Division of Material Science (Physics), Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
Biochemistry. 2010 Sep 7;49(35):7504-15. doi: 10.1021/bi100607c.
Model calculations of linear dichroism (LD) and circular dichroism (CD) spectra were conducted for the chlorosomes of green sulfur bacteria, Chlorobium phaeobacteroides and Chlorobium tepidum, on the basis of the theory of delocalized exciton. The chlorosomes of Chl. phaeobacteroides and Chl. tepidum contain bacteriochlorophylls (BChls) e and c as the major light-harvesting pigments, respectively. The excitonic couplings among the Soret and Q(y) transitions were considered on the basis of the "rodlike" structural model for the pigment self-aggregates in a chlorosome. Trial simulations were conducted by assuming the B(x) and B(y) transition-dipole vectors to be parallel to the molecular x- and y-axes, respectively. The simulation at this stage could nicely reproduce the larger splitting of the Soret band and more significant enhancement of the Q(y) band upon formation of chlorosome in the BChl e-containing chlorosome than in the BChl c-containing one. Intensity borrowing was indicated to be the key mechanism inducing the enhancement of the Q(y) band in the BChl e-containing chlorosomes. However, the simulated LD and CD spectra in the Soret region showed qualitative disagreement from the observed ones. To resolve the deviations, the directions of the B(x) and B(y) transition-dipole vectors and the orientations of the molecular planes of BChls were adjusted in the next stage. The fine-tuning of these parameters resulted in a striking agreement between the observed and simulated CD and LD spectra over the whole spectral range studied. The best fit was obtained when the B(x) and B(y) transition-dipole vectors were assumed to be rotated 25 degrees clockwise from the molecular x- and y-axes and the molecular planes in the pigment aggregate were tilted 5 degrees from that assumed in the original model without alteration in the direction of the molecular y-axis. The calculated spectral profiles were affected little by the change in the curvatures of the rod surface, showing that the optical spectra of chlorosomes were determined essentially by the local pigment arrangement, but not by the higher-order structure, of the aggregate.
基于离域激子理论，对绿硫细菌的菌绿体——Chlorobium phaeobacteroides 和 Chlorobium tepidum 的线性二色性（LD）和圆二色性（CD）光谱进行了模型计算。Chl. phaeobacteroides 和 Chl. tepidum 的菌绿体分别以细菌叶绿素（BChl）e 和 c 作为主要的光捕获色素。根据菌绿体中色素自聚集的“棒状”结构模型，考虑了 Soret 和 Q(y)跃迁之间的激子耦合。通过假设 B(x)和 B(y)跃迁偶极矢量分别平行于分子的 x 和 y 轴，进行了试模拟。在这个阶段的模拟可以很好地再现 BChl e 含量较高的菌绿体中 Soret 带的较大分裂和 Q(y)带的更显著增强，而在 BChl c 含量较高的菌绿体中则不然。强度借贷被认为是诱导 BChl e 含量较高的菌绿体中 Q(y)带增强的关键机制。然而，模拟的 LD 和 CD 光谱在 Soret 区域与观察结果存在定性分歧。为了解决这些偏差，在下一阶段调整了 B(x)和 B(y)跃迁偶极矢量的方向以及 BChl 分子平面的取向。这些参数的微调导致在整个研究的光谱范围内观察到的和模拟的 CD 和 LD 光谱之间产生了惊人的一致性。当假设 B(x)和 B(y)跃迁偶极矢量相对于分子的 x 和 y 轴顺时针旋转 25 度，并且色素聚集体中的分子平面相对于原始模型中没有改变的方向倾斜 5 度时，获得了最佳拟合。计算的光谱轮廓受棒状表面曲率变化的影响很小，表明菌绿体的光学光谱主要由聚集的局部色素排列决定，而不是由聚集的高级结构决定。
