Dissociation of Valine Cluster Cations.

Independently of the preparation method, for cluster cations of aliphatic amino acids, the protonated form M _n H ⁺ 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 _n H ⁺ via collision-induced dissociation experiments and ab initio calculations with the aim to elucidate the formation of M _n H ⁺ -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 -1} H ⁺ and [M _n -COOH] ⁺ ions. With increasing internal energy, the M _{n -1} H ⁺ formation channel becomes dominant. M _{n -1} H ⁺ ions then fragment nearly exclusively by monomer loss, describing the high abundance of protonated clusters in the mass spectra of amino acid clusters.