Baral Swapnil, Phillips Matthew, Yan Han, Avenso Joseph, Gundlach Lars, Baumeier Björn, Lyman Edward
Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, United States.
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States.
J Phys Chem B. 2020 Apr 2;124(13):2643-2651. doi: 10.1021/acs.jpcb.0c00121. Epub 2020 Mar 24.
Lipophilic dyes such as laurdan and prodan are widely used in membrane biology due to a strong bathochromic shift in emission that reports the structural parameters of the membrane such as area per molecule. Disentangling of the factors which control the spectral shift is complicated by the stabilization of a charge-transfer-like excitation of the dye in polar environments. Predicting the emission therefore requires modeling both the relaxation of the environment and the corresponding evolution of the excited state. Here, an approach is presented in which (i) the local environment is sampled by a classical molecular dynamics (MD) simulation of the dye and solvent, (ii) the electronically excited state of prodan upon light absorption is predicted by numerical quantum mechanics (QM), (iii) the iterative relaxation of the environment around the excited dye by MD coupled with the evolution of the excited state is performed, and (iv) the emission properties are predicted by QM. The QM steps are computed using the many-body Green's function in the approximation and the Bethe-Salpeter equation with the environment modeled as fixed point charges, sampled in the MD simulation steps. The comparison to ultrafast time-resolved transient absorption measurements demonstrates that the iterative molecular mechanics (MM)/QM approach agrees quantitatively with both the polarity-dependent shift in emission and the time scale over which the charge transfer state is stabilized. Together the simulations and experimental measurements suggest that the evolution into the charge transfer state is slower in amphiphilic solvents.
诸如劳丹（laurdan）和普罗丹（prodan）等亲脂性染料在膜生物学中被广泛应用，这是因为其发射光谱存在强烈的红移，该红移反映了膜的结构参数，例如每个分子的面积。由于染料在极性环境中类似电荷转移激发的稳定化，控制光谱移动的因素的解析变得复杂。因此，预测发射需要对环境的弛豫和激发态的相应演化进行建模。在此，提出了一种方法，其中：（i）通过对染料和溶剂进行经典分子动力学（MD）模拟来采样局部环境；（ii）通过数值量子力学（QM）预测普罗丹在光吸收时的电子激发态；（iii）通过MD进行激发染料周围环境的迭代弛豫并结合激发态的演化；（iv）通过QM预测发射特性。QM步骤使用多体格林函数在近似下以及贝叶斯 - 萨尔皮特方程进行计算，环境建模为固定点电荷，在MD模拟步骤中采样。与超快时间分辨瞬态吸收测量的比较表明，迭代分子力学（MM）/QM方法在发射的极性依赖性位移以及电荷转移态稳定的时间尺度方面都与实验结果定量相符。模拟和实验测量共同表明，在两亲性溶剂中向电荷转移态的演化较慢。
