Potapov Alexey, Goldfarb Daniella
Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
Inorg Chem. 2008 Nov 17;47(22):10491-8. doi: 10.1021/ic8011316. Epub 2008 Oct 24.
The coordination of bicarbonate to Mn (2+) is the simplest model system for the coordination of Mn (2+) to carboxylate residues in a protein. Recently, the structure of such a complex has been investigated by means of X-band pulse EPR (electron paramagnetic resonance) experiments ( Dasgupta, J. ; et al. J. Phys. Chem. B 2006, 110, 5099 ). Based on the EPR results, together with electrochemical titrations, it has been concluded that the Mn (2+) bicarbonate complex consists of two bicarbonate ligands, one of which is monodentate and other bidentate, but only the latter has been observed by the pulsed EPR techniques. The X-band measurements, however, suffer several drawbacks. (i) The zero-field splitting (ZFS) term of the spin Hamiltonian affects the nuclear frequencies. (ii) There are significant contributions from ENDOR (electron nuclear double resonance) lines of the M S not equal +/- (1)/ 2 manifolds. (iii) There are overlapping signals of (23)Na. All these reduce the uniqueness of the data interpretation. Here we present a high-field ENDOR investigation of Mn (2+)/NaH (13)CO 3 in a water/methanol solution that eliminates the above difficulties. Both Davies and Mims ENDOR measurements were carried out. The spectra show that a couple of slightly inequivalent (13)C nuclei are present, with isotropic and anisotropic hyperfine couplings of A iso1 = 1.2 MHz, T perpendicular1 = 0.7 MHz, A iso2 = 1.0 MHz, T perpendicular2 = 0.6 MHz, respectively. The sign of the hyperfine coupling was determined by variable mixing time (VMT) ENDOR measurements. These rather close hyperfine parameters suggest that there are either two distinct, slightly different, carbonate ligands or that there is some distribution in conformation in only one ligand. The distances extracted from T perpendicular1 and T perpendicular2 are consistent with a monodentate binding mode. The monodentate binding mode and the presence of two ligands were further supported by DFT calculations and (1)H ENDOR measurements. Additionally, (23)Na ENDOR resolved at least two types of (23)Na (+) in the Mn (2+)-bicarbonate complex, thus suggesting that the bicarbonate bridges two positively charged metal ions.
碳酸氢根与Mn(2+)的配位是蛋白质中Mn(2+)与羧酸根残基配位的最简单模型体系。最近，通过X波段脉冲EPR（电子顺磁共振）实验对这种配合物的结构进行了研究（Dasgupta, J.等人，《物理化学杂志B》，2006年，第110卷，第5099页）。基于EPR结果以及电化学滴定，得出结论：Mn(2+)碳酸氢根配合物由两个碳酸氢根配体组成，其中一个是单齿的，另一个是双齿的，但脉冲EPR技术仅观察到了后者。然而，X波段测量存在几个缺点。(i) 自旋哈密顿量的零场分裂（ZFS）项会影响核频率。(ii) M S不等于±(1)/ 2流形的ENDOR（电子核双共振）线有显著贡献。(iii) 存在(23)Na的重叠信号。所有这些都降低了数据解释的唯一性。在此，我们展示了对水/甲醇溶液中Mn(2+)/NaH(13)CO 3的高场ENDOR研究，该研究消除了上述困难。进行了戴维斯和米姆斯ENDOR测量。光谱表明存在一对略有不等价的(13)C核，各向同性和各向异性超精细耦合分别为A iso1 = 1.2 MHz，T perpendicular1 = 0.7 MHz，A iso2 = 1.0 MHz，T perpendicular2 = 0.6 MHz。超精细耦合的符号通过可变混合时间（VMT）ENDOR测量确定。这些相当接近的超精细参数表明，要么存在两个不同的、略有差异的碳酸根配体，要么仅在一个配体中存在构象分布。从T perpendicular1和T perpendicular2提取的距离与单齿结合模式一致。单齿结合模式和两个配体的存在通过DFT计算和(1)H ENDOR测量得到了进一步支持。此外，(23)Na ENDOR在Mn(2+) - 碳酸氢根配合物中分辨出至少两种类型的(23)Na(+)，因此表明碳酸氢根桥接了两个带正电荷的金属离子。
