Real light emitter in the bioluminescence of the calcium-activated photoproteins aequorin and obelin: light emission from the singlet-excited state of coelenteramide phenolate anion in a contact ion pair

Fluorescence of the phenolate anion (3(O)(-)) and the amide anion (5(N)(-)) of coelenteramide analogues in ion pairs with various counter cations was systematically investigated to elucidate the ionic structure of the light emitter in the bioluminescence of the calcium-activated photoproteins aequorin and obelin. The fluorescent properties of 3(O)- in an ion pair with a conjugate acid of an organic base (BASE-H+) were varied depending on the structural variation of the ion pair and the solvent polarity. In particular, the fluorescence of 3(O)- in the ion pair with the conjugate acid of n-butylamine (NBA-H+) indicates that the singlet-excited state of 3(O)(-) ((1)3(O)(-)*) and NBA-H+ make a contact ion pair in which the fluorescence emission maxima of 3(O)- is sensitive to the solvent polarity and the fluorescence quantum yields of 3(O)- increase in a less polar solvent. The results also confirm that (1)3(O)-* is a twisted intramolecular charge transfer state. By contrast, the fluorescence of 5(N)(-) in an ion pair depends little on the BASE-H+ or the solvent polarity. Based on these results, we conclude that the light emitter in aequorin and obelin bioluminescences is the singlet-excited state of coelenteramide phenolate anion 2(O)(-)((1)2(O)(-)*) in a contact ion pair with an imidazolium side chain of a histidine residue, which is located at the less polar active sites of the photoproteins. We also propose a mechanism for the bioluminescence reaction, including the chemiexcitation process to give (1)2(O)(-)*. (c) 2006 Elsevier Ltd. All rights reserved.