Department of Chemical Physics, Lund University , SE-22 100, Lund, Sweden.
J Phys Chem B. 2013 Sep 26;117(38):11391-6. doi: 10.1021/jp403863c. Epub 2013 May 23.
Protein is a soft material with inherently large structural disorder. Consequently, the bulk spectroscopies of photosynthetic pigment protein complexes provide averaged information where many details are lost. Here we report spectroscopy of single light-harvesting complexes where fluorescence excitation and detection polarizations are both independently rotated. Two samples of peripheral antenna (LH2) complexes from Rhodopseudomonas acidophila were studied. In one, the complexes were embedded in polyvinyl alcohol (PVA) film; in the other, they were anchored on the glass surface and covered by the PVA film. LH2 contains two rings of pigment molecules-B800 and B850. The B800 excitation polarization properties of the two samples were found to be very similar, indicating that orientation statistics of LH2s are the same in these two very different preparations. At the same time, we found a significant difference in B850 emission polarization statistics. We conclude that the B850 band of the anchored sample is substantially more disordered. We argue that both B800 excitation and B850 emission polarization properties can be explained by the tilt of the anchored LH2s due to the spin-casting of the PVA film on top of the complexes and related shear forces. Due to the tilt, the orientation statistics of two samples become similar. Anchoring is expected to orient the LH2s so that B850 is closer to the substrate. Consequently, the tilt-related strain leads to larger deformation and disorder in B850 than in B800.
蛋白质是一种具有固有较大结构无序性的软物质。因此，光合色素蛋白复合物的体相光谱提供了平均信息，其中许多细节丢失了。在这里，我们报告了单个光捕获复合物的光谱，其中荧光激发和检测偏振都可以独立旋转。我们研究了来自嗜酸红假单胞菌的两个外周天线（LH2）复合物的样本。在一个样本中，复合物嵌入聚乙烯醇（PVA）膜中；在另一个样本中，它们固定在玻璃表面上，并被 PVA 膜覆盖。LH2 包含两个色素分子环-B800 和 B850。两个样本的 B800 激发偏振特性非常相似，表明在这两种非常不同的制剂中，LH2s 的取向统计相同。同时，我们发现 B850 发射偏振统计存在显著差异。我们得出结论，固定样本的 B850 带明显更无序。我们认为，由于 PVA 膜在复合物顶部旋涂以及相关的剪切力，导致固定样本的 B800 激发和 B850 发射偏振特性都可以用固定的 LH2s 倾斜来解释。由于倾斜，两个样本的取向统计变得相似。固定预计会使 LH2s 定向，从而使 B850 更接近基底。因此，与倾斜相关的应变会导致 B850 比 B800 更大的变形和无序。
