Dissociation of Valine Cluster Cations

Abstract

Independently of the preparation method, for cluster cations of aliphatic amino acids, the protonated form M_nH⁺ is always the dominant species. This is a surprising fact considering that in the gas phase, they dissociate primarily by the loss of 45 Da, i.e., the loss of the carboxylic group. In the present study, we explore the dissociation dynamics of small valine cluster cations M_n⁺ and their protonated counterparts M_nH⁺ via collision-induced dissociation experiments and ab initio calculations with the aim to elucidate the formation of M_nH⁺-type cations from amino acid clusters. For the first time, we report the preparation of valine cluster cations M_n⁺ in laboratory conditions, using a technique of cluster ion assembly inside He droplets. We show that the M_n⁺ cations cooled down to He droplet temperature can dissociate to form both M_n-1H⁺ and [M_n-COOH]⁺ ions. With increasing internal energy, the M_n-1H⁺ formation channel becomes dominant. M_n-1H⁺ ions then fragment nearly exclusively by monomer loss, describing the high abundance of protonated clusters in the mass spectra of amino acid clusters.

Figure 1

Scheme of the experiment and multistep process of ion assembly in He droplets enabling preparation of cold clusters.

Figure 2

Mass spectra at different evaporation cell conditions and without Ar gas in the collision cell of a Q-TOF instrument. (upper panel) Spectrum at low evaporation cell pressure showing valine and valine cluster ions with attached He from the droplets or water impurities. (bottom panel) All He atoms stripped from the cluster ions and the same fragment ions are observedas reported in previous studies with amino acid doped He droplets.

Figure 3

Relative intensities of individual product ions after CID of valine dimer cations and protonated valine dimers as a function of collision energy in the center of the mass frame.

Figure 4

Relative intensities of individual product ions after CID of valine trimer cations and protonated valine trimers as a function of the collision energy in the center of the mass frame.

Figure 5

(a–d) Dissociation reactions of valine dimers and protonated valine dimer cations. Calculated at the B3LYP+D2/aug-cc-pVDZ level of theory. M06-2X/aug-cc-pVDZ results are included for comparison in the Supporting Information. Color code: carbon - green, oxygen - red, nitrogen - blue, and hydrogen - white.