The fluorescence of dihydroxyphenylalanine: the effects of protonation‐deprotonation

The fluorescence excitation and emission spectra and quantum yields of 3,4‐dihydroxyphenylalanine (DOPA), its methyl ester (DOPAM) and the N‐acetylated derivative (DOPANA) in aqueous solution at pH values ranging from 3.0 to 10.0 have been determined and the excitation spectra related to their absorption spectra. The fluorescence quantum yield of the phenolic form of DOPA is 0.09. However, the fluorescence yield is substantially reduced for the conjugate base forms, i.e. deprotonation of the amine group and the phenolic hydroxyl group. The fluorescence spectra are dominated by the phenolic forms of DOPA, DOPAM and DOPANA from pH 3.0 to 10.0 with excitation and emission spectral maxima at 280 and 315 nm respectively. The excitation maxima for all the compounds correspond to the absorption maxima of their phenolic forms. The phenolate anions of DOPA and DOPAM exhibit weak fluorescence with emission maxima at 325 and 330 nm respectively.