Wanko M, Hoffmann M, Strodel P, Koslowski A, Thiel W, Neese F, Frauenheim T, Elstner M
Department of Theoretical Physics, University of Paderborn, D-33098 Paderborn, Germany.
J Phys Chem B. 2005 Mar 3;109(8):3606-15. doi: 10.1021/jp0463060.
Rhodopsins can modulate the optical properties of their chromophores over a wide range of wavelengths. The mechanism for this spectral tuning is based on the response of the retinal chromophore to external stress and the interaction with the charged, polar, and polarizable amino acids of the protein environment and is connected to its large change in dipole moment upon excitation, its large electronic polarizability, and its structural flexibility. In this work, we investigate the accuracy of computational approaches for modeling changes in absorption energies with respect to changes in geometry and applied external electric fields. We illustrate the high sensitivity of absorption energies on the ground-state structure of retinal, which varies significantly with the computational method used for geometry optimization. The response to external fields, in particular to point charges which model the protein environment in combined quantum mechanical/molecular mechanical (QM/MM) applications, is a crucial feature, which is not properly represented by previously used methods, such as time-dependent density functional theory (TDDFT), complete active space self-consistent field (CASSCF), and Hartree-Fock (HF) or semiempirical configuration interaction singles (CIS). This is discussed in detail for bacteriorhodopsin (bR), a protein which blue-shifts retinal gas-phase excitation energy by about 0.5 eV. As a result of this study, we propose a procedure which combines structure optimization or molecular dynamics simulation using DFT methods with a semiempirical or ab initio multireference configuration interaction treatment of the excitation energies. Using a conventional QM/MM point charge representation of the protein environment, we obtain an absorption energy for bR of 2.34 eV. This result is already close to the experimental value of 2.18 eV, even without considering the effects of protein polarization, differential dispersion, and conformational sampling.
视紫红质可以在很宽的波长范围内调节其发色团的光学性质。这种光谱调谐的机制基于视网膜发色团对外部应力的响应以及与蛋白质环境中带电、极性和可极化氨基酸的相互作用，并与其激发时偶极矩的巨大变化、大的电子极化率以及结构灵活性相关。在这项工作中，我们研究了计算方法在模拟吸收能量随几何结构变化和外加外部电场变化方面的准确性。我们说明了吸收能量对视网膜基态结构的高度敏感性，而视网膜基态结构会因用于几何结构优化的计算方法的不同而有显著变化。对外部场的响应，特别是对在量子力学/分子力学（QM/MM）联合应用中模拟蛋白质环境的点电荷的响应，是一个关键特征，而以前使用的方法，如含时密度泛函理论（TDDFT）、完全活性空间自洽场（CASSCF）、哈特里 - 福克（HF）或半经验单激发组态相互作用（CIS），都不能恰当地体现这一特征。本文针对细菌视紫红质（bR）进行了详细讨论，bR可使视网膜气相激发能量发生约0.5 eV的蓝移。作为这项研究的结果，我们提出了一种程序，该程序将使用密度泛函理论方法进行结构优化或分子动力学模拟与对激发能量的半经验或从头算多参考组态相互作用处理相结合。使用蛋白质环境的传统QM/MM点电荷表示法，我们得到bR的吸收能量为2.34 eV。即使不考虑蛋白质极化、微分色散和构象采样的影响，这个结果也已经接近2.18 eV的实验值。
