Scheme 1.

Possible reaction mechanisms for NH₃ loss from the protonated dipeptides AlaAsn (AN), AsnAla (NA), AlaGln (AQ), and GlnAla (QA). Both ammonia loss from the side chain (left column) as well as from the N-terminus (right column) are taken into account. This Scheme only considers NH₃ loss induced by nucleophilic attack leading to 5- or 6-membered ring products. Mechanisms leading to energetically less favorable 4-,7-, or higher membered ring structures have been considered but are not shown here. Analogous reactions are indicated by arrows of the same color. The dark and light brown arrows for QA indicate two different reactions leading to the same product