Nanocrystalline Aggregation of Serine Detected by Electrospray Ionization Mass Spectrometry:  Origin of the Stable Homochiral Gas-Phase Serine Octamer

Abstract

Portions published previously in: Julian R. R.; Hodyss R.; Kinnear B.; Jarrold M. F.;Beauchamp J. L.

Gas-phase dissociation pathways of multiply charged peptide clusters

“…Some observations have been made of amino acid and peptide clusters formed by ESI of the type [M m + nH] n+ where m > n [18,20,[22][23][24][25][26][27][28]40,47]. Much of this work has focused on octamers of the amino acid serine that have been found to form with a preference for homochirality [20,22,[25][26][27]48]).…”

“…Investigators have explored a wide variety of materials, including charged droplets of various liquids [2,3], clusters of noble metals [4][5][6][7][8], metal-ligand clusters [1,[9][10][11], highly charged clusters of noble gasses [12,13], small molecules [14,15], and ultracold clusters of transition metals [16]. Recently, investigators have formed noncovalently bound clusters of biologically important molecules, including clusters of amino acids and small peptides [17][18][19][20][21][22][23][24][25][26][27][28], and have studied their dissociation processes [18,20,[26][27][28]. Multiply charged "clusters" of proteins consisting of multimeric, noncovalently associated protein complexes with a high degree of solution-phase structural specificity have been liberated into the gas phase [29][30][31][32][33][34][35].…”

“…These ions are indistinguishable in low resolution mass spectra from the monomer and larger clusters and could potentially complicate the interpretation of dissociation spectra [17,19,21,23,46]. While the formation of such highly charged molecular clusters by ESI is interesting in its own right and indicative of the gentle nature of the ESI process, the elucidation of dissociation pathways for clusters of the type [M n + nH] n+ is more complicated as the fragments are likely to be at the same m/z as the parent ion.Some observations have been made of amino acid and peptide clusters formed by ESI of the type [M m + nH] n+ where m > n [18,20,[22][23][24][25][26][27][28]40,47]. Much of this work has focused on octamers of the amino acid serine that have been found to form with a preference for homochirality [20,22,[25][26][27]48]).…”

“…The analogy between these clusters and liquid drops is justified by the cluster's approximately constant densities and fixed boundaries [6]. Clusters of the amino acids arginine [18] and serine [20,27] have also been measured to undergo dissociation by neutral evaporation and charge separation by fission, although the liquid-drop model was not applied to these clusters.The dissociation processes of multiply charged noble metal clusters [5][6][7][8]59] is empirically similar to the dissociation of LE. In noble metal cluster dissociation, evaporation of a neutral metal atom competes with asymmetric fission, which is typically the ejection of a positively charged trimer or other small species.…”

“…Much of this work has focused on octamers of the amino acid serine that have been found to form with a preference for homochirality [20,22,[25][26][27]48]). Clusters of serine as large as [44Ser + 4H] 4+ were isolated by Beauchamp and coworkers and collisionally activated in an ion trap mass spectrometer [20]. The cluster dissociation revealed that the large serine clusters dissociate by a combination of fission and evaporation, but the specific dissociation pathways are difficult to determine because the primary fragment ions of noncovalently bound complexes are often sufficiently activated to undergo subsequent dissociation.…”