Abstract
Independently of the preparation method, for cluster cations of aliphatic amino acids, the protonated form MnH+ 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 Mn+ and their protonated counterparts MnH+ via collision-induced dissociation experiments and ab initio calculations with the aim to elucidate the formation of MnH+-type cations from amino acid clusters. For the first time, we report the preparation of valine cluster cations Mn+ in laboratory conditions, using a technique of cluster ion assembly inside He droplets. We show that the Mn+ cations cooled down to He droplet temperature can dissociate to form both Mn-1H+ and [Mn–COOH]+ ions. With increasing internal energy, the Mn-1H+ formation channel becomes dominant. Mn-1H+ ions then fragment nearly exclusively by monomer loss, describing the high abundance of protonated clusters in the mass spectra of amino acid clusters.
Highlights
Understanding of physical, chemical, and biological processes in living organisms often requires exploration of fundamental properties of isolated biomolecular ions or their van der Waals complexes
Valine cluster cations were prepared by ion assembly in He droplets
Amino acid vapor is picked up by charged He droplets, and the charge is transferred from He+ (IE = 24.59 eV)[19] and resp
Summary
Understanding of physical, chemical, and biological processes in living organisms often requires exploration of fundamental properties of isolated biomolecular ions or their van der Waals complexes. A significant improvement was brought by invention of the electrospray technique[1] where ions are formed on the liquid/ aerosol interface at atmospheric pressure and reach sufficient ion intensities for transport to vacuum and further analysis.[2] the control of the ion state in electrospray sources is not trivial and introduces a high level of uncertainty when interpreting the results. Parameters such as solution ions, capillary temperature, collisional activation, and charge transfer processes may result in the formation of excited, isomerized, or metastable ion species far from their electronic or vibrational ground state. Complicated ion transport and trapping systems have to be used to cool the ions for spectroscopy or comparison with theory.[3,4]
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