetate (≥99.9%), and ammonium
hydrogen carbonate (≥99.9%) were supplied by Sigma-Aldrich
(St. Louis, MO, USA). Propan-2-ol was purchased from Scharlab (Barcelona,
Spain), while acetonitrile and water were supplied by Sigma-Aldrich
(all of them were of LC-MS quality grade). Trypsin and Glu-C (both
sequencing grade modified) were provided by Promega (Madison, WI,
USA). The ESI low concentration (ESI-L) tuning mix for tuning and
calibration of the mass spectrometer was obtained from Agilent Technologies
(Waldbronn, Germany).