The effects of resonance delocalization and the extent of pi-system on ionization energies of model fluorescent proteins chromophores
Recent developments in the design and application of redox-active fluorescent proteins (FPs) stimulated an interest in the electronic structure of the FP chromophores in ionized and electron-detached states. Here, we report the results of a computational study of the electron-detached and ionized states of model chromophores of green and red FPs. We focus on the analysis of the effects of the phenolate OH-group position (ortho, meta, para) on relative energies of the chromophores in the ground electronic states as well as in the ionized/detached states. We found that, similarly to green chromophore's meta-substituted forms, the red chromophores with the OH group in meta position exhibit lower vertical detachment energies and greater ionization energies. Moreover, the effect is stronger for the red anionic chromophores. The differences in detachment energies in meta species relative to their para counterparts are 0.47 and 0.25 eV for the red and green chromophores, respectively. The observed trends are explained by a combination of resonance stabilization and the electronegativity of the acylimine group in the red chromophores. The analysis is supported by the computed charge and spin density delocalization patterns.