Pearlstein R M, Hemenger R P
Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830.
Proc Natl Acad Sci U S A. 1978 Oct;75(10):4920-4. doi: 10.1073/pnas.75.10.4920.
The low-temperature 800-nm band absorption and circular dichroism spectra of the bacteriochlorophyll (Bchl) a-protein from Prosthecochloris aestuarii strain 2K are analyzed theoretically. These spectra show considerable structure that is attributed primarily to resonance (exciton) interactions among the lowest singlet transitions of the Bchl a molecules contained in each protein. We calculate these spectra from the known arrangement of the Bchl molecules in the protein. With the conventional assignment of the lowest singlet transition of Bchl a as Q(y) (y-polarized), agreement of calculated spectra with experiment is poor. All of our attempts, based on this conventional assignment, to improve the theoretical fits to absorption and circular dichroism spectra simultaneously are unsuccessful. However, by making the simple but unconventional assumption that the lowest singlet transition in each of the Bchl a molecules in each protein is x-polarized rather than y-polarized, we find good agreement between calculated and observed spectra. If these results are not fortuitous, they indicate that there is a systematic error in the protein structural model, that the conventional assignments of Bchl a transitions are incorrect, or that the protein environment provides a sufficiently strong perturbation to rotate the lowest singlet transition moment direction by approximately 90 degrees , presumably by changing the order of certain of the Bchl a orbitals.
对来自河口原绿球菌2K菌株的细菌叶绿素（Bchl）a蛋白的低温800纳米波段吸收光谱和圆二色光谱进行了理论分析。这些光谱显示出相当多的结构，这主要归因于每个蛋白质中所含Bchl a分子最低单重态跃迁之间的共振（激子）相互作用。我们根据蛋白质中Bchl分子的已知排列来计算这些光谱。按照Bchl a最低单重态跃迁的传统归属为Q(y)（y偏振），计算光谱与实验结果的吻合度很差。基于这种传统归属，我们所有同时改进吸收光谱和圆二色光谱理论拟合的尝试都未成功。然而，通过做出一个简单但非传统的假设，即每个蛋白质中每个Bchl a分子的最低单重态跃迁是x偏振而非y偏振，我们发现计算光谱与观测光谱之间有很好的吻合度。如果这些结果不是偶然的，它们表明在蛋白质结构模型中存在系统误差，Bchl a跃迁的传统归属是不正确的，或者蛋白质环境提供了足够强的微扰，使最低单重态跃迁矩方向旋转了大约90度，大概是通过改变某些Bchl a轨道的顺序。
