In a study by Marek et al., the phenolate form of coelenteramide was identified as a potential emitter in RLuc8. This emission is hypothesized to be attributed to the deprotonation of the R2-6-(p-hydroxyphenyl) substituent, influenced by the nearby aspartic acid D162, which is only 2.9 Å away. On the other hand, the RLuc8-D162A and RLuc8-D120A mutants see the pyrazine and amide ion as light emitters, as shown in Scheme 1. This latter finding corroborates earlier research by Shimomura and Teranishi, 20 who investigated the luminescence of various biomolecular states of coelenteramide in different solvent polarities, and in the absence of the protein environment. While the X-ray crystallography method used in the above study has provided insights into the luminescence mechanism, it is crucial to understand the molecular details associated with the luminescent reaction. This is because X-ray crystallography captures either a single spatial structure or, in the presence of defects, an average of different conformations at a specific moment in time. In particular the previous studies did not explore the relationship between the proton transfer coordinate to the emission spectra.