ORIENTATION OF THE CHROMOPHORES IN THE REACTION CENTER OF RHODOPSEUDOMONAS-VIRIDIS - COMPARISON OF LOW-TEMPERATURE LINEAR DICHROISM SPECTRA WITH A MODEL DERIVED FROM X-RAY CRYSTALLOGRAPHY
ORIENTATION OF THE CHROMOPHORES IN THE REACTION CENTER OF RHODOPSEUDOMONAS-VIRIDIS - COMPARISON OF LOW-TEMPERATURE LINEAR DICHROISM SPECTRA WITH A MODEL DERIVED FROM X-RAY CRYSTALLOGRAPHY
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DOI：
10.1016/0005-2728(85)90138-0
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发表时间：
1985-01-01
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影响因子：
--
通讯作者：
BRETON, J
中科院分区：
文献类型：
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作者：
BRETON, J
在10 K下，记录了绿色红球藻（Rhodopteriocyanviridis）反应中心的线性二色性（LD）和吸收（A）光谱。样品在挤压聚丙烯酰胺凝胶中取向，并且主要供体P处于还原或（化学）氧化状态。在这两种状态下反应中心的LD光谱有利于P的二聚体模型，其中两个非平行QY跃迁之间的激子耦合导致在990 nm处的主跃迁（平行于膜平面）和在850 nm处的另一个较小振子强度的跃迁（倾斜约100 nm）。60.degree.膜平面外）。这些分配与先前在室温下的LD研究中提出的分配（Paillotin，G.，Vermeglio，A.和布雷东。J.（1979）Biochim. Biophys. Acta 545，249-264）。P的主要QX激子组分具有在620 nm处的宽吸收峰，并且其对应于表现出与负责850 nm跃迁的那些相同取向的偶极子。根据目前的LD研究和CD数据的化学氧化-减-还原反应中心，我们提出，次要QX激子组件的P是面向接近膜平面和吸收约660 nm。这两个单体的细菌叶绿素表现出积极的LD为他们的QY转换（在834 nm处未解决）和他们的QX转换（在600和607 nm处解决），表明这些分子的平面只是稍微倾斜的膜平面。这两种细菌脱镁叶绿素表现出强的负LD，其QY跃迁的LD/A值相同（在790和805 nm处分辨），其QX跃迁的LD为小的正LD（在534和544 nm处分辨），表明这两种分子强烈地倾斜出膜平面，每个QY跃迁倾斜约500 nm。50.degree.从那架飞机上下来这些LD数据与从X射线晶体学导出的结构模型的比较（Deisenhofer，J.，Epp，O.，三木，K.，Huber R.和Mitchel，H.等人（1984）J. Mol. Biol.180，385-398）清楚地表明，在以下条件下，两种技术的结果之间存在良好的一致性：（i）反应中心的C-2对称轴沿着膜法线延伸;（ii）激子偶联仅存在于初级供体特殊对中;以及（iii）单体细菌叶绿素和细菌脱藻素的光学跃迁方向不会受到色素之间相互作用的显着干扰。此外，类胡萝卜素被检测到在孤立的反应中心的取向，而垂直于C-2对称轴。最后，比较这些数据与类似的细菌叶绿素a的反应中心的红球藻sphaeroides 241点朝向的发色团的几何排列，这是从观察到的反应中心的Rps的一个无法区分。绿色的
Linear dichroism (LD) and absorption (A) spectra of reaction centers from Rhodopseudomonas viridis included in the native chromatophores or reconstituted in planar aggregates have been recorded at 10 K. The samples were oriented in squeezed polyacrylamide gels and the primary donor P was in the reduced or (chemically) oxidized state. The LD spectra of reaction centers in these two states are in favor of a dimeric model of P in which excitonic coupling between the two non-parallel QY transitions leads to a main transition at 990 nm (parallel to the membrane plane) and another one of smaller oscillator strength at 850 nm (tilted at approx. 60.degree. out of the membrane plane). These assignments are in close agreement with the ones proposed in a previous LD study at room temperature (Paillotin, G., Vermeglio, A. and Breton. J. (1979) Biochim. Biophys. Acta 545, 249-264). The main QX excitonic component of P has a broad absorption peaking at 620 nm and it corresponds to dipoles exhibiting the same orientation as those responsible for the 850 nm transition. On the basis of the present LD study and of CD data of chemically oxidized-minus-reduced reaction centers, we proposed that the minor QX excitonic component of P is oriented close to the membrane plane and absorbs around 660 nm. The two monomeric bacteriochlorophylls exhibit a positive LD for both their QY transitions (unresolved at 834 nm) and their QX transitions (resolved at 600 and 607 nm), indicating that the planes of these molecules are only slightly tilted out of the membrane plane. The two bacteriopheophytins exhibit strong negative LD with identical LD/A values for their QY transitions (resolved at 790 and 805 nm) and small positive LD for their QX transitions (resolved at 534 and 544 nm), demonstrating that these two molecules are strongly tilted out of the membrane plane with each of the QY transitions tilted at approx. 50.degree. out of that plane. A comparison of these LD data with the structural model derived from X-ray crystallography (Deisenhofer, J., Epp, O., Miki, K., Huber. R. and Mitchel, H. (1984) J. Mol. Biol. 180, 385-398) clearly suggests that a good agreement exists between the results of the two techniques under the following conditions: (i) the C-2 symmetry axis of the reaction center runs along the membrane normal; (ii) excitonic coupling is present only in the primary donor special pair; and (iii) the direction of the optical transitions of the monomeric bacteriochlorophylls and of the bacteriopheophytins is not significantly perturbed by the interactions among the pigments. In addition, a carotenoid is detected in the isolated reaction center with an orientation rather perpendicular to the C-2 symmetry axis. Finally, a comparison of these data with similar ones obtained on the bacteriochlorophyll a-containing reaction center of Rhodopseudomonas sphaeroides 241 points towards a geometrical arrangement of the chromophores which is indistinguishable from the one observed in the reaction center of Rps. viridis.