Cerezo Javier, García-Iriepa Cristina, Santoro Fabrizio, Navizet Isabelle, Prampolini Giacomo
Departamento de Química and Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain.
CNR - Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organo Metallici (ICCOM-CNR), SS di Pisa, Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy.
Phys Chem Chem Phys. 2023 Feb 8;25(6):5007-5020. doi: 10.1039/d2cp05701h.
We present a computational investigation of the absorption spectrum in water of 5,5-spirocyclopropyl-oxyluciferin (5,5-CprOxyLH), an analogue of the emitter compound responsible for the bioluminescence in fireflies. Several factors participate in determining the 5,5-CprOxyLH's spectral shape: (i) the contribution of the four close-energy excited states, which show significant non-adiabatic couplings, (ii) the flexible molecular structure and (iii) the specific interactions established with the surrounding environment, which strongly couple the protic solvent dynamics with the dye's spectral response. To tackle the challenge to capture and dissect the role of all these effects we preliminarily investigate the role of non-adiabatic couplings with quantum dynamics simulations and a linear vibronic coupling model in the gas phase. Then, we account for both the molecular flexibility and solvent interactions by resorting to a mixed quantum classical protocol, named Adiabatic Molecular Dynamics generalized Vertical Gradient (Ad-MD|gVG), which is built on a method recently proposed by some of us. It is rooted in the partition between stiff degrees of freedom of the dye, accounted for at the vibronic level within the harmonic approximation, and flexible degrees of freedom of the solute (and of the solvent), described classically through a sampling based on Molecular Dynamics (MD). Ad-MD|gVG avoids spurious effects arising in the excited state Hessians due to non-adiabatic couplings, and can therefore be applied to account for the contributions of the first four excited states to the 5,5-CprOxyLH absorption spectrum. The final simulated spectrum is in very good agreement with the experiment, especially when the MD is driven by a refined quantum-mechanically derived force-field. More importantly, the origin of each separate contribution to the spectral shape is appropriately accounted for, paving the way to future applications of the method to more complex systems or alternative spectroscopies, as emission or circular dichroism.
我们对5,5-螺环丙基-氧化荧光素（5,5-CprOxyLH）在水中的吸收光谱进行了计算研究，5,5-CprOxyLH是萤火虫生物发光发射体化合物的类似物。有几个因素参与决定5,5-CprOxyLH的光谱形状：（i）四个近能激发态的贡献，它们显示出显著的非绝热耦合；（ii）灵活的分子结构；（iii）与周围环境建立的特定相互作用，这将质子溶剂动力学与染料的光谱响应强烈耦合。为了应对捕捉和剖析所有这些效应作用的挑战，我们首先用量子动力学模拟和气相中的线性振子耦合模型研究非绝热耦合的作用。然后，我们采用一种名为绝热分子动力学广义垂直梯度（Ad-MD|gVG）的混合量子经典协议来考虑分子灵活性和溶剂相互作用，该协议基于我们中的一些人最近提出的一种方法。它基于染料刚性自由度的划分，在简谐近似下在振子水平上考虑，以及溶质（和溶剂）的灵活自由度，通过基于分子动力学（MD）的采样进行经典描述。Ad-MD|gVG避免了由于非绝热耦合在激发态海森矩阵中产生的虚假效应，因此可用于解释前四个激发态对5,5-CprOxyLH吸收光谱的贡献。最终模拟光谱与实验结果非常吻合，特别是当MD由精细的量子力学推导力场驱动时。更重要的是，对光谱形状的每个单独贡献的来源都得到了适当解释，为该方法未来应用于更复杂的系统或替代光谱学（如发射光谱或圆二色光谱）铺平了道路。
