Wittig reactions on photoprotoporphyrin IX: new synthetic models for the special pair of the photosynthetic reaction center.
Wittig reactions on photoprotoporphyrin IX: new synthetic models for the special pair of the photosynthetic reaction center.
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光原卟啉 IX 的维蒂希反应：光合反应中心特殊对的新合成模型。
DOI：
10.1021/jo991254
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
R. Pandey
中科院分区：
文献类型：
--
作者：
G. Zheng;M. Shibata;T. Dougherty;R. Pandey
第一个例子螺二氢卟酚二聚体与固定的距离和方向作为潜在的“特殊对”的光合反应中心的模型进行了讨论。为了制备这种新结构，所需的“间隔基”5的维蒂希试剂与光原卟啉IX二甲酯3反应以产生中间体二聚体6，其在分子内[4 + 2]狄尔斯-阿尔德环加成反应中得到意想不到的螺二氢卟酚-二氢卟酚二聚体9。二聚体6在酸催化条件下脱水，以定量产率产生相应的螺二氢卟啉-卟啉二聚体16。通过NMR和模型研究证实了联苯型各向异性效应引起的二聚体6的不对称性。通过二氢卟啉3与对甲基苄基溴的鏻盐10反应形成二氢苯并卟啉14以及从二氢卟啉7中分离出8-菲乙烯基卟啉19进一步证实了我们提出的形成螺二氢卟啉-二氢卟啉二聚体9的机理。按照类似的方法，二氢卟酚3与4，4 ′-双氯甲基联苯的双鏻盐反应产生共轭二氢卟酚二聚体25。从这些二聚体获得的光谱数据表明，在这些化合物中，各个发色团不是作为一个单独的分子，而是作为一个单一的大环。为了研究二聚体9所表现出的π-π相互作用是否类似于反应中心（RC）中细菌叶绿素的结构排列，我们研究了用于表征四吡咯大环中π-π相互作用的几何参数。从晶体学坐标9开始，进行分子力学能量最小化以获得模型二聚体结构。利用表征单个吡咯环重叠的几何参数，将模型结构与光合反应中心特殊对的晶体学坐标进行了比较。结果表明，螺二氢卟酚的环A和二氢卟酚的环C在我们的模型二聚体9模拟环A-环A的相互作用中发现的晶体学特殊对，这是战略性地放置在周围的光合反应中心蛋白质基质。
A first example of spirochlorin-chlorin dimer with fixed distances and orientations as potential model for the "special pair" of the photosynthetic reaction center is discussed. For the preparation of such a novel structure, the Wittig reagent of the desired "spacer" 5 was reacted with photoprotoporphyrin IX dimethyl ester 3 to produce the intermediate dimer 6, which on intramolecular [4 + 2] Diels-Alder cycloaddition gave an unexpected spirochlorin-chlorin dimer 9. Dehydration of dimer 6 under acid-catalyzed conditions generated the corresponding spirochlorin-porphyrin dimer 16 in quantitative yield. The asymmetry in dimer 6 caused by the biphenyl-type anisotropic effect was confirmed by NMR and model studies. The formation of dihydrobenzoporphyrin 14 by reacting chlorin 3 with the phosphonium salt of p-methylbenzylbromide 10 and isolation of 8-phenanthrenevinylporphyrin 19 from chlorin 7 further confirmed our proposed mechanism for the formation of a spirochlorin-chlorin dimer 9. Following a similar approach, chlorin 3 on reacting with bis-phosphonium salt of 4, 4'-bischloromethylbiphenyl produced conjugated chlorin dimer 25. The spectroscopic data obtained from these dimers suggest that, in these compounds, the individual chromophores are not behaving as an individual molecule, but as a single macrocycle. To examine whether the pi-pi interaction exhibited by dimer 9 resembles the structural arrangement of bacteriochlorophylls in reaction center (RC), we investigated the geometrical parameters used to characterize the pi-pi interactions in tetrapyrrolic macrocycles. Starting from the crystallographic coordinates of 9, the molecular mechanics energy minimization was performed to obtain the model dimer structure. The geometrical parameters that measure the single pyrrole ring overlap were used to compare the model structure with the crystallographic coordinates of the special pair in photosynthetic reaction center. The results indicated that the ring A of spirochlorin and the ring C of chlorin in our model dimer 9 mimic the ring A-ring A interaction found in the crystallographic special pairs, which are strategically placed by the surrounding photosynthetic reaction center protein matrix.