Furosine

CML, CEL,
MG-H₁,
G-H₁, MOLD, GOLD, N(6)-{2-[(5-amino-5-carboxypentyl)amino]-2-oxoethyl}lysine
(GOLA), glycolic acid lysine amide (GALA), pyrraline, argpyrimidine
(Arg-p), pentosidine, d₄-CML, and furosine were purchased
form Iris Biotech (Marktredwitz, Germany). MGO (40% in aqueous solution),
2,3-hexanedione, arginine, lysine, d₄-lysine, LAN, 2-methylquinoxaline,
2,3-dimethylquinoxaline, and furosine were purchased from Sigma (St.
Louis, MO, USA). GO (40% in aqueous solution), diacetyl (DA), quinoxaline,
and o-phenylenediamine (OPD) were purchased from
Alfa Aesar (Great Britain, UK). 2-(1′,2′,3′,4′-Tetrahydroxybutyl)quinoxaline
was purchased from Biosynth Carbosynth (St. Gallen, Switzerland).
Glucosone, 3-deoxygalactosone (3-DGal), and 3-DG were purchased from
Cayman (Ann Arbor, MI, USA). 2-(2′,3′,4′-trihydroxybutyl)quinoxaline
was purchased from Toronto Research Chemicals (North York, Canada).
Perchloric acid was purchased from Honeywell Fluka (Charlotte, NC,
USA). All solvents used in this study are analytical or LC–MS
grade and sourced from Sigma.

All chemicals of the highest quality available were provided by Sigma-Aldrich (Munich/Steinheim, Germany), unless otherwise indicated. Pronase E was purchased from Sigma-Aldrich. Carboxypeptidase Y and leucine aminopeptidase were prepared as described previously^{47 (link),48 (link)}. N⁶-formyl lysine, N⁶-acetyl lysine, furosine, methionine sulfoxide, methionine sulfone, o-tyrosine, o,o-dityrosine, and citrulline were purchased from Sigma-Aldrich. Synthesis and structure elucidation of commercially unavailable authentic reference standards was described previously^{9 (link),13 (link),19 (link),20 (link)}. Structural formulas of analytes are included in the supporting information (Figure S2).

Furosine and TFA were purchased from Sigma–Aldrich (San Francisco, CA, USA). Mannose, glucosamine, ribose, glucuronic acid, galacturonic acid, aminogalactose, glucose, and galactose were purchased from Yuan-ye Biotechnology Co., Ltd. (Shanghai, China). The purity of these reagents was ˃99%. A total of 17 amino acid standards, ninhydrin (NIN), and a citric acid buffer solution were purchased from Hitachi Inc. (Hitachi Co., Osaka, Japan). HPLC-grade acetonitrile was purchased from Fisher Scientific (Waltham, MA, USA), and C18 Sep-Pak^® SPE tubes were purchased from Sepax (Sepax technology, Cork, Ireland). Ultrapure water was obtained by using a super-pure water system (Sichuan, China). All other reagents used in this study were of analytical grade and were purchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China).