Roadrnap for fragmentation of protonated methylarginines in the gas phase

Up to MS4 collision-induced fragmentation study of protonated pseudomolecular ions of L-arginine (Arg), omega-N-G-monomethylarginine (MMA), omega-N-G,N-G-asymmetric dimethylarginine (aDMA) and omega-N-G,N-G'-symmetric dimethylarginine (sDMA) were performed with Fourier transform ion cyclotron resonance mass spectrometry. The parent pseudomolecular ions were generated in a home-built atmospheric-pressure matrix-assisted laser desorption/ionization source, and the admixing of nitrocellulose with the matrix significantly reduced the interfering matrix peaks in the resulting mass spectra. While protonated isobaric molecules (aDMA, sDMA) could be easily distinguished by low-order tandem mass spectrometry, MS4 was needed for unequivocal revelation of some previously unreported fragmentation pathways. Among a total of 44 fragment ions generated from the two isomeric DMAs, 5 were uniquely and exclusively produced from just one of these precursor ions that allowed the precursor ion structures to be differentiated. The unsurpassed advantage of high resolution and mass accuracy of FTICR MS enabled us to identify fragmentation channels that yield fragment ions differing in mass by only similar to 0.01 Da. The sub-ppm mass accuracies facilitated in proposing chemical structures for all the fragments observed. Many hitherto unreported isobaric fragments from the methylated arginines are observed for the first time. (C) 2015 Elsevier B.V. All rights reserved.