Premium
Reaction competition in the fragmentation of protonated dipeptides
Author(s) -
Harrison Alex. G.,
Csizmadia Imre G.,
Tang TingHua,
Tu YaPing
Publication year - 2000
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/1096-9888(200006)35:6<683::aid-jms994>3.0.co;2-d
Subject(s) - chemistry , protonation , proton affinity , amino acid , fragmentation (computing) , stereochemistry , amine gas treating , medicinal chemistry , ion , computational chemistry , organic chemistry , biochemistry , computer science , operating system
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 σ α 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.