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 penultimate lysine, the coupling was carried out using Fmoc-Lys(Boc)-OH (2 eq), DIC (4 eq), ethyl cyanohydroxyiminoacetate (2 eq) at 75 °C (155 W) for 0.5 min and at 90 °C (30 W) for further 4 min. For the last lysine, the coupling was carried out using Fmoc-Lys(ivDde)-OH (3 eq), DIC (6 eq), ethyl cyanohydroxyiminoacetate (3 eq) at 75 °C (155 W) for 1 min and at 90 °C (30 W) for further 7 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 peptidyl resin was removed from the automated synthesizer and the trimethine cyanine dye was manually coupled using 3 eq of compound 1, 6 eq of DIC, 3 eq of ethyl cyanohydroxyiminoacetate, 16 h. For the ivDde group cleavage, the resin was treated with 4% hydrazine hydrate (v/v) in DMF for 45 min and then the solution filtrated off. The procedure was repeated twice. Then, glutamic acid was coupled using 4 eq of Fmoc-Glu(OtBu)-OH, 4 eq of HBTU, 4 eq of HOBt and 8 eq of DIPEA.