Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, CA 90024.
Proc Natl Acad Sci U S A. 1989 Jul;86(14):5376-9. doi: 10.1073/pnas.86.14.5376.
The presence of a biphasic circular dichroism (CD) observed in the visible absorption spectrum of retinal in bacteriorhodopsin (bR) has been believed for many years to be due to excitonic-type interaction within the trimeric structure of the retinal in the protein membrane. In the present work, we present data and discuss previous observations that strongly suggest the absence of such an excitonic interaction. The magnetic CD spectrum of the trimer is found to be similar to that of the monomer and shows no sign of absorption to the doubly degenerate state predicted to be present from the exciton theory. This, together with the previous observations on the CD spectra of the photocycle intermediates of bR as well as the linear polarization studies of the fluorescence and the daughter absorption, sheds doubt on the presence of exciton interaction and thus suggests the presence of neither an antenna system nor a viable special reaction center in bR. Possible explanation for the observed biphasic nature of the CD spectrum is given in terms of having more than one type of bR with different protein conformations around the retinals in the trimer giving each similar absorption maximum but opposite signs for its rotary dispersion power.
多年来，人们一直认为菌紫质（bR）中视黄醛的可见吸收光谱中存在的双相圆二色性（CD）是由于蛋白质膜中视黄醛的三聚体结构内的激子型相互作用。在本工作中，我们提出了数据并讨论了先前的观察结果，这些结果强烈表明不存在这种激子相互作用。发现三聚体的磁圆二色性光谱与单体的光谱相似，并且没有出现从激子理论预测的双简并态吸收的迹象。这一点，以及 bR 的光循环中间体的 CD 光谱以及荧光和子吸收的线性偏振研究的先前观察结果，使人们对激子相互作用的存在产生了怀疑，因此表明 bR 中既没有天线系统也没有可行的特殊反应中心。根据观察到的 CD 光谱的双相性质，给出了可能的解释，即三聚体中的视黄醛周围的蛋白质构象有不止一种类型的 bR，每种 bR 的吸收最大值相似，但旋转色散能力的符号相反。
