The unimolecular chemistry of [Zn(amino acid)2-H]+ in the gas phase: H2 elimination when the amino acid is a secondary amine

Abstract

The unimolecular chemistry of the [Zn(Pro-H)(Pro)]⁺ complex following collisional or infrared multiple photon activation was studied, and interestingly was found to lose H₂ as one of the main dissociation pathways. Furthermore a second dehydrogenation step, forming [Zn(Pro-H)(Pro)-2H₂]⁺, was also observed. When proline was substituted for sarcosine, also a secondary amine, a single dehydrogenation was observed. In contrast, [Zn(Gly-H)(Gly)]⁺ and [Zn(Ala-H)(Ala)]⁺ were found to lose H₂O as their primary fragmentation route with no dehydrogenation observed. Tandem mass spectrometry, deuterium substitution, and infrared spectroscopy were used to determine the origin of the H atoms in the losses of H₂, as well as for other fragmentation routes, including the loss of H₂O. The hydrogen atoms for H₂ loss from [Zn(Pro-H)(Pro)]⁺ was found to originate on the amine group and primarily from C5 on the non-deprotonated proline, with a smaller contribution from the C2 hydrogen. Both hydrogens for H₂O loss were determined to be from labile hydrogens. Potential energy surfaces were computed for the H₂ loss and H₂O loss routes for both [Zn(Pro-H)(Pro)]⁺ and [Zn(Gly-H)(Gly)]⁺ and were compared. For [Zn(Pro-H)(Pro)]⁺, H₂ loss was found to be the pathway with the lower energy requirement than for H₂O loss, and the opposite was found for [Zn(Gly-H)(Gly)]⁺. The greater basicities of proline and sarcosine are most likely responsible for stabilizing the 3 coordinate Zn²⁺ transition states en route to H₂ loss, compared to those complexes formed with the much less basic glycine or alanine.