Aznar Constantino P, Britt R David
Department of Chemistry, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, USA.
Philos Trans R Soc Lond B Biol Sci. 2002 Oct 29;357(1426):1359-65; discussion 1365-7. doi: 10.1098/rstb.2002.1144.
The pulsed EPR methods of electron spin echo envelope modulation (ESEEM) and electron spin echo-electron nuclear double resonance (ESE-ENDOR) are used to investigate the proximity of exchangeable hydrogens around the paramagnetic S(2)-state Mn cluster of the photosystem II oxygen-evolving complex. Although ESEEM and ESE-ENDOR are both pulsed electron paramagnetic resonance techniques, the specific mechanisms by which nuclear spin transitions are observed are quite different. We are able to generate good simulations of both (1)H ESE-ENDOR and (2)H ESEEM signatures of exchangeable hydrogens at the S(2)-state cluster. The convergence of simulation parameters for both methods provides a high degree of confidence in the simulations. Several exchangeable protons-deuterons with strong dipolar couplings are observed. In the simulations, two of the close ( approximately 2.5 A) hydrogen nuclei exhibit strong isotropic couplings and are therefore most probably associated with direct substrate ligation to paramagnetic Mn. Another two of the close ( approximately 2.7 A) hydrogen nuclei show no isotropic couplings and are therefore most probably not contained in Mn ligands. We suggest that these proximal hydrogens may be associated with a Ca(2+)-bound substrate, as indicated in recent mechanistic proposals for O(2) formation.
电子自旋回波包络调制（ESEEM）和电子自旋回波 - 电子核双共振（ESE - ENDOR）的脉冲电子顺磁共振方法用于研究光系统II析氧复合物的顺磁S(2)态锰簇周围可交换氢的接近程度。虽然ESEEM和ESE - ENDOR都是脉冲电子顺磁共振技术，但观察核自旋跃迁的具体机制却大不相同。我们能够对S(2)态簇处可交换氢的（1）H ESE - ENDOR和（2）H ESEEM信号进行良好的模拟。两种方法模拟参数的一致性为模拟提供了高度的可信度。观察到几个具有强偶极耦合的可交换质子 - 氘核。在模拟中，两个距离较近（约2.5埃）的氢核表现出强各向同性耦合，因此很可能与底物直接与顺磁锰的连接有关。另外两个距离较近（约2.7埃）的氢核没有表现出各向同性耦合，因此很可能不包含在锰配体中。我们认为，这些近端氢可能与Ca(2+)结合的底物有关，正如最近关于O(2)形成的机制研究所表明的那样。
