Avila Ferrer Francisco J, Davari Mehdi D, Morozov Dmitry, Groenhof Gerrit, Santoro Fabrizio
Physical Chemistry, Faculty of Science, University of Málaga, Málaga 29071 (Spain); Istituto di Chimica dei Composti Organo Metallici, Consiglio Nazionale delle Ricerche (ICCOM-CNR), UOS di Pisa, Area della Ricerca via G. Moruzzi 1, 56124 Pisa (Italy).
Chemphyschem. 2014 Oct 20;15(15):3246-57. doi: 10.1002/cphc.201402485. Epub 2014 Sep 18.
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.
绿色荧光蛋白发色团类似物对羟基苄叉-2,3-二甲基咪唑啉酮(HBDI)和3,5-叔丁基-HBDI(35Bu)的振动光谱在真空中相似,但在水或乙醇中却有很大差异。为了理解这种差异,我们使用可极化连续介质模型(PCM)来计算这些发色团的振动分辨溶液光谱,以考虑溶剂对(谐波)势能面(PES)的影响。与实验结果一致,我们发现振动谱带随溶剂极性的增加而增加,但我们既无法重现谱带展宽,也无法重现HBDI和35Bu吸收光谱之间的巨大差异。为了解决溶液光谱的非均匀展宽问题,我们采用了两种方法。第一种方法是使用特定状态的PCM实现方式,根据光激发时的溶剂重组能来估计极性展宽。第二种方法是根据分子动力学轨迹中垂直激发能的方差来估计展宽。尽管我们发现35Bu在乙醇中的线形与预期吻合良好,在水中的吻合程度稍低,但我们对HBDI展宽的估计却严重偏低。为了解决这一差异,我们研究了HBDI在水中的PES,发现与PCM结果相反,在明确的溶剂中基态几何结构并非平面。我们还发现非平面几何结构会增强激发时的分子内电荷转移。因此,溶剂重组和展宽要大得多,我们推测HBDI在水中的光谱更宽是由于非平面几何结构的存在。