Fluorescence properties of phenolate anions of coelenteramide analogues: the light-emitter structure in aequorin bioluminescence

To elucidate the ionic structure of the excited state of light-emitter coelenteramide in aequorin bioluminescence, the fluorescent properties of phenolate anions of coelenteramide analogues were investigated. Fluorescence of phenolate anion in non-polar solvents was observed by electronic excitation of a 1: 1 hydrogen-bonded complex of a coelenteramide analogue with a hydrogen-bond donor molecule such as n-butylamine. In polar solvents, the phenolate anion was directly generated using a base, and its fluorescence was studied. These results confirm that the singlet-excited state of phenolate anion of coelenteramide has an intramolecular CT character, and that its fluorescence emission wavelength changes depending upon solvent polarity. The fluoro-substituent effect on the fluorescent property of phenolate anions was also clarified to help in explaining the bioluminescent property of fluorinated semi-synthetic aequorin. These results consistently support the assignment that the phenolate anion is the ionic structure of the excited light-emitter in BFP during AQ bioluminescence. (C) 2001 Elsevier Science B.V. All rights reserved.