Towards understanding the redox properties of model chromophores from the green fluorescent protein family: An interplay between conjugation, resonance stabilization, and solvent effects
The redox properties of model chromophores from the green fluorescent protein family are characterized computationally using density functional theory with a long-range corrected functional, the equation-of-motion coupled-cluster method, and implicit solvation models. The analysis of electron donating abilities of the chromophores reveals an intricate interplay between the size of the chromophore, conjugation, resonance stability, presence of heteroatoms, and solvent effects. Our best estimates of the gas-phase vertical detachment energies of the deprotonated (i.e., anionic) model red, green, and blue chromophores are 3.27 eV, 2.67 eV, and 2.75 eV, respectively. Vertical ionization energies of the respective neutral species are 7.64 eV, 7.38 eV, and 7.70 eV, respectively. The standard reduction potentials (E0red) of the anionic/neutral model chromophores in acetonitrile are: 0.38/1.44 V (red), 0.22/1.26 V (green), and -0.12/1.05 V (blue) suggesting, counter-intuitively, that the red chromophore is more difficult to oxidize than the green and blue ones (in either neutral or deprotonated form).