Dinh Thanh-Chung, Renger Thomas
Institut für Theoretische Physik, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria.
J Chem Phys. 2015 Jan 21;142(3):034104. doi: 10.1063/1.4904928.
A challenge for the theory of optical spectra of pigment-protein complexes is the equal strength of the pigment-pigment and the pigment-protein couplings. Treating both on an equal footing so far can only be managed by numerically costly approaches. Here, we exploit recent results on a normal mode analysis derived spectral density that revealed the dominance of the diagonal matrix elements of the exciton-vibrational coupling in the exciton state representation. We use a cumulant expansion technique that treats the diagonal parts exactly, includes an infinite summation of the off-diagonal parts in secular and Markov approximations, and provides a systematic perturbative way to include non-secular and non-Markov corrections. The theory is applied to a model dimer and to chlorophyll (Chl) a and Chl b homodimers of the reconstituted water-soluble chlorophyll-binding protein (WSCP) from cauliflower. The model calculations reveal that the non-secular/non-Markov effects redistribute oscillator strength from the strong to the weak exciton transition in absorbance and they diminish the rotational strength of the exciton transitions in circular dichroism. The magnitude of these corrections is in a few percent range of the overall signal, providing a quantitative explanation of the success of time-local convolution-less density matrix theory applied earlier. A close examination of the optical spectra of Chl a and Chl b homodimers in WSCP suggests that the opening angle between Qy transition dipole moments in Chl b homodimers is larger by about 9(∘) than for Chl a homodimers for which a crystal structure of a related WSCP complex exists. It remains to be investigated whether this change is due to a different mutual geometry of the pigments or due to the different electronic structures of Chl a and Chl b.
色素 - 蛋白质复合物光谱理论面临的一个挑战是色素 - 色素和色素 - 蛋白质耦合强度相当。迄今为止，要平等对待这两者，只能通过计算成本高昂的方法来实现。在此，我们利用基于正常模式分析得出的光谱密度的最新结果，该结果揭示了在激子态表示中激子 - 振动耦合对角矩阵元的主导地位。我们使用累积量展开技术，该技术精确处理对角部分，在久期和马尔可夫近似中包含非对角部分的无穷级数求和，并提供一种系统的微扰方法来纳入非久期和非马尔可夫修正。该理论应用于一个模型二聚体以及来自花椰菜的重组水溶性叶绿素结合蛋白（WSCP）的叶绿素（Chl）a和Chl b同二聚体。模型计算表明，非久期/非马尔可夫效应在吸收中从强激子跃迁到弱激子跃迁重新分配了振子强度，并降低了圆二色性中激子跃迁的旋转强度。这些修正的幅度在总信号的百分之几范围内，为早期应用的无时间局部卷积密度矩阵理论的成功提供了定量解释。对WSCP中Chl a和Chl b同二聚体光谱的仔细研究表明，Chl b同二聚体中Qy跃迁偶极矩之间的张角比存在相关WSCP复合物晶体结构的Chl a同二聚体大约9（°）。这种变化是由于色素的相互几何结构不同还是由于Chl a和Chl b的电子结构不同，仍有待研究。
