Investigation of Fragmentation of Tryptophan Nitrogen Radical Cation

This work describes investigation of the fragmentation mechanism of tryptophan N-indolyl radical cation, H3N(+)-TrpN(•) (m/z 204) studied via DFT calculations and several gas-phase experimental techniques. The main fragment ion at m/z 131, shown to be a mixture of up to four isomers including 3-methylindole (3MI) π-radical cation, was found to undergo further loss of an H atom to yield one of the two isomeric m/z 130 ions. 3-Methylindole radical cation generated independently (via CID of [Cu(II)(terpy)3MI](•2+)) displayed gas-phase reactivity partially similar to that of the m/z 131 fragment, further confirming our proposed mechanism. CID of deuterated tryptophan N-indolyl radical cation (m/z 208) suggested that up to six H atoms are involved in the pathway to formation of the m/z 131 ion, consistent with hydrogen atom scrambling during CID of protonated Trp.