A 15N labelling,FD and FAB study of ammonia loss from protonated arginine molecules

The positive ion field desorption (FD) spectrum of arginine taken at the best anode temperature only contains a peak due to M+H]⁺ ions. At higher emitter temperatures a considerable amount of fragmentation is induced and the M+H? NH₃]⁺ ions become most abundant. Specific ¹⁵N labelling reveals that the eliminated ammonia molecule, exclusively, contains one of the terminal nitrogen atoms of the guanidyl group. This also applies to the ammonia loss from metastably decomposing M+H]⁺ ions. The positive ion fast atom bombardment (FAB) spectrum shows more fragmentation than the FD spectrum. In contrast with the FD results, the M+H]⁺ ions generated upon FAB with ion lifetimes <10^?6 s eliminate both ammonia containing one of the terminal nitrogen atoms of the guanidyl group and ammonia containing the α-amino group in the ratio of 1.35, as found by ¹⁵N labelling. The metastably decomposing M+H]⁺ ions, however, eliminate only the former ammonia molecule. In the negative ion FD and FAB spectra no other peak than that corresponding to the M? H]^? ion is observed. Some attention has been paid to the thermal degradation of arginine on the basis of a few Curie-point pyrolysis experiments.