The peptide chain was synthesized by SPPS on Rink Amide MBHA resin. Each amino acid coupling step was carried out with Fmoc-protected amino acid (2 eq), DIC (4 eq), ethyl cyanohydroxyiminoacetate (2 eq) in DMF at 75 °C (155 W) for 0.25 min and at 90 °C (30 W) for further 2 min. For the last lysine, the coupling was carried out at 75 °C (155 W) for 0.5 min and at 90 °C (30 W) for a further 4 min. After each coupling step, the respective Fmoc protecting group was removed by 10% Piperazine (w/v) in ethanol:NMP (1:9). After the final Fmoc deprotection, the cyanine dye scaffold was manually coupled using 3 eq of compound 1 or 2, 6 eq of DIC, 3 eq of ethyl cyanohydroxyiminoacetate for 16 h at room temperature. Cleavage was performed in TFA/H2O/TIS 95:2.5:2.5 solution for 2 h. The resin was removed by filtration and the filtrate was concentrated by N2 flow. The crude product was precipitated with cold diethyl ether, centrifuged (4,000 g for 10 min) and the supernatant was discarded. The precipitate was dissolved in water, filtrated through a 0.22 µm syringe filter and purified by RP-HPLC. Lyophilization of the pure product fractions afforded the desired compound as red powder in case of labelling with compound 1, or blue powder in case of labelling with compound 2, which were characterized by LC-MS (Supplementary Figure ).