Davari Mehdi D, Ferrer Francisco J Avila, Morozov Dmitry, Santoro Fabrizio, Groenhof Gerrit
Theoretical and Computational Biophysics Department, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, Göttingen 37077 (Germany).
Chemphyschem. 2014 Oct 20;15(15):3236-45. doi: 10.1002/cphc.201402355. Epub 2014 Sep 1.
In this work we present the vibrationally resolved optical absorption spectrum of p-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI), the green fluorescent protein (GFP) chromophore, computed at several levels of theory, including time-dependent DFT with various functionals and basis sets, CASSCF, CASPT2 and XMCQDPT2. We also investigated what happens to the spectrum if the ground- and excited-state geometries are optimized at different levels of theory (mixed approach), as has been used previously. The vibrationally resolved absorption spectra obtained by DFT, CASPT2 and XMCQDPT2 are very similar and consist of a main absorption peak and a shoulder that is ∼1500 cm(-1) higher in energy. The vibrational progression increases moderately with temperature. These spectra are in qualitative agreement with experimental action spectra, but much narrower and lack the long tail in the blue, even at high temperatures. Because our calculated emission spectra, which are equally narrow, are in good agreement with the emission of green fluorescent protein at 253 K, we argue that the action spectrum are too broad to be considered as the absorption spectrum. The CASSCF method and the mixed approaches overestimate the vibrational progressions with respect to CAM-B3LYP, CASPT2 and XMCQDPT2, due to inaccuracies in the geometric S0 →S1 displacements. Finally, we computed the vibronic spectra of four chromophore analogues with different substitutions on the rings and found that these substitutions hardly affect the lineshape in vacuum.
在这项工作中，我们展示了对羟基苯亚甲基-2,3-二甲基咪唑啉酮（HBDI，绿色荧光蛋白（GFP）发色团）在多个理论水平下计算得到的振动分辨光学吸收光谱，这些理论水平包括使用各种泛函和基组的含时密度泛函理论（TD-DFT）、完全活性空间自洽场（CASSCF）、完全活性空间微扰理论二级近似（CASPT2）和扩展多组态准简并微扰理论二级近似（XMCQDPT2）。我们还研究了如果像之前那样在不同理论水平下优化基态和激发态几何结构（混合方法），光谱会发生什么变化。通过DFT、CASPT2和XMCQDPT2得到的振动分辨吸收光谱非常相似，由一个主要吸收峰和一个能量高约1500 cm⁻¹的肩峰组成。振动进展随温度适度增加。这些光谱与实验光电流谱在定性上一致，但更窄，即使在高温下也缺乏蓝色的长尾。由于我们计算得到的同样狭窄的发射光谱与253 K下绿色荧光蛋白的发射很好地吻合，我们认为光电流谱太宽而不能被视为吸收光谱。由于几何结构从S₀到S₁位移的不准确，CASSCF方法和混合方法相对于CAM-B3LYP、CASPT2和XMCQDPT2高估了振动进展。最后，我们计算了在环上有不同取代的四种发色团类似物的振转光谱，发现这些取代在真空中几乎不影响线形。
