Cystine cys2

Standards of amino acids (AA) namely phenylalanine (Phe), tryptophan (Trp), isoleucine (Ile), leucine (Leu), asparagine (Asn), methionine (Met), valine (Val), proline (Pro), tyrosine (Tyr), alanine (Ala), threonine (Thr), glycine (Gly), serine (Ser), glutamic acid (Glu), aspartic acid (Asp), arginine (Arg), glutamine (Gln), lysine (Lys), histidine (His), cysteine (Cys) and cystine (Cys2),were purchased from Sigma-Aldrich (Steinheim, Germany). Acetonitrile (ACN) and water (LC-MS grade) was purchased from Carlo Erba (Milan, Italy). Formic acid (LC-MS additive), TGA (thioglycolic acid; ≥98%), and ammonium formate (NH₄HCO₂) were purchased from Sigma-Aldrich (Steinheim, Germany). Analytical grade hydrochloric acid (HCl) 37% w/w was also supplied from Carlo Erba (Milan, Italy). The chromatographic column HILIC amide BEH, Acquity UPLC 1.7 m, 2 × 150 mm (Waters) were used.

Cystine (Cys)₂, methionine (Met), N-acetylcysteine (NAC), o-phthaldialdehyde (OPA) and tris-(2-carboxyethyl)phosphine (TCEP) were received from Sigma Aldrich Company (St. Louis, MO, USA). HPLC gradient grade acetonitrile (MeCN), hydrochloric acid (HCl), sodium hydrogen phosphate heptahydrate (Na₂HPO₄·7H₂O), sodium dihydrogen phosphate dihydrate (NaH₂PO₄·2H₂O) and sodium hydroxide (NaOH) were from J.T. Baker (Deventer, The Netherlands). Perchloric acid (PCA) and trichloroacetic acid (TCA) were from Merck (Darmstadt, Germany). The derivatization reagent, 2-chloro-1-methylquinolinium tetrafluoroborate (CMQT) was synthesized in our laboratory [28 (link)]. Deionized water was produced in our laboratory.