几个荧光蛋白发色团双光子吸收性质的理论研究

实验测得的荧光蛋白的单、双光子吸收光谱在低频和高频区域都表现出明显不同的特征。为了揭示这些不同点的起源和研究荧光蛋白的构–效关系，我们详细研究了三种荧光蛋白发色团(一种增强型蓝绿色荧光蛋白的中性发色团和两种红色荧光蛋白的阴离子发色团)的单、双光子吸收特性，分别计算了纯的和振动分辨的电子谱。计算结果表明：光谱线形与计算采用的交换相关密度泛函及谱截面计算所采用的近似关系密切；如果在计算光谱截面时，我们利用长程修正的交换相关泛函CAM-B3LYP来计算几何和电子结构参数，然后把Franck-Condon (FC)效应和包含Herzberg-Teller (HT)效果的电-声耦合效应都考虑进去，理论计算的光谱与实验测定的光谱可以很好地符合；对于两种离子态的发色团，HT电-声耦合效应使得对应于基态到第一激发态跃迁的双光子吸收最强峰相对于单光子吸收的最强峰发生了蓝移，但HT电-声耦合效应对高频的双光子吸收谱没有太大的影响；分子内电荷转移是导致高频区的双光子吸收明显强于单光子吸收的主要原因。

Two-Photon Absorption Properties of Chromophores of a Few Fluorescent Proteins: a Theoretical Investigation

The experimentally-measured two-photon absorption (TPA) spectra of fluorescent proteins (FPs) show quite different characteristics with one-photon absorption (OPA) spectra in both the low-and high-frequency regions. To reveal the mechanism that results in the discrepancies between OPA and TPA spectra, and to obtain the fundamental structure–property relationships of FPs, here we conduct a theoretical study of OPA and TPA properties of three FP chromophores, including a neutral chromophore in enhanced cyan fluorescent protein (ECFP) and two anionic FP chromophores in DsRed2 and TagRFP. Both the pure electronic and vibrationally-resolved TPA spectra have been calculated. The calculated spectra were found to be highly dependent on the density functional theory exchange-correlation functional used. The experimental spectral lineshapes of vibronic spectra can be well produced when the Franck-Condon (FC) scattering and Herzberg-Teller (HT) vibronic coupling effects were taken into account and the structure parameters produced by CAM-B3LYP were applied in the theoretical calculations. The HT effects affect the low-frequency absorption bands corresponding to the electronic transition from S₀ to S₁ for two anionic chromophores, leading to a blue-shift of the TPA maximum relative to OPA maximum, while the HT effect is insignificant in the higher-frequency region of the TPA spectra. The intramolecular charge-transfer character of higher-lying excited states explains why the TPA spectra in the higher-frequency region are much stronger than those in the low-frequency region.