Katz J J, Shipman L L, Norris J R
Ciba Found Symp. 1978(61):1-40.
Evidence from electron paramagnetic resonance (e.p.r.) studies suggests that the unpaired spin in oxidized P700+. or P865+. is shared by two special chlorophyll a (Chl a) or bacteriochlorophyll a (Bchl a) molecules respectively. Three classes of models have been proposed for special pair reaction centre chlorophyll: asymmetric, in which one Chl a (or Bchl a) acts as electron donor to a second acting as acceptor; models with translational symmetry only; and models with C2 symmetry. Models with C2 symmetry have been synthesized in vitro with two chlorophyll macrocycles tied together by a covalent link. The singlet and triplet states of the in vitro models have been characterized by e.p.r., nuclear magnetic resonance, and optical studies involving absorption, emission, and lasting behaviour. The fact that lasting occurs only from the folded configuration of the linked dimers suggests the availability of a highly effective nonradiative decay path from the S1 state of the excited open dimer. A radical-pair mechanism that accounts for the unusual spin polarization of the special pair triplet is proposed for the primary photochemistry in the reaction centre.
电子顺磁共振（e.p.r.）研究的证据表明，氧化态的P700 +或P865 +中的未成对自旋分别由两个特殊叶绿素a（Chl a）或细菌叶绿素a（Bchl a）分子共享。针对特殊对反应中心叶绿素，已提出三类模型：不对称模型，其中一个Chl a（或Bchl a）作为电子供体，另一个作为受体；仅具有平移对称性的模型；以及具有C2对称性的模型。具有C2对称性的模型已在体外合成，其中两个叶绿素大环通过共价键连接在一起。体外模型的单重态和三重态已通过e.p.r.、核磁共振以及涉及吸收、发射和持久行为的光学研究进行了表征。持久现象仅从连接二聚体的折叠构型发生这一事实表明，从激发的开放二聚体的S1态存在一条高效的非辐射衰变路径。针对反应中心的初级光化学，提出了一种自由基对机制，该机制解释了特殊对三重态的异常自旋极化。
