Turner D L, Brennan L, Messias A C, Teodoro M L, Xavier A V
Department of Chemistry, University of Southampton, UK.
Eur Biophys J. 2000;29(2):104-12. doi: 10.1007/s002490050255.
Experimental magnetic susceptibility tensors are reported for eight haems c with bis-His coordination. These data, obtained by fitting the dipolar shifts of backbone protons in the tetrahaem cytochromes c(3) from Desulfovibrio vulgaris and D. gigas, are analysed together with published values for other haem proteins. The x and y axes are found to rotate in the opposite sense to the axial ligands and are also counter-rotated with respect to the frontier molecular orbitals of the haem. The magnetic z-axis is close to the normal to the haem plane in each case. The magnitudes of the magnetic anisotropies are used to derive crystal field parameters and the rhombic splitting, V, is correlated with the dihedral angle between the axial ligands. Hence, it is apparent that the axial ligands are the dominant factor in determining the variation in magnetic properties between haems, and it is confirmed that "high g(max)" EPR signals are a reliable indicator of near-perpendicular ligands. These results are in full agreement with the analysis of non-Curie effects and electronic structure in the His-Met coordinated cytochromes c and C(551). Collectively, they show that the orientations of axial ligands to the haem may be estimated from single-crystal EPR data, from (13)C NMR shifts of the haem substituents, or from NMR dipolar shifts of the polypeptide.
报告了8种具有双组氨酸配位的血红素c的实验磁化率张量。这些数据是通过拟合普通脱硫弧菌和巨大脱硫弧菌的四血红素细胞色素c(3)中主链质子的偶极位移获得的，并与其他血红素蛋白的已发表值一起进行了分析。发现x轴和y轴的旋转方向与轴向配体相反，并且相对于血红素的前沿分子轨道也反向旋转。在每种情况下，磁z轴都接近垂直于血红素平面的方向。利用磁各向异性的大小来推导晶体场参数，菱形分裂V与轴向配体之间的二面角相关。因此，很明显轴向配体是决定血红素之间磁性性质变化的主要因素，并且证实“高g(max)”EPR信号是近垂直配体的可靠指标。这些结果与对组氨酸-甲硫氨酸配位的细胞色素c和C(551)中的非居里效应和电子结构的分析完全一致。总的来说，它们表明可以从单晶EPR数据、血红素取代基的(13)C NMR位移或多肽的NMR偶极位移来估计血红素轴向配体的取向。
