Low energy fragmentation of protonated glycine. An ab initio theoretical study

Abstract

A common fragmentation of protonated α-amino acids is the loss of 46 u corresponding to the formation of an immonium ion. From protonated glycine, the fragmentation pathways leading to the loss of 46 u were investigated by means of ab initio calculations at various levels of theory: B3LYP/6-31G∗, MP2(FC)/6-31G∗, MP2(FC)/6-311+G(2d,2p)//MP2(FC)/6-31G∗, and MP2(FC)/6-311+G(2d,2p)//MP2(FC)/6-31G∗+ZPVE(MP2(FC)/6-31G∗. Several neutral species may correspond to 46 u: formic acid (HCOOH), carbon dioxide and dihydrogen (CO 2 + H 2) from N-protonated glycine; dihydroxycarbene [C(OH) 2] from the CO-protonated isomer; and water and carbon monoxide (H 2O + CO) from the OH-protonated form. The difference in energy between the N-, CO-, and OH-protonated forms is calculated to be 0, 112, and 122 kJ mol −1 at the highest level of theory. The fragmentation of lowest critical energy is the consecutive loss of water and carbon monoxide that was the mechanism previously admitted. For ions having long lifetimes this reaction is in competition with a loss of CO. This fragmentation and the consecutive losses of H 2O + CO arise through the same determining step that is the isomerization of N-protonated glycine [GlyH +(N)] into OH-protonated glycine located 153 kJ mol −1 higher than GlyH +(N). At high energy, the loss of dihydroxycarbene may occur. Its formation from N-protonated glycine requires 313 kJ mol −1. The fragmentation is preceded by an isomerization of N-protonated glycine into CO-protonated glycine. Elimination of formic acid is ruled out by the present calculations.