Gas-phase dissociation pathways of multiply charged peptide clusters

Abstract

Numerous studies of cluster formation and dissociation have been conducted to determine properties of matter in the transition from the condensed phase to the gas phase using materials as diverse as atomic nuclei, noble gasses, metal clusters, and amino acids. Here, electrospray ionization is used to extend the study of cluster dissociation to peptides including leucine enkephalin with 7-19 monomer units and 2-5 protons, and somatostatin with 5 monomer units and 4 protons under conditions where its intramolecular disulfide bond is either oxidized or reduced. Evaporation of neutral monomers and charge separation by cluster fission are the competing dissociation pathways of both peptides. The dominant fission product for all leucine enkephalin clusters studied is a proton-bound dimer, presumably due to the high gas-phase stability of this species. The branching ratio of the fission and evaporation processes for leucine enkephalin clusters appears to be determined by the value of z 2 /n for the cluster where z is the charge and n the number of monomer units in the cluster. Clusters with low and high values of z 2 /n dissociate primarily by evaporation and cluster fission respectively, with a sharp transition between dissociation primarily by evaporation and primarily by fission measured at a z 2 /n value of ~0.5. The dependence of the dissociation pathway of a cluster on z 2 /n is similar to the dissociation of atomic nuclei and multiply charged metal clusters indicating that leucine enkephalin peptide clusters exist in a state that is more disordered, and possibly fluid, rather than highly structured in the dissociative transition state. The branching ratio, but not the dissociation pathway of [somatostatin 5 + 4H] 4+ is altered by the reduction of its internal disulfide bond indicating that monomer conformational flexibility plays a role in peptide cluster dissociation.Bulk properties of matter and the properties of isolated gas-phase atoms and molecules can have radically different characteristics in terms of structure, work function, ion solvation, etc. (for a review see reference [1]). One motivation behind cluster research is that the transitional characteristics between bulk and molecular properties can be discovered by studying intermediate states of matter. 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 ...

Do ionic charges in ESI MS provide useful information on macromolecular structure?

“…Dissociation of such complexes in the gas phase usually proceeds via the so-called asymmetric charge partitioning, where a single polypeptide chain is ejected from a metastable complex and carries a disproportionately high number of charges [13]. An example of such a process is presented in Figure 2, where mild collisional activation of large protein complexes (14-meric molecular chaperon GroEL) in the ESI interface results in ejection of a highly charged monomeric ion from the complex.…”

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

“…In a double resonance experiment [77,78], a frequency corresponding to the cyclotron resonance frequency of an ion suspected to be an intermediate in a consecutive dissociation process is continuously irradiated over the course of a dissociation experiment. The disappearance of lower mass ions confirms that the irradiated ion is an intermediary in the formation of those ions.…”

Further studies on the origins of asymmetric charge partitioning in protein homodimers

“…It has also been demonstrated that the degree of asymmetric charge partitioning for protein homodimers depends on the conformational flexibility of the proteins in the complex, the charge state of the complex, the internal energy deposition, and the composition of the electrospray solutions from which the complexes are formed [56]. For larger clusters consisting of identical subunits, fission was observed to occur with preferential formation of protonated dimers, and with increasing charge state and cluster size, increasing formation of protonated monomer was observed [57]. The extent of fission versus neutral loss for these clusters could be related to that observed for multiply charged metal clusters with one important difference: Differences in conformational flexibility of the monomer sub-units within a cluster results in differences in the branching ratios of the various dissociation processes observed for these multiply protonated biomolecule clusters [57].…”