*2.5. Radiochemical Synthesis of [64Cu]***1***–***4**

DOTA-compounds **1**–**4** were dissolved in metal free water at 1 µg/µL, and the [ <sup>64</sup>Cu]CuCl<sup>2</sup> (1–10 µL of 0.5 M HCl, **1** and **2**: 174–255.3 MBq, **3** and **4**: 51–55 MBq) was diluted with 1.0 M ammonium acetate (NH4OAc, Sigma-Aldrich) aqueous solution (pH = 8.0) to 0.27 µL/MBq. Peptides **1** and **2** were added to the NH4OAc buffered [64Cu]CuCl<sup>2</sup> such that the starting molar activity of the reaction was between 18.5 and 20 GBq/µmol. The starting molar activity for compounds **3** and **4** was between 15.9 and 17.1 GBq/µmol. The reaction mixtures were vortexed and warmed to 37 ◦C for 30 min. The radiochemical purity was assessed by quenching an aliquot of the reaction (≤1 µL; 0.74–3.7 MBq) with 0.1 M EDTA (50 µL) and analyzed by analytical RP-HPLC. Product identity was confirmed by cold spike RP-HPLC, i.e., co-injection of the radiolabeled product with authenticated respective [NatCu]Cu **1**–**4** reference standard of each compound (Figures S6, S9, S12 and S15). [ NatCu]Cu **1**–**4** reference standards were produced via reaction of DOTA-compounds **1**–**4** (0.1–0.5 mg) with excess CuCl<sup>2</sup> (Sigma-Aldrich, 1–6 mg) in water (50 µL) for 30 min at room temperature, and purified directly by RP-HPLC and confirmed by MALDI-TOF (Figures S7, S10, S13 and S16).
