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.