D'Angelo Paola, Lapi Andrea, Migliorati Valentina, Arcovito Alessandro, Benfatto Maurizio, Roscioni Otello Maria, Meyer-Klaucke Wolfram, Della-Longa Stefano
Dipartimento di Chimica, Universita di Roma La Sapienza, P. le A. Moro 5, 00185 Rome, Italy.
Inorg Chem. 2008 Nov 3;47(21):9905-18. doi: 10.1021/ic800982a. Epub 2008 Oct 7.
A full quantitative analysis of Fe K-edge X-ray absorption spectra has been performed for hemes in two porphynato complexes, that is, iron(III) tetraphenylporphyrin chloride (Fe(III)TPPCl) and iron(III) tetraphenylporphyrin bis(imidazole) (Fe(III)TPP(Imid)2), in two protein complexes whose X-ray structure is known at atomic resolution (1.0 A), that is, ferrous deoxy-myoglobin (Fe(II)Mb) and ferric aquo-myoglobin (Fe(III)MbH2O), and in ferric cyano-myoglobin (Fe(III)MbCN), whose X-ray structure is known at lower resolution (1.4 A). The analysis has been performed via the multiple scattering approach, starting from a muffin tin approximation of the molecular potential. The Fe-heme structure has been obtained by analyzing independently the Extended X-ray Absorption Fine Structure (EXAFS) region and the X-ray Absorption Near Edge Structure (XANES) region. The EXAFS structural results are in full agreement with the crystallographic values of the models, with an accuracy of +/- 0.02 A for Fe-ligand distances, and +/-6 degrees for angular parameters. All the XANES features above the theoretical zero energy (in the lower rising edge) are well accounted for by single-channel calculations, for both Fe(II) and Fe(III) hemes, and the Fe-N p distance is determined with the same accuracy as EXAFS. XANES evaluations of Fe-5th and Fe-6th ligand distances are determined with 0.04-0.07 A accuracy; a small discrepancy with EXAFS (0.01 to 0.05 A beyond the statistical error), is found for protein compounds. Concerns from statistical correlation among parameters and multiple minima in the parameter space are discussed. As expected, the XANES accuracy is slightly lower than what was found for polarized XANES on Fe(III)MbCN single crystal (0.03-0.04 A), and states the actual state-of-the-art of XANES analysis when used to extract heme-normal parameters in a solution spectrum dominated by heme-plane scattering.
已对两种卟啉配合物中的血红素进行了Fe K边X射线吸收光谱的全定量分析，这两种卟啉配合物分别是氯化铁（III）四苯基卟啉（Fe(III)TPPCl）和铁（III）四苯基卟啉双（咪唑）（Fe(III)TPP(Imid)2）；还对两种蛋白质配合物进行了分析，其X射线结构在原子分辨率（1.0 Å）下已知，即亚铁脱氧肌红蛋白（Fe(II)Mb）和铁水合肌红蛋白（Fe(III)MbH2O）；以及对铁氰化肌红蛋白（Fe(III)MbCN）进行了分析，其X射线结构在较低分辨率（1.4 Å）下已知。该分析通过多重散射方法进行，从分子势的 muffin tin 近似开始。通过独立分析扩展X射线吸收精细结构（EXAFS）区域和X射线吸收近边结构（XANES）区域获得了Fe-血红素结构。EXAFS结构结果与模型的晶体学值完全一致，Fe-配体距离的精度为±0.02 Å，角度参数的精度为±6度。对于Fe(II)和Fe(III)血红素，单通道计算很好地解释了理论零能量以上（在较低上升边缘）的所有XANES特征，并且Fe-N p距离的确定精度与EXAFS相同。Fe-第五和Fe-第六配体距离的XANES评估精度为0.04 - 0.07 Å；对于蛋白质化合物，发现与EXAFS存在小的差异（超出统计误差0.01至0.05 Å）。讨论了参数之间的统计相关性和参数空间中的多个极小值问题。正如预期的那样，XANES精度略低于在Fe(III)MbCN单晶上的偏振XANES精度（0.03 - 0.04 Å），并说明了在由血红素平面散射主导的溶液光谱中用于提取血红素正常参数时XANES分析的实际技术水平。
