Reaction competition in the fragmentation of protonated dipeptides

Abstract

A major low-energy fragmentation reaction of many protonated dipeptides involves cleavage of the amide bond resulting in formation of either the y1″ ion or the a1 ion. For a series of protonated dipeptides H—Val—Xxx—OH it is observed that log(y1″/a1) is a linear function of the proton affinity of the variable C-terminal amino acid. For the series of protonated dipeptides H—Xxx—Phe—OH log(a1/y1″) gives a poor correlation with the proton affinity or gas-phase basicity of H—Xxx—OH. However, a good limited correlation of log(a1/y1″) with the Taft–Topsom σα for the alkyl group is observed when Xxx is an aliphatic amino acid. It is proposed that fragmentation occurs by initial formation of a proton-bound complex of an aziridinone and an amino acid which may fragment to form either a protonated amino acid (y1″) or an N-protonated aziridinone with the corresponding neutrals being an aziridinone and an amino acid. Ab initio calculations show that the N-protonated aziridinone is unstable and fragments by loss of CO to form the a1 immonium ion. However, the proton-bound complex of an aziridinone and an amine base is a stable species which exists in a potential well. Copyright © 2000 John Wiley & Sons, Ltd.