Exendin4(9-39) was employed as scaffold for modification with fluorophore. Using solid-phase peptide synthesis (SPSS), Exendin4(9-39)-S39C (S39C-Ex4) was generated, bearing a Cterminal serine to cysteine substitution for functionalization via the introduced thiol handle. CF488A-, Cy3-, CPY-and Cy7-conjugated versions were produced using cysteine-maleimide reactions and termed LUXendin492, LUXendin551, LUXendin615 and LUXendin762, respectively (Figure ), according to their maximal absorption values. Spectral properties were determined using UV/Vis and fluorescence spectroscopy (Figure and C) (Table ) and were in line with known properties of the fluorophores used. Full compound characterization and purity assessment are provided in the Supplementary Information.