An ICP-MS, ESI-MS and molecular modelling investigation of homogeneous gallium affinity tagging (HMAT) of phosphopeptides

• Selective and sensitive label for the identification of phosphopeptides.
• Selective detection of phosphopeptide by LC–ICP-MS after Ga tagging.
• Experimental and theoretical characterisation of Ga–phosphopeptide complex.
• Elucidation of energetically favoured structures Ga–phosphopeptide complex.
• Insight into differential binding of phosphorylated molecules to metals.

Protein phosphorylation and de-phosphorylation, provide one of the most common signalling pathways within cells, being involved in regulating cellular processes, mediating enzyme inhibition, protein–protein recognition and protein degradation. Compared with normal proteomics, phosphoproteomics poses some additional challenges requiring more initial separation and additional sensitivity to detect and quantify potentially ultra-low abundance species. In this work, the selective detection of phosphopeptides is described based on the incorporation of a metal tag, gallium–N,N-biscarboxymethyl lysine (Ga-LysNTA), in solution before separation and detection by liquid chromatography coupled to inductively coupled plasma mass spectrometry (LC–ICP-MS). Experimental and theoretical characterisation of the resulting Ga–phosphopeptide complex is presented based on linear ion trap electrospray ionisation mass spectrometry (ESI-MS), Fourier transform mass spectrometry (FT-MS) and molecular modelling data. Linear ion trap electrospray ionisation mass spectrometry (ESI-MS) was employed to study the interaction of the gallium tag with platelet derived growth factor beta receptor (ß-PDGF), a small phosphopeptide. In addition high resolution Fourier transform mass spectrometry (FT-MS) was used for accurate mass determination and multistage tandem mass spectrometry of the gallium–ß-PDGF complex identified the fragmentation pathway. Finally, molecular modelling was used to investigate the energetically favoured structures of both the Ga-LysNTA material and the ß-PDGF–Ga-LysNTA complex.

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