Fragmentation mechanisms of α-amino acids protonated under electrospray ionization: a collisional activation and ab initio theoretical study

Abstract

The ionic complexes formed by electrospray of methanol/water solutions of all the α amino acids (AA) were studied by collisional activation in a triple quadrupole mass spectrometer. The fragmentation common to all protonated AA, except tryptophan, lysine, and arginine, is the well known sequential loss of H 2O and CO yielding an immonium ion. For GlyH + it is argued that formation of CH 2NH 2 + involves the most stable N-protonated form from which a proton is transferred to the hydroxy group. For the amino acids bearing a functional group on their side chain, formation of the immonium ion is in competition either with the loss of ammonia from the amino terminus or with the loss of a small molecule from the side chain. Extensive ab initio calculations at the MP2/6-31G∗ level have been carried out to determine the various fragmentation pathways of SerH + and CysH +. These calculations are further used to validate an empirical determination of thermochemical data based on experimental heats of formation and Benson increments. Such approximate data are used to interpret the fragmentations of protonated Met, Thr, Asn, Asp, Gln, and Glu. They are in agreement with an initial protonation at the N terminus of these amino acids. On the other hand, side chain protonation is expected to occur for His, Trp, Lys, and Arg. With increasing collision energy, proton transfer to less basic sites X (X = SH, SCH 3, OH, NH 2 … ) can occur. All primary fragmentations start with an elongation of the C– +XH bond. This elongation may be assisted by a cyclisation stabilizing the incoming carbocation. The competitive fragmentations of each protonated amino acid are governed by a combination of enthalpic factors [bond dissociation energies (BDE) of the various C– +XH bonds and the energy of the final states associated with each HX loss] and activation barriers associated with rearrangements.