Dimethyl ether chemical ionization mass spectrometry of alpha-amino acids

Dimethyl ether (DME) is a useful reagent gas for the characterization of a variety of diverse biologically important or environmentally significant classes of compounds. In this work the gas-phase ion-molecule reactions of DME with 23 alpha-amino acids were investigated and the collision-induced dissociation (CID) fragmentations of the protonated molecules and their most prominent adduct ions were studied. The identities and relative abundances of the adduct ions varied widely and, not unexpectedly, were dependent on the nature of the R substituent in H2NCH(R)CO2H. With a few exceptions, notably serine and threonine, protonated molecules and [M + 13]+ adduct ions were highly abundant, and in most cases methoxymethylene cations [M + 45]+ were also prominent. The solvated methoxymethylene cation [M + 91]+ was seen in very modest abundance or not at all, except for serine and threonine, when it was the most abundant. CID fragmentations of the protonated molecules generated in the DME plasma showed similar characteristics to those generated by fast atom bombardment in that sequential elimination of H2O and CO to the iminium ion was the predominant process in the majority of cases, and was also accompanied by the loss of NH3 in the cases of cysteine, glutamine, ornithine and lysine. Loss of NH3 alone was the predominant process for tryptophan, and for arginine and methionine the fragmentations were dominated by the guanidino and methylthio substituents, respectively.