Positive and negative ion fast atom bombardment tandem mass spectrometry (FAB-CID MS/MS) has established the fragmentation pattern of the 18 imidazolone derivatives which are related to the structure of the GFP (green fluorescent protein) chromophore in the jellyfish Aequorea bioluminescence. Ring cleavage of the C5-N1 and C2-N3 bonds is characteristics of these compounds, but for NH derivatives cleavage of the methylidene group as well as the C4-C5 and C2-N3 bonds also occurred in the positive ion mode, while ring cleavage of the C2-N3 and C4-C5 bonds occurred commonly in the negative ion mode. On the basis of the CID-MS/MS (collision induced dissociation tandem mass spectrometry) the structures of protonated molecules produced upon FAB are discussed.
When the structure of the fragment ion has a p-quinonoid moiety, distinct peaks due to radical anion species such as [M-H-CH₃]^-· are observed in their CID-MS/MS of parent anions [M-H]^-, which seems to be characteristics of the conjugated p-hydroxyphenyl structures.