## 3 Experimental Section
### General Procedure for Synthesis of Mimosine Dipeptides  
Fmoc-amino acids (l- or d-form) (2.35 mmol) were dissolved in a solution of HOBt (2.35 mmol, 360 mg) and DIC (2.35 mmol, 365 \(\upmu\)L), followed by preactivation in 5 mL of dimethylformamide (DMF). The solution was then added to Wang resin (0.5 g) which was swollen in 2 mL of dichloromethane (CH\({}_{2}\)Cl\({}_{2}\)) for 30 min. 4-Dimethylaminopyridine (DMAP) (0.235 mmol, 29 mg) was then suspended in 500 \(\upmu\)L of DMF and added to the mixture prior to stirring for 3 h at room temperature. This procedure was repeated twice, and the resin was washed with DMF, CH\({}_{2}\)Cl\({}_{2}\), methanol (MeOH), and dried under reduced pressure. A small portion of the resin (5 mg) was used for analysis of Fmoc-group content. Fmoc protection was removed by adding 25% piperidine and shaking for 45 min at room temperature. To couple the mimosine to the amino acids, a solution of Fmoc-mimosine (4 equiv relative to amino acid on resin), HOBt (4 equiv), HBTU (3.6 equiv), and DIPEA (8 equiv) was added to the amino acid-resin suspended in DMF, and the mixture was stirred at room temperature for 1 h. The Kaiser's test was used to assess the completeness of the coupling reaction. The final cleavage was performed by shaking the resin vigorously in 95% trifluoroacetic acid (TFA) for 90 min and then filtering and washing with TFA. Ice-cold diethyl ether was used to precipitate the combined washings. The resulting precipitate was filtered, washed with diethyl ether, and dried under a vacuum to obtain the desired mimosine dipeptides. All crude peptides were a white solid, and their yields are presented in Table 1. All samples were purified using high-performance liquid chromatography (HPLC) (Shimadzu, Kyoto, Japan) before being used in enzyme inhibition assays. The major peaks were collected using a Phenomenex (150 mm \(\times\) 4.6 mm; 4 \(\upmu\)m) column with mobile phase of solvent A (0.1% TFA in water) and solvent B (0.1% TFA in acetonitrile). The flow rate and absorbance wavelength were set at 1 mL/min and 210 nm, respectively. The purified compounds were identified using LC-MS (ESI): _m/z_ [M - H]\({}^{+}\) 344.2, 344.3, 268.1, 268.3, 383.2, 382.3, 296.2, 296.4, 294.1, 294.4 for Mi-l-Phe, Mi-d-Phe, Mi-l-Ala, Mi-d-Ala, Mi-l-Trp, Mi-d-Trp, Mi-l-Val, Mi-d-Val, Mi-l-Pro, and Mi-d-pro, respectively. Representative data for some of the synthesized mimosine dipeptides are as follows.  
Data for Mi-l-Phe. \({}^{1}\)H-NMR (400 MHz, D\(\alpha\)D) \(\delta\): 3.27-3.14 (dd, 2H, CH\({}_{2}\)), 3.80 (s, 1H, CH-NH\({}_{2}\)), 3.99 (q, 1H, CH-NH\({}_{2}\)), 4.17 (q, 1H, CH), 6.59 (d, 1H, CH), 7.33 (s, 1H, CH), 7.38 (s, 1H, CH), 7.43 (s, 1H, CH), 7.65 (s, 1H, CH). \({}^{13}\)C-NMR \(\delta\): 36.30, 55.99, 71.12, 97.49, 114.79, 127.65, 129.07, 129.32, 135.05, 140.2, 148.2, 173.89, 181.52. LC-MS (ESI): m/z 344.2 [M - H]\({}^{+}\).  
Data for Mi-l-Ala. \({}^{1}\)H-NMR (400 MHz, D\(\alpha\)D) \(\delta\): 1.48 (d, 3H, CH\({}_{3}\)), 3.78 (dd, 1H, CH-NH\({}_{2}\)), 4.20-4.17 (q, 1H, CH-NH\({}_{2}\)), 4.49-4.45 (q, 1H, CH\({}_{2}\)), 4.50-4.64 (q, 1H, CH), 6.60 (d, 1H, CH), 7.67 (s, 1H, CH). \({}^{13}\)C-NMR \(\delta\): 16.08, 42.80, 50.47, 97.79, 114.82, 127.65, 129.07, 130.70, 138.52, 140.2, 148.2, 175.75, 181.55. LC-MS (ESI): m/z 268.1 [M - H]\({}^{+}\).  
Data for Mi-l-Trp. \({}^{1}\)H-NMR (400 MHz, D\(\alpha\)D) \(\delta\): 3.27 (d, 2H, CH\({}_{2}\)), 3.32 (d, 2H, CH\({}_{2}\)), 3.50 (q, 1H, CH-NH\({}_{2}\)), 4.05 (q, 1H, CH-NH\({}_{2}\)), 4.15 (q, 1H, CH), 4.45 (q, 1H, CH), 6.57 (d, 1H, CH), 7.17 (s, 1H, CH), 7.26 (d, 1H, CH), 7.74-7.30 (m, 4H, benzene). \({}^{13}\)C-NMR \(\delta\): 26.32, 48.07, 55.00, 68.44, 90.77, 98.99, 107.04, 111.89, 118.39, 119.41, 122.08, 124.97, 132.77, 136.34, 138.31, 148.2, 174.43, 181.52. LC-MS (ESI): m/z 383.2 [M - H]\({}^{+}\).