Peptide Synthesis and Labeling Protocol

All materials were used according to the manufacturer's instructions unless otherwise noted. The following were purchased from Sigma‐Aldrich, St. Louis, MO, USA: acetonitrile (MeCN), methanol (MeOH), triethylamine (TEA), dimethyl sulphoxide (DMSO), diethyl ether, cyanocobalamin (B₁₂), triisopropylsilane, 1,1′‐carbonyl‐di‐(1,2,4‐trizole), n‐methyl‐pyrrolidone, 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide, 1‐hydroxybenzo‐triazole and dihydroxybenzoic acid. The following were purchased from CEM Corporation, Matthews, NC, USA: Fmoc‐Rink Amide Protide non‐preloaded resin (LL), N,N′‐diisopropylcarbodiimide, ethyl cyanohydroxyiminoacetate (Oxyma), Fmoc protected amino acids: Asn(Trt); Gln(OtBu); Tyr(tBu); and Trp(Boc). The following were purchased from VWR, Radnor, PA, USA: 7‐trifloroacetic acid (TFA) and dimethylformamide (DMF). The following was purchased from BroadPharm, San Diego, CA, USA: sulpho‐DBCO‐amine. The following was purchased from Thermo Fisher, Waltham, MA, USA: AlexaFluor 564 (DBCO‐AF546). The following was purchased from Lumiprobe: Sulfo‐cyanine5 NHS ester. The following was purchased from Lumiprobe, Hunt Valley, MD, USA: alpha‐cyano‐4‐hydroxycinnamic acid.