胆红素分子激发态性质的密度泛函理论研究

Density functional theory studies on the excited-state properties of Bilirubin molecule

胆红素是人体胆汁中的主要色素,与人体健康密切相关.结合荧光蛋白的胆红素分子代表一类新型荧光发色团,在生物成像和生物传感领域有着重要应用.本文结合隐式溶剂模型和线性响应含时密度泛函理论方法计算了胆红素分子最低单重激发态的垂直激发能、振子强度和垂直发射能.以实验测量值和高水平RIADC(2)计算值作为参考,系统考察了一系列密度泛函方法的预测表现,结果发现最优化调控区间分离密度泛函方法整体表现最优,预测的绝对误差和相对误差最小.这得益于泛函中包含适宜的准确交换项比例能够产生既不离域也不局域的电子结构.在最优化调控泛函方法计算的波函数基础上,基于空穴-电子分析和片段间电荷转移方法定量表征了胆红素分子最低单重激发态,发现其具有杂化局域-电荷转移的激发特征.相信本工作可以为今后研究胆红素分子的激发态动力学过程和光谱性质提供重要理论依据,该最优化调控理论模型也可以为接下来其他生物分子体系的激发态性质研究提供可靠、高效的理论工具.

Bilirubin is the main pigment in human bile,which is closely related to human health.Bilirubin combining with fluorescent protein represents a new type of fluorescent chromophore and has important applications in the field of biological imaging and biosensor.Due to the lack of efficient and accurate electronic structure methods,the electronic structure and excited-state properties of bilirubin molecule are not characterized quantitatively and accurately.Firstly,the vertical absorption energy,oscillator strength and vertical emission energy of the lowest singlet excited state of bilirubin molecule are calculated by combining the implicit solvent model and the linear response time-dependent density functional theory(TDDFT)method.Compared to the experimental data and high-level RI-ADC(2)calculation,the prediction performance of a series of density functional methods is systematically investigated.The results show that the optimally-tuned range separated density functional method has the best overall performance and the minimum absolute and relative errors.This is obviously due to the fact that the suitable proportion of exact exchange included in density functionals can produce neither delocalized nor localized electronic structures.Based on the produced wavefunction by the optimally-tuned method,the excited-state characteristics of the S1 state of bilirubin molecule indicate a hybrid local and charge transfer excitation,based on the quantitative characterization using hole-electron analysis and interfragment charge transfer method.This work can provide a theoretical basis for the study of excited-state dynamics and spectral properties of bilirubin molecules and the optimally tuned range-separated DFT method also provide a reliable and efficient theoretical tool to study the excited-state properties of other biomolecular systems in the future.