Formation and characterization of iron-oligonucleotide complexes with matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry

Iron-containing oligonucleotide negative ions can be generated by matrix-assisted laser desorption/ionization from a stainless steel target disk (by either defocusing the laser beam or by mixing iron salts such as FeCl₃ with the matrix compound during the sample preparation). High resolution mass measurements reveal the presence of both Fe²⁺ (as M + Fe - 3H)^- and Fe³⁺ (as M + Fe - 4H)^- in the metal-oligonucleotide ions. The presence of Fe³⁺ is unexpected, and must involve replacement of protons from the nucleic bases or ribose groups as well as the phosphate groups of the oligonucleotides. Inspection of a range of small oligonucleotides and mononucleotides reveals that the presence of both Fe²⁺ and Fe³⁺ in the iron-biomolecule complexes is dependent on the number of acidic hydrogens that can be replaced in the oligonucleotide or nucleotide. Collisional dissociation of several metal-tetranucleotide ions revealed that the presence of the iron ion alters the fragmentation observed. The iron atom was observed to be present in all of the fragment ions, and, whenever possible, seemed to enhance the abundance of fragment ions containing both iron and a guanine nucleic base. These results suggest that iron may serve as a useful probe for characterizing phosphorylated biomolecules.