CHP Synthesis.

CHP was synthesized as described previously using standard SPPS methods.^18,37 All peptides were synthesized on a Focus XC peptide synthesizer (AAPPTec) using TentaGel R Ram resins and standard Fmoc chemistry with HBTU/DIEA activation. Nonfluorescent CHP was prepared with the sequence Ac-C-Ahx-NB(GPO)₉. Fluorescent CHP was prepared with the sequence Ac-C-Ahx-K(Fluoro)-Ahx-NB(GPO)₉, where Fluoro was either IR680 dye (NIRF) (Li-COR) or 5(6)-carboxyfluorescein (CF) (Sigma-Aldrich). Fluorescent tags were conjugated on resin as described previously.¹⁸ Cleaved peptides were purified by RP-HPLC (Agilent) on a C18 column with a 5–50% gradient of Water with 0.1% trifluoroacetic acid (TFA) and acetonitrile. Peptide mass and purity was confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) (UltrafleXtreme, Bruker Daltronics) and HPLC.