The Effects of Resonance Delocalization and the Extent of System on Ionization Energies of Model Fluorescent Proteins Chromophores

Recent advances in the design and application of redox-active fluorescent proteins (FPs) stimulated an interest in the electronic structure of the ionized/electron-detached FP chromophores. 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, and para) on relative energies of the chromophores in the ground as well as in the ionized/detached electronic states. We found that, similarly to the green chromophore, the red chromophores with the OH group in meta position have lower vertical detachment energies (DE) and greater ionization energies relative to the ortho and para forms. Moreover, the effect is stronger for the red anionic chromophores. The differences in DE 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 due to a combined effect 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. VC 2014 Wiley Periodicals, Inc.