The Mn2+-Bicarbonate Complex in a Frozen Solution Revisited by Pulse W-Band ENDOR
The Mn2+-Bicarbonate Complex in a Frozen Solution Revisited by Pulse W-Band ENDOR
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DOI：
10.1021/ic8011316
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发表时间：
2008-11-17
影响因子：
4.6
通讯作者：
Goldfarb, Daniella
中科院分区：
文献类型：
--
作者：
Potapov, Alexey;Goldfarb, Daniella
碳酸氢根与Mn 2+的配位是Mn 2+与蛋白质中羧酸残基配位的最简单模型系统。最近，已经通过X波段脉冲EPR（电子顺磁共振）实验研究了这种络合物的结构（Dasgupta，J.;等人J. Phys. Chem. B 2006，110，5099）。根据EPR结果，结合电化学滴定，可以得出结论，Mn 2+碳酸氢根络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，但只有后者已被观察到的脉冲EPR技术。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项影响核频率。(ii)M-S不等于+/-1/2流形的ENDOR（电子核双共振）谱线有重要贡献。(iii)有Na-23的重叠信号。所有这些都降低了数据解释的唯一性。在这里，我们提出了一个高场ENDOR调查Mn 2 +/（NaHCO 3）-C-13在水/甲醇溶液中，消除了上述困难。进行Davies和Mims ENDOR测量。谱图表明存在一对稍不等价的C-13核，其各向同性和各向异性超精细耦合分别为A（iso 1）= 1.2MHz，T-垂直于1 = 0.7MHz，A（iso 2）= 1.0MHz，T-垂直于2 = 0.6MHz。超精细耦合的符号由可变混合时间（VMT）ENDOR测量确定。这些相当接近的超精细参数表明，要么有两个不同的，略有不同，碳酸酯配体或有一些构象分布在只有一个配体。从垂直于1的T和垂直于2的T提取的距离与单齿结合模式一致。通过DFT计算和H-1 ENDOR测量进一步支持了单齿结合模式和两个配体的存在。此外，Na-23 ENDOR在Mn 2 +-碳酸氢根络合物中分辨了至少两种类型的Na-23（+），从而表明碳酸氢根桥接两个带正电荷的金属离子。
The coordination of bicarbonate to Mn2+ is the simplest model system for the coordination of Mn2+ 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 Mn2+ 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 Na-23. All these reduce the uniqueness of the data interpretation. Here we present a high-field ENDOR investigation of Mn2+/(NaHCO3)-C-13 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 C-13 nuclei are present, with isotropic and anisotropic hyperfine couplings of A(iso1) = 1.2 MHz, T-perpendicular to 1 = 0.7 MHz, A(iso2) = 1.0 MHz, T-perpendicular to 2 = 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-perpendicular to 1 and T-perpendicular to 2 are consistent with a monodentate binding mode. The monodentate binding mode and the presence of two ligands were further supported by DFT calculations and H-1 ENDOR measurements, Additionally, Na-23 ENDOR resolved at least two types of Na-23(+) in the Mn2+-bicarbonate complex, thus suggesting that the bicarbonate bridges two positively charged metal ions.