Synthesis and analysis of bioorthogonal pHLIP-Tz. (A) Synthesis of Tz-maleimide; Reagents and conditions: (i) maleic anhydride, BOP, Et3N, room temperature, 4.5 h, 34%; (B) HPLC and ESI-MS traces of Tz-maleimide; (C) Synthesis of pHLIP-Tz from Tz-maleimide Reagents and conditions: (ii) pHLIP, PBS, room temperature, 3 h, 77%; (D) HPLC and ESI-MS traces of pHLIP-Tz.