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

作者：Gholami Ameneh; Fridgen Travis D^*

来源：Physical Chemistry Chemical Physics, 2014, 16(7): 3134-3143.

DOI：10.1039/c3cp53716a

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 H-2 as one of the main dissociation pathways. Furthermore a second dehydrogenation step, forming [Zn(Pro-H)(Pro)-2H(2)](+), 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 H-2, as well as for other fragmentation routes, including the loss of H2O. The hydrogen atoms for H-2 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 H-2 loss and H2O loss routes for both [Zn(Pro-H)(Pro)](+) and [Zn(Gly-H)(Gly)](+) and were compared. For [Zn(Pro-H)(Pro)](+), H-2 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 H-2 loss, compared to those complexes formed with the much less basic glycine or alanine.

• 出版日期2014