Tandem mass spectral decompositions of protonated N-acyloligoalanines and N-acyloligoglycines as models for those of the protonated free oligopeptides.

The intensities of the y-type and b-type ions in the tandem mass spectra of the [M + 1]+ ions of acylated oligomers of alanine and glycine give evidence that these ions are not structurally similar to the [M + 1]+ ions of the free oligomers. For example, the intensities do not follow statistical or linear free energy relationships as would be expected if they were homologous. Instead there are sharp differences in intensities: the y3 ions are considerably more intense than expected, and the b2 ions are less intense. The ions whose intensities differ sharply seem to involve cleavage where the amount of stabilization of a product of cyclization changes when the terminal nitrogen is acylated.