Favre Daniel, Bobst Cedric E, Eyles Stephen J, Murakami Heide, Crans Debbie C, Kaltashov Igor A
Department of Chemistry, University of Massachusetts-Amherst, Amherst, MA.
Department of Biochemistry and Molecular Biology, University of Massachusetts-Amherst, Amherst, MA.
Inorg Chem Front. 2022 Apr 7;9(7):1556-1564. doi: 10.1039/d1qi01618k. Epub 2022 Feb 14.
Decavanadate (VO or V10) is a paradigmatic member of the polyoxidometalate (POM) family, which has been attracting much attention within both materials/inorganic and biomedical communities due to its unique structural and electrochemical properties. In this work we explored the utility of high-resolution electrospray ionization (ESI) mass spectrometry (MS) and ion exclusion chromatography LC/MS for structural analysis of V10 species in aqueous solutions. While ESI generates abundant molecular ions representing the intact V10 species, their isotopic distributions show significant deviations from the theoretical ones. A combination of high-resolution MS measurements and hydrogen/deuterium exchange allows these deviations to be investigated and interpreted as a result of partial reduction of V10. While the redox processes are known to occur in the ESI interface and influence the oxidation state of redox-active analytes, the LC/MS measurements using ion exclusion chromatography provide unequivocal evidence that the mixed-valence V10 species exist in solution, as extracted ion chromatograms representing V10 molecular ions at different oxidation states exhibit distinct elution profiles. The spontaneous reduction of V10 in solution is seen even in the presence of hydrogen peroxide and has not been previously observed. The susceptibility to reduction of V10 is likely to be shared by other redox active POMs. In addition to the molecular V10 ions, a high-abundance ionic signal for a VO anion was displayed in the negative-ion ESI mass spectra. None of the VO cations were detected in ESI MS, and only a low-abundance signal was observed for VO anions with a single negative charge, indicating that the presence of abundant VO anions in ESI MS reflects gas-phase instability of VO anions carrying two charges. The gas-phase origin of the VO anion was confirmed in tandem MS measurements, where mild collisional activation was applied to V10 molecular ions with an even number of hydrogen atoms (HVO ), resulting in a facile loss of HO molecules and giving rise to VO as the lowest-mass fragment ion. Water loss was also observed for VO anions carrying an odd number of hydrogen atoms (e.g., HVO ), followed by a less efficient and incomplete removal of an OH radical, giving rise to both HVO and VO fragment ions. Importantly, at least one hydrogen atom was required for ion fragmentation in the gas phase, as no further dissociation was observed for any hydrogen-free V10 ionic species. The presented workflow allows a distinction to be readily made between the spectral features revealing the presence of non-canonical POM species in the bulk solution from those that arise due to physical and chemical processes occurring in the ESI interface and/or the gas phase.
十钒酸盐（VO₁₀）是多金属氧酸盐（POM）家族中的典型成员，由于其独特的结构和电化学性质，在材料/无机和生物医学领域都备受关注。在这项工作中，我们探索了高分辨率电喷雾电离（ESI）质谱（MS）和离子排斥色谱-液相色谱/质谱联用技术在水溶液中V₁₀物种结构分析中的应用。虽然ESI能产生代表完整V₁₀物种的大量分子离子，但其同位素分布与理论值存在显著偏差。高分辨率MS测量与氢/氘交换相结合，使得这些偏差得以研究，并被解释为V₁₀部分还原的结果。虽然已知氧化还原过程发生在ESI界面并影响氧化还原活性分析物的氧化态，但使用离子排斥色谱的液相色谱/质谱测量提供了明确的证据，表明混合价态的V₁₀物种存在于溶液中，因为代表不同氧化态V₁₀分子离子的提取离子色谱图呈现出不同的洗脱曲线。即使在过氧化氢存在的情况下，溶液中的V₁₀也会自发还原，这是之前未曾观察到的。V₁₀对还原的敏感性可能也存在于其他氧化还原活性的POM中。除了分子V₁₀离子外，负离子ESI质谱中还显示出VO₄³⁻阴离子的高丰度离子信号。在ESI MS中未检测到VO阳离子，对于单负电荷的VO阴离子仅观察到低丰度信号，这表明ESI MS中大量VO₄³⁻阴离子的存在反映了带两个电荷的VO阴离子在气相中的不稳定性。VO₄³⁻阴离子的气相来源在串联质谱测量中得到证实，其中对具有偶数个氢原子的V₁₀分子离子（H₂VO₁₀）施加温和的碰撞活化，导致容易失去H₂O分子并产生VO₄³⁻作为最低质量的碎片离子。对于带有奇数个氢原子的VO阴离子（例如HVO₄²⁻）也观察到了失水现象，随后OH自由基的去除效率较低且不完全，产生了HVO₄²⁻和VO₄³⁻碎片离子。重要的是，气相中的离子碎片化至少需要一个氢原子，因为对于任何无氢的V₁₀离子物种都未观察到进一步的解离。所提出的工作流程能够轻松区分揭示本体溶液中存在非典型POM物种的光谱特征与那些由ESI界面和/或气相中发生的物理和化学过程引起的光谱特征。
