Leucine

Urea was purchased from Solaribio (Beijing, China). The L-AAs discussed in our work were obtained from Macklin (Shanghai, China), including alanine (Ala), phenylalanine (Phe), glycine (Gly), proline (Pro), lysine (Lys), cysteine (Cys), serine (Ser), histidine (His), arginine (Arg), leucine (Leu), isoleucine (Ile), valine (Val), methionine acid (Met), aspartic acid (Asp), asparagine (Asn), glutamic acid (Glu), glutamine (Gln), tryptophan (Trp), threonine (Thr), and tyrosine (Tyr). Peptides were synthesized by Anhui Guoping Pharmaceutical Co., Ltd. (Anqing, China), including alanyl-alanine (Ala-Ala), phenylalanyl-phenylalanine (Phe-Phe), and alanyl-glycyl-proline (Ala-Gly-Pro). Cell culture media DMEM and RPMI 1640 were purchased from Corning (USA).

All reagents were not purified further before use. Methylbenzene, hydrochloric acid (HCl), lysine (Lys), arginine (Arg), titanium tetrachloride (TiCl₄), tetrabutyltitanate, gold (III) chloride (HAuCl₄·3H₂O), leucine (Leu), cysteine (Cys), proline (Pro), glycine (Gly), barium hydroxide octahydrate (Ba(OH)₂·8H₂O), formic acid, ethylene diamine tetraacetic disodium (EDTA−2Na), chloroplatinic acid (H₂PtCl₆), 3,3′,5,5′−tetramethylbenzidine (TMB, ≥99%), L−histidine (L−His), glutathione (GSH), glutamic acid (Glu), and fetal bovine serum were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Sodium borohydride (NaBH₄), disodium hydrogen phosphate (Na₂HPO₄), p−Benzoquinone (p−BQ), isopropanol (IPA), potassium nitrate (KNO₃), citric acid, ascorbic acid (AA), and sodium nitrate (NaNO₃) were purchased from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China).