The lineshape of the electronic spectrum of the green fluorescent protein chromophore, Part II: Solution phase

The vibronic spectra of the green fluorescent protein chromophore analogues p-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI) and 3,5-tert-butyl-HBDI (35Bu) are similar in the vacuum, but very different in water or ethanol. To understand this difference, we have computed the vibrationally resolved solution spectra of these chromophores, using the polarizable continuum model (PCM) to account for solvent effects on the (harmonic) potential energy surfaces (PES). In agreement with experiment, we found that the vibrational progression increases with the polarity of the solvent, but we could neither reproduce the broadening, nor the large difference between the absorption spectra of HBDI and 35Bu. To account for the inhomogeneous broadening of the solution spectra, we used two approaches. In the first, we estimated the polar broadening from the solvent reorganization energy upon photo-excitation, using the state-specific PCM implementation. In the second, we estimated the broadening from the variance of the vertical excitation energies in molecular dynamics trajectories. Although we found good agreement for the lineshape of 35Bu in ethanol, and to a lesser extent in water, we highly underestimated the broadening for HBDI. To resolve this discrepancy, we explored the PES of HBDI in water and found that in contrast to the PCM result, the ground-state geometry is not planar in explicit solvent. We furthermore found that nonplanar geometries enhance the intramolecular charge transfer upon excitation. Therefore, the solvent reorganization and broadening are much larger and we speculate that the much broader spectrum of HBDI in water is due to the population of nonplanar geometries.