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.

Institut für Ionenphysik und Angewandte Physik Universität Innsbruck Technikerstr 25 Innsbruck A 6020 Austria

J Heyrovský Institute of Physical Chemistry v v i The Czech Academy of Sciences Dolejškova 3 Prague 18223 Czech Republic

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