4.1. Peptide synthesis, purification, and characterization

Peptides were synthesized on a microwave‐equipped Liberty Peptide Synthesizer (CEM®) using standard Fmoc protection and HBTU/HOBt activation chemistry as previously described.^{19, 24, 25} Briefly, rink amide resin (Advanced ChemTech, 100–200 mesh, 0.2 mmol g⁻¹) was used as the solid support to provide C‐terminal amide peptides. Peptides were treated with 20% acetic anhydride in dimethylformamide (DMF) to give N‐terminal acetyl sequences. Side chain deprotection and cleavage from the solid support were accomplished by treatment with trifluoroacetic acid (TFA), triisopropylsilane (TIPS), and water (95:2.5:2.5, v/v/v) (room temperature, 1 h). Cleavage solutions were concentrated to 10% of the reaction volume after which peptides were precipitated in ethyl ether and isolated by centrifugation; precipitated peptides were washed by resuspension in ether. The isolated solid peptide was then dissolved in DMSO for purification by HPLC.

Purification of synthetic peptides was conducted using a Shimadzu LC‐AD HPLC instrument over a reverse phase C18 stationary phase (Waters, BEH300, 10 μm, 19 × 250 mm). A binary gradient of acetonitrile and water with 0.1% TFA at 10 ml min⁻¹ was used as the mobile phase, and eluent was monitored by UV absorbance at 215 and 254 nm. Fractions were collected and lyophilized and then analyzed by analytical HPLC (reverse phase C18, Waters, BEH300, 10 μm, 4.6 × 250 mm) to confirm purity (Figures S1–S10). Peptide identity was confirmed by matrix‐assisted laser desorption ionization‐time of flight (MALDI‐TOF) mass spectroscopy (Figures S11–S20). Ac‐(FKFE)₂‐NH₂ m/z 1162.28 (1162.34 calcd for [MH]⁺), m/z 1185.25 (1185.32 calcd for [MNa]⁺), m/z 1201.20 (1201.43 calcd for [MK]⁺). Ac‐(HphKHphE)₂‐NH₂ m/z 1219.58 (1219.45 calcd for [MH]⁺), m/z 1241.57 (1241.43 calcd for [MNa]⁺), m/z 1257.55 (1257.54 calcd for [MK]⁺). Ac‐(WKWE)₂‐NH₂ m/z 1319.51 (1319.49 calcd for [MH]⁺), m/z 1341.50 (1341.47 calcd for [MNa]⁺), m/z 1357.47 (1357.58 calcd for [MK]⁺). Ac‐(1‐NalK1‐Nal‐E)₂‐NH₂ m/z 1363.47 (1363.58 calcd for [MH]⁺), m/z 1385.46 (1385.56 calcd for [MNa]⁺), m/z 1401.43 (1401.67 calcd for [MK]⁺). Ac‐(2‐NalK2‐Nal‐E)₂‐NH₂ m/z 1363.54 (1363.58 calcd for [MH]⁺), m/z 1385.51 (1385.56 calcd for [MNa]⁺), m/z 1401.47 (1401.67 calcd for [MK]⁺). Ac‐(BipKBipE)₂‐NH₂ m/z 1467.17 (1467.73 calcd for [MH]⁺), m/z 1489.16 (1489.71 calcd for [MNa]⁺), m/z 1505.13 (1505.82 calcd for [MK]⁺). Ac‐WKFEFKFE‐NH₂ m/z 1202.85 (1202.38 calcd for [MH]⁺), m/z 1224.85 (1224.36 calcd for [MNa]⁺), m/z 1240.83 (1240.47 calcd for [MK]⁺). Ac‐(WKFE)₂‐NH₂ m/z 1241.78 (1241.42 calcd for [MH]⁺), m/z 1263.49 (1263.40 calcd for [MNa]⁺), m/z 1279.47 (1279.51 calcd for [MK]⁺). Ac‐BipKFEFKFE‐NH₂ m/z 1239.59 (1239.44 calcd for [MH]⁺), m/z 1261.53 (1261.42 calcd for [MNa]⁺), m/z 1277.51 (1277.53 calcd for [MK]⁺). Ac‐(BipKFE)₂‐NH₂ m/z 1314.87 (1315.54 calcd for [MH]⁺), m/z 1336.85 (1337.52 calcd for [MNa]⁺), m/z 1352.81 (1353.63 calcd for [MK]⁺). See Figures S1–S20 and Table S1 for HPLC and MALDI‐MS data.