Figure 1

(a) Relative optical intensities of marine luciferases to the synthetic luciferins. The optical intensities of five marine luciferases to each luciferin were normalized to the maximal intensity. The synthetic luciferins were grouped into five categories according to their chemical structures. Inset a shows a Western blot analysis indicating relative protein amounts between the marine luciferases. The house-keeping protein, β-actin, was stained with a specific antibody. (b) The detailed chemical structures of synthetic coelenterazine analogues. Compound ① (native CTZ; nCTZ) and its derivatives are categorized into five groups: Group 1 contains an ethynyl group at the C-6 position. The distinctive feature between groups 2 and 3 is the presence or absence of the hydroxyl (OH) group at the C-2 phenyl ring. Both groups 2 and 3 share extension of the double bond conjugated system at the C-6 position. Group 4 analogues lack the OH group at the C-2 and/or C-6 positions, upon comparison with nCTZ. Group 5 has an OH group at the C8 position. The characteristic functional groups in the chemical structures are highlighted in red.