Hirsh D J, Beck W F, Innes J B, Brudvig G W
Department of Chemistry, Yale University, New Haven, Connecticut 06511.
Biochemistry. 1992 Jan 21;31(2):532-41. doi: 10.1021/bi00117a033.
The stable tyrosine radical YD. (tyrosine 160 in the D2 polypeptide) in photosystem II (PSII) exhibits nonexponential electron spin-lattice relaxation transients at low temperature. As previously reported, the tetranuclear Mn complex in PSII significantly enhances the spin-lattice relaxation of YD.. However, in Mn-depleted PSII membranes, the spin-lattice relaxation transients of YD. are also nonexponential, and progressive power saturation (P 1/2) experiments show that it does not behave like an isolated tyrosine radical. A model is developed to treat the interaction of two paramagnets in a rigid lattice at a fixed distance apart but with a random orientation in a magnetic field. This model describes the spin-lattice relaxation of a radical in proximity to another paramagnetic site in terms of three relaxation rate constants: the "intrinsic" relaxation rate, the relaxation rate due to scalar exchange, and the relaxation rate due to dipole-dipole interactions. The intrinsic and the scalar exchange relaxation rates are isotropic and together contribute a single rate constant to the spin-lattice relaxation transients. However, the dipolar relaxation rate is orientation dependent. Each orientation contributes a different dipolar relaxation rate constant to the net spin-lattice relaxation rate constant. The result is a superposition of single-exponential recoveries, each with a different net rate constant, causing the observed saturation-recovery transients to be non-(single)-exponential. Saturation-recovery relaxation transients of YD. are compared with those of a model tyrosine radical, generated by UV photolysis of L-tyrosine in a borate glass. From this comparison, we conclude that scalar exchange does not make a significant contribution to the spin-lattice relaxation of YD. in Mn-depleted PSII. We account for the nonexponential relaxation transients obtained from YD. in Mn-depleted PSII membranes in terms of dipolar-induced relaxation enhancement from the non-heme Fe(II). From simulations of the spin-lattice relaxation transients, we obtain the magnitude of the magnetic dipolar interaction between YD. and the non-heme Fe(II), which can be used to calculate the distance between them. Using data on the non-heme Fe(II) in the reaction center of Rhodobacter sphaeroides to model the non-heme Fe(II) in PSII, we calculate a YD.-Fe(II) distance of greater than or equal to 38 A in PSII. This agrees well with the distance predicted from the structure of the bacterial reaction center.
光系统II（PSII）中的稳定酪氨酸自由基YD.（D2多肽中的酪氨酸160）在低温下表现出非指数型电子自旋 - 晶格弛豫瞬态。如先前报道，PSII中的四核锰复合物显著增强了YD.的自旋 - 晶格弛豫。然而，在锰耗尽的PSII膜中，YD.的自旋 - 晶格弛豫瞬态也是非指数型的，并且渐进功率饱和（P 1/2）实验表明它的行为不像孤立的酪氨酸自由基。开发了一个模型来处理在刚性晶格中两个顺磁体在固定距离但在磁场中具有随机取向的相互作用。该模型根据三个弛豫速率常数描述了靠近另一个顺磁位点的自由基的自旋 - 晶格弛豫：“本征”弛豫速率、标量交换引起的弛豫速率以及偶极 - 偶极相互作用引起的弛豫速率。本征和标量交换弛豫速率是各向同性的，并且一起对自旋 - 晶格弛豫瞬态贡献一个单一的速率常数。然而，偶极弛豫速率是取向依赖的。每个取向对净自旋 - 晶格弛豫速率常数贡献不同的偶极弛豫速率常数。结果是单指数恢复的叠加，每个都具有不同的净速率常数，导致观察到的饱和 - 恢复瞬态是非（单）指数型的。将YD.的饱和 - 恢复弛豫瞬态与通过硼酸盐玻璃中L - 酪氨酸的紫外光解产生的模型酪氨酸自由基的进行比较。通过这种比较，我们得出结论，标量交换对锰耗尽的PSII中YD.的自旋 - 晶格弛豫没有显著贡献。我们根据非血红素Fe(II)的偶极诱导弛豫增强来解释从锰耗尽的PSII膜中的YD.获得的非指数弛豫瞬态。通过对自旋 - 晶格弛豫瞬态的模拟，我们获得了YD.与非血红素Fe(II)之间磁偶极相互作用的大小，这可用于计算它们之间的距离。利用球形红细菌反应中心中非血红素Fe(II)的数据来模拟PSII中的非血红素Fe(II)，我们计算出PSII中YD. - Fe(II)的距离大于或等于38埃。这与从细菌反应中心结构预测的距离非常吻合。
