Distinction of alpha- and beta-aspartyl and alpha- and gamma-glutamyl peptides by fast atom bombardment/tandem mass spectrometry.

A fast atom bombardment/collisional activation/linked-scan at constant B/E (tandem mass spectrometric) method is described which can distinguish between alpha- and beta-aspartyl and alpha- and gamma-glutamyl underivatized peptides. The method is based upon differences in loss of CO from aspartyl or glutamyl B-fragment ions (IB) in these isomers which are rationalized from the stability of the resultant A-fragment ions (IA). It was observed that the ratio of IB:IA which was used in this determination was dependent upon the collision cell pressure. The higher the collision cell pressure, the larger the difference between the IB:IA ratios for these linkages.