Electrospray ionization tandem mass spectrometric and electron impact mass spectrometric characterization of mycosporine-like amino acids.

Positive-ion mass spectral fragmentations of seven mycosporine-like amino acids (MAAs) are reported and discussed. The MAAs studied are small compounds composed of a cycloheximine ring substituted with amino acid or amino alcohol units. Techniques used include electron impact (EI) and electrospray ionization (ESI) with tandem mass spectrometry (MS/MS). ESI-MS/MS showed unusual small radical losses, generally resulting from the loss of a methyl group with the exception of shinorine and porphyra for which the initial losses were 30 and 44 Da, respectively. As expected from structural similarities, porphyra, shinorine and palythinol displayed similar fragmentation patterns, while palythenic acid and palythene fragmented in a similar manner. Overall, the ESI-MS/MS fragmentations at m/z <200 exhibited a distinctive pattern for all seven MAAs with characteristic ions at m/z 137, 168, 186, and 197 or 199. Several ions were observed for each of the MAAs analyzed, and together provide a useful and potentially diagnostic pattern for identification of MAAs and as an aid in structure elucidation of novel MAAs. For GC/EI-MS analysis, trimethylsilyl (TMS) derivatives were made. The EI-MS fragmentation patterns of TMS-MAAs showed many features typical of TMS-derivatized alpha-amines. The precursor TMS-MAA ion was not detected, but a [M-90](+ radical) ion was the highest-mass intense peak observed for palythine, palythinol and shinorine, while palythene gave a [M-116](+ radical) ion. Besides determining the number of acidic hydrogens, EI-MS of TMS-derivatized MAAs will aid in structure elucidation of novel MAAs.