Yeagle Gregory J, Gilchrist M Lane, McCarrick Robert M, Britt R David
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, USA.
Inorg Chem. 2008 Mar 17;47(6):1803-14. doi: 10.1021/ic701680c.
Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is employed to measure the strength of the hyperfine coupling of magnetic nuclei to the paramagnetic (S = 1/2) S2 form of photosystem II (PSII). Previous X-band-frequency ESEEM studies indicated that one or more histidine nitrogens are electronically coupled to the tetranuclear manganese cluster in the S2 state of PSII. However, the spectral resolution was relatively poor at the approximately 9 GHz excitation frequency, precluding any in-depth analysis of the corresponding bonding interaction between the detected histidine and the manganese cluster. Here we report ESEEM experiments using higher X-, P-, and Ka-band microwave frequencies to target PSII membranes isolated from spinach. The X- to P-band ESEEM spectra suffer from the same poor resolution as that observed in previous experiments, while the Ka-band spectra show remarkably well-resolved features that allow for the direct determination of the nuclear quadrupolar couplings for a single I = 1(14)N nucleus. The Ka-band results demonstrate that at an applied field of 1.1 T we are much closer to the exact cancellation limit (alpha iso = 2nu(14)N) that optimizes ESEEM spectra. These results reveal hyperfine (alpha iso = 7.3 +/- 0.20 MHz and alpha dip = 0.50 +/- 0.10 MHz) and nuclear quadrupolar (e(2)qQ = 1.98 +/- 0.05 MHz and eta = 0.84 +/- 0.06) couplings for a single (14)N nucleus magnetically coupled to the manganese cluster in the S 2 state of PSII. These values are compared to the histidine imidazole nitrogen hyperfine and nuclear quadrupolar couplings found in superoxidized manganese catalase as well as (14)N couplings in relevant manganese model complexes.
多频电子自旋回波包络调制（ESEEM）光谱用于测量磁核与光系统II（PSII）顺磁性（S = 1/2）S2形式之间的超精细耦合强度。先前的X波段频率ESEEM研究表明，一个或多个组氨酸氮原子在PSII的S2状态下与四核锰簇发生电子耦合。然而，在约9 GHz的激发频率下，光谱分辨率相对较差，无法对检测到的组氨酸与锰簇之间的相应键合相互作用进行任何深入分析。在此，我们报告了使用更高的X、P和Ka波段微波频率对从菠菜中分离出的PSII膜进行的ESEEM实验。X波段到P波段的ESEEM光谱与先前实验中观察到的分辨率一样差，而Ka波段光谱显示出分辨率极高的特征，可直接确定单个I = 1（14）N核的核四极耦合。Ka波段的结果表明，在1.1 T的外加磁场下，我们更接近优化ESEEM光谱的精确抵消极限（αiso = 2ν（14）N）。这些结果揭示了与PSII的S2状态下与锰簇磁耦合的单个（14）N核的超精细（αiso = 7.3 +/- 0.20 MHz和αdip = 0.50 +/- 0.10 MHz）和核四极（e（2）qQ = 1.98 +/- 0.05 MHz和η = 0.84 +/- 0.06）耦合。将这些值与超氧化锰过氧化氢酶中发现的组氨酸咪唑氮超精细和核四极耦合以及相关锰模型配合物中的（14）N耦合进行了比较。
