Search

The gas phase structures of several proteins have been studied by electron transfer dissociation (ETD) with and without prior collisional heating after electrospraying these proteins from native-like solutions into a quadrupole ion trap mass spectrometer. Without prior collisional heating, we find that ETD fragmentation is mostly limited to regions of the protein that are not spanned by the salt bridges known to form in solution. When protein ions are collisionally heated before ETD, new product ions are observed, and in almost all cases, these new ions arise from protein regions that are spanned by the salt bridges. Together these results confirm the existence of salt bridges in protein ions and demonstrate that a sufficient amount energy is required to disrupt these salt bridges in the gas phase. More interestingly, we also show that different salt bridges require different collisional activation voltages to be disrupted, suggesting that they have variable stabilities in the gas phase. These stabilities appear to be influenced by the gas-phase basicities of the involved residues and the presence of nearby charged residues. We also find that higher collisional activation voltages are needed to enable the formation of new product from sites spanned by multiple salt bridges. [ABSTRACT FROM AUTHOR]

The Li+ complexes of the isomeric α-dipeptide radicals H2N—•CH—C(O)—NH—CH2COOH (•GlyGly) and H2N—CH2—C(—O)NH—•CH—COOH (•GlyGly) are formed in the gas phase from the isomeric complexes [PheGly + Li]+ and [GlyPhe + Li]+, respectively, via homolytic cleavage of the corresponding benzyl side chains. The isomers undergo distinctively different reactions upon collisionally activated dissociation (CAD) and, hence, represent unique, non-interconverting species. The investigation of deuterated isotopomers and of dipeptide radicals with Ala residues permits complete elucidation of the dissociation pathways of the radical complexes. The majority of reactions observed are promoted by the radical site, with the location of the unpaired electron playing an important role in the types of reactions taking place. Analogous differences are found for dilithiated complexes of •GlyGly and GlyGly•, in which the COOH termini are derivatized to COO−Li+ salt bridges. Density functional theory calculations confirm that the lithiated and dilithiated α-dipeptide radicals have distonic character; the radical is largely localized on the N- or C-terminal α-C atom and the charge is largely localized on the metal ions. In the most stable conformers, the Li+ ion(s) are bound between the amide carbonyl and C-terminal carbonyl (or carboxylate) groups. Theory predicts a higher thermodynamic stability for the complexes of the N-terminal radical •GlyGly, as reflected by the significantly higher yield, with which these complexes are formed (from their PheGly precursors), compared to the GlyGly• complexes. [Copyright &y& Elsevier]

Abstract: A series of singly charged dilithiated peptides [Pep−H+2Li]+, has been prepared in situ by electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) and their collisionally activated dissociations (CAD) have been examined by quadrupole ion trap or quadrupole/time-of-flight tandem mass spectrometry, respectively. In the [Pep−H+2Li]+ derivatives, an acidic proton of the peptide is replaced by a lithium cation, while the other lithium cation provides the charge. Upon CAD, the Li+ ions become mobile along the backbone of the peptide, inducing fragmentations that produce sequence ions. Density functional theory calculations carried out for the dipeptide complex [PheGly−H+2Li]+ predict the existence of several isomers, including ions with lithium carboxylate and lithium amide structures with one or two salt bridges. Because the relative energies of these isomers are relatively small, all may be populated upon typical CAD experiments. This explains the presence of protonated, singly lithiated as well as dilithiated fragments in the CAD spectra of [Pep−H+2Li]+. In general, the CAD spectra contain almost complete, structurally diagnostic y n **, c n ** and a n * ion series, which allow for definitive sequence determination (the number of * indicates the number of metal ions in the fragments). The formation of C- as well as N-terminal products from [Pep−H+2Li]+ agrees well with both carboxylate and deprotonated amide structures being populated upon CAD, consistent with the theoretical prediction. Besides promoting sequence-specific fragmentations, dilithiation of the precursor ions also enables the elimination of radicals (at low collision energies) to form dilithiated peptide α-backbone radicals. Very similar characteristics are observed for singly charged disodiated peptides. [Copyright &y& Elsevier]

The dissociation mechanisms of various types of cluster ions are elucidated by direct detection and characterization of the neutral products evaporated from the cluster ions upon collisionally activated dissociation. The cluster ions investigated include proton- or sodium ion-bound oligomers of ionic and protic molecules. The experiments are complemented by quantum chemistry methods at the semiempirical PM3 level for insight on the geometries and energetics of the cluster ions and the neutral oligomers evaporated from them. The combined experimental and computational data reveal that the structure of the decomposing cluster ion determines whether complete (neutral) oligomers are eliminated from the cluster ion or whether the cluster ion “shrinks” by sequential monomer losses. An intact oligomer is eliminated only if its components already interact with each other in the cluster ion, for example, through hydrogen bonds or ion pairs (salt bridges). From the cluster ions that allow for such interligand interactions, complete oligomer and consecutive monomer evaporation are competitive, the yield of intact oligomer (dimer, trimer, etc.) loss increasing with the binding energy of oligomer in respect to the separated monomers. Ionic interactions (salt bridges) in the neutral clusters generally bring upon higher binding energies than hydrogen bonding. [Copyright &y& Elsevier]

A novel gas phase rearrangement reaction has been discovered for [M+H]+ ions of arginine-containing dipeptides. In the case of Gly-Arg and Arg-Gly, this leads to identical tandem mass spectra (MS/MS) thereby precluding their sequence assignment. Density functional theory (DFT) calculations and further multistage mass spectrometry experiments suggest a mechanism which involves the formation of salt bridges for Gly-Arg and Arg-Gly which then undergo ring closure followed by ring opening to form a mixed anhydride. Prevention of salt bridge formation switches off this reaction and yields different MS/MS spectra which allow sequence assignment. This can be achieved by preforming CID on the deprotonated dipeptides or their protonated methyl esters. [Copyright &y& Elsevier]