Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama 2-12-1-S8, Tokyo 152-8552, Japan.
J Am Chem Soc. 2011 May 4;133(17):6703-10. doi: 10.1021/ja111475z. Epub 2011 Apr 8.
Chlorosomes are light-harvesting antennae of photosynthetic bacteria containing large numbers of self-aggregated bacteriochlorophyll (BChl) molecules. They have developed unique photophysical properties that enable them to absorb light and transfer the excitation energy with very high efficiency. However, the molecular-level organization, that produces the photophysical properties of BChl molecules in the aggregates, is still not fully understood. One of the reasons is heterogeneity in the chlorosome structure which gives rise to a hierarchy of structural and energy disorder. In this report, we for the first time directly measure absorption linear dichroism (LD) on individual, isolated chlorosomes. Together with fluorescence-detected three-dimensional LD, these experiments reveal a large amount of disorder on the single-chlorosome level in the form of distributions of LD observables in chlorosomes from wild-type bacterium Chlorobaculum tepidum . Fluorescence spectral parameters, such as peak wavelength and bandwidth, are measures of the aggregate excitonic properties. These parameters obtained on individual chlorosomes are uncorrelated with the observed LD distributions and indicate that the observed disorder is due to inner structural disorder along the chlorosome long axis. The excitonic disorder that is also present is not manifested in the LD distributions. Limiting values of the LD parameter distributions, which are relatively free of the effect of structural disorder, define a range of angles at which the excitonic dipole moment is oriented with respect to the surface of the two-dimensional aggregate of BChl molecules. Experiments on chlorosomes of a triple mutant of Chlorobaculum tepidum show that the mutant chlorosomes have significantly less inner structural disorder and higher symmetry, compatible with a model of well-ordered concentric cylinders. Different values of the transition dipole moment orientations are consistent with a different molecular level organization of BChl's in the mutant and wild-type chlorosomes.
类囊体是光合细菌的光收集天线，含有大量自行聚集的细菌叶绿素 (BChl) 分子。它们具有独特的光物理性质，能够高效地吸收光并传递激发能。然而，产生聚集态 BChl 分子光物理性质的分子水平组织仍未完全理解。原因之一是类囊体结构的异质性，导致结构和能量无序的层次结构。在本报告中，我们首次直接测量了单个分离的类囊体的吸收线二向色性 (LD)。与荧光探测的三维 LD 一起，这些实验揭示了在野生型嗜热盐杆菌的类囊体中，单类囊体水平存在大量无序，表现为 LD 观测值在类囊体中的分布。荧光光谱参数，如峰值波长和带宽，是聚集体激子性质的度量。在单个类囊体上获得的这些参数与观察到的 LD 分布无关，表明观察到的无序是由于类囊体长轴上的内部结构无序引起的。激子无序也存在，但在 LD 分布中没有表现出来。LD 参数分布的限制值，相对不受结构无序的影响，定义了激子偶极矩相对于二维 BChl 分子聚集表面取向的角度范围。对嗜热盐杆菌三重突变体的类囊体的实验表明，突变体类囊体的内部结构无序程度显著降低，对称性更高，与有序同心圆柱模型兼容。不同的跃迁偶极矩取向值与突变体和野生型类囊体中 BChl 的不同分子水平组织一致。
