Ion-molecule reaction is binary collision between ions (cation or anion) with neutral molecules and further result in chemical reaction if there is any. Theoretically, chemical bond dissociation and formation can be observed during the collision due to substantial change in internal energy of the ion complex and further activate various reaction channels. Ion-molecule reaction study is common in atmospheric chemistry, radiation chemistry, etc. Recently, we found that ion-molecule reaction is efficient to study radical isomerization in molecular peptide ion [M]●+ especially in tyrosyl-containing molecular peptide. We setup a customized triple quadrupole linear ion trap mass spectrometer for ion molecule reaction between tyrosyl containing molecular peptide ion with an odd electron neutral molecule NO2 as shown below.
Interestingly, we found the collision is effective to probe the radical isomerization in tyrosyl-containing molecular peptide via radical recombination. We performed density functional theory calculation to characterize the various reaction mechanisms by using Transition State Theory (TST) and further explain the competitive reaction channels that responsible for the product ions that observed in mass spectrometer. Both experimental and theoretical studies concluded that the tyrosine-containing peptide radical cation undergoes radical isomerization upon formation of the π-canonical radical tautomer to produce phenoxy radical tautomer. Unexpectedly, the ion molecule reaction of phenoxy radical with NO2 result in formation of 3-nitrosyl peptide which is an important bio-molecule in biological system.
