Reaction competition in the fragmentation of protonated dipeptides

A major low-energy fragmentation reaction of many protonated dipeptides involves cleavage of the amide bond resulting in formation of either the y(1)" ion or the a(1) ion. For a series of protonated dipeptides H-Val-Xxx-OH it is observed that log(y(1)"/a(1)) 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(a(1)/y(1)") gives a poor correlation with the proton affinity or gas-phase basicity of H-Xxx-OH. However, a good limited correlation of log(a(1)/y(1)") with the Taft-Topsom sigma(alpha) 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 (y(1)") 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 a(1) 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.