Dl aspartic acid

All reagents were of analytical grade. Methanol (MeOH), acetonitrile (ACN), and acetic acid (HAc) were acquired from Scharlau Chemie (Barcelona, Spain). Triethylamine (TEA) and 9-fluorenylmethoxycarbonyl (FMOC) chloride were obtained from Fluka (Buchs, Switzerland). Ammonium hydroxide (NH₃·H₂O), boric acid (H₃BO₃) and pentane were from Sigma (St. Louis, Missouri, USA), and ammonium acetate was from Merck (Darmstadt, Germany). 10,11-dihydroquinidine, 2,2’-azobisisobutyronitrile (AIBN), 3-(trimethoxysilyl)-propylmethacrylate (γ-MAPS), vinyltrimethoxysilane (VTMS), cetyltrimethylammonium bromide (CTAB), tetramethoxysilane (TMOS) and ethylene glycol (EG) were acquired from Aladdin Chemicals (Shanghai, China). dl-arginine, dl-histidine, dl-lysine, dl-serine, dl-threonine, dl-asparagine, dl-glutamine, dl-cysteine, dl-proline, dl-alanine, dl-valine, dl-leucine, dl-methionine, dl-phenylalanine, dl-tyrosine, d-tryptophan, l-tryptophan, dl-ornithine, dl-citrulline standards were from Fluka (Buchs, Switzerland), while dl-isoleucine, dl-carnitine, dl-aspartic acid, dl-glutamic acid, dl-norvaline, l-norvaline, dl-norleucine, dl-DOPA, dl-pyroglutamic acid and dl-methionine sulfone were obtained from Sigma (St. Louis, Missouri, USA). FMOC-amino acids were synthesized as reported previously [29 (link),30 (link)]. The dietary supplements were obtained in capsule form from online sources.

DL-aspartic acid, D-asparagine and sucrose were acquired from Sigma-Aldrich (catalog numbers A9006, 441597, and S9378, ≥99% purity, St. Louis, MO, USA). Uric acid (SRM 913b, 99.8 ± 0.2 g/100 g) was acquired from National Institute of Standards and Technology (Gaithersburg, MD, USA). CO₂ was used as carrier gas and O₂ were used to achieve a complete oxidation after catalytic post-combustion process and control the atmosphere inside the furnace during the course of an analysis, for protein and moisture/ash determinations, respectively (UHP gas, Praxair Technology, Inc., Uruca, San José, Costa Rica). In the case of protein analysis, the reducing agent (converting nitrogen oxides to molecular nitrogen) was used as recommended by the manufacturer (EAS REDUCTOR® and EAS REGAINER®, Elementar, Lagenselbold, Germany).