Moving on to our findings for the emission energy of RLuc8-D120A mutant, the calculated emission spectra (Figure ) has a maximum at 456 nm . The position of this maximum represents a modest red shift of 26 nm, which aligns well with the experimentally observed value of 430 nm. Notably, our calculated value is consistent with the luminescence results obtained by Shimomura and Teranishi, who reported an emission maximum for the amide-deprotonated emitter in the range of 435-458 nm. The correlation between the calculated emission energy and experimental data strongly suggests that the amide ion is the principal luminescent species in this mutant. This is attributed to the Asp120 position being unoccupied by aspartate, which precludes the usual proton transfer, making the formation of neutral or phenolate emitters unfavorable.