Study of metal complexes of a tripodal hydroxypyridinone ligand by electrospray tandem mass spectrometry

A review of recent trends in electrospray ionisation-mass spectrometry for the analysis of metal-organic ligand complexes

Electrospray ionisation-mass spectrometry (ESI-MS) is used in a wide variety of fields to examine the formation, stoichiometry and speciation of complexes involving metals and organic ligands. This article reviews the literature in this area over the past 5 years, examining trends in ESI-MS use and novel applications that enhance the scope of the technique. ESI-MS can provide direct information on changes in speciation with metal:ligand ratio and pH, identify metal oxidation state directly and allow insight into competitive interactions in ternary systems. However, both the instrumental set-up and artefacts in the electrospraying process can affect the species distribution observed, and changes in solution chemistry can affect the relative ion intensity of species. Therefore, ESI-MS data is at its most powerful when corroborated by data from other experimental techniques, such as pH potentiometry. The challenges in interpreting direct ESI-MS data quantitatively are discussed in detail, with reference to differences in the ion intensities of species, signal suppression and quantifying species distributions. The use of HPLC-ESI-MS is also reviewed, highlighting challenges and applications. Overall, the need for more standard reporting of quality assurance data is discussed, to strengthen the applications of ESI-MS to metal-organic ligand complexes further.

Separation and identification of phytosiderophores and their metal complexes in plants by zwitterionic hydrophilic interaction liquid chromatography coupled to electrospray ionization mass spectrometry

A sensitive method for the separation of different phytosiderophores (PS) of the mugineic acid (MA) family, and the candidate ligand for intracellular metal transport in plants nicotianamine (NA), and respective metal complexes in plants by zwitterionic hydrophilic interaction liquid chromatography (ZIC-HILIC) coupled to electrospray ionization mass spectrometry (ESI-MS) is described. Separation of mugineic acid, 2'-deoxymugineic acid (DMA), 3-epi-hydroxymugineic acid (epi-HMA), nicotianamine, Fe(III)-DMA, Fe(III)-NA, M(II)-DMA, and M(II)-NA complexes (M(II)=Zn(II), Cu(II), Ni(II), and Fe(II)), was achieved within 22 min on the ZIC-HILIC column by using a gradient elution with a mobile phase consisting of ammonium acetate and acetonitrile at pH 7.3, at a flow rate of 0.15 mL/min. The on-line coupling to ESI-MS in the negative ionization mode enables the detection of these compounds in the micromol/L range, which is the relevant concentration range in real plant samples. DMA-complexes of Fe(III), Zn, and Cu in wheat root, and an NA-complex of Ni in Arabidopsis were detected and identified by the proposed method. Even in the case of partial coelution of some divalent metal complexes, the identification is possible by their distinct mass spectra. The stability of metal complexes during separation was checked by injecting ethylenediaminetetraacetic acid (EDTA) after each run of metal-phytosiderophore complexes. Good stability of divalent-phytosiderophores, except for Fe(II)-complexes, was observed. During gradient separation, Fe(III)-complexes are partly dissociated (<20%), but a good sensitivity of Fe(III)-DMA in real plant samples is still achieved. In order to avoid instability problems with the separation of Fe-complexes, an isocratic separation is proposed, which allows the separation of ferrous and ferric complexes in 2 min.

Different iron-chelating properties of pyochelin diastereoisomers revealed by LC/MS

Pyochelin is a siderophore and virulence factor common to Burkholderia cepacia and several Pseudomonas strains. It is isolated from bacterial media as a mixture of two epimers, which readily equilibrate in most solvents. Experiments based on high-performance liquid chromatography/electrospray ionization mass spectrometry are reported here, allowing the investigation of the different Fe(III)-chelating properties of pyochelin diastereomers in solution without the need for labourious isolation. It is demonstrated in this study that only one of the two pyochelin diastereomers is able to chelate Fe(III); no Fe(III) complexes of the other diastereomer could be detected. The Fe(III)-pyochelin complex exhibited a 1:1 metal-to-siderophore ratio and no evidence for other stoichiometries was found.

Analysis of iron(II)/iron(III) phytosiderophore complexes by nano-electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Nano-electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (nano-ESI-FTICRMS) was employed for the analysis of the phytosiderophore 2'-deoxymugineic acid (DMA) and the candidate ligand for the intracellular iron transport in plants nicotianamine (NA). Due to the zwitterionic nature of NA and DMA, complementary mass spectra were obtained in positive and negative ionization modes. The technique was also used for speciation of their complexes with Fe(II) and Fe(III), respectively. The species observed at pH 7.3 are the 1:1 Fe-ligand complexes and no evidence for the existence of dimeric complexes was observed. NA and DMA differ only by one mass unit. Consequently, in the system NA + DMA + Fe(II)/Fe(III), there are pairs of iron species (i.e. NA-Fe(II) and DMA-Fe(III)) with the same nominal mass, which differ only by approximately 0.02 mass units. It is shown that high-resolution MS accompanied by accurate mass data analysis allows the unequivocal identification of all four iron species (NA-Fe(II), NA-Fe(III), DMA-Fe(II), DMA-Fe(III)) in one solution without separation. We also addressed the possible alteration of the oxidation state of chelated iron under nano-ESI conditions, but no redox reactions were observed under optimized conditions.