Doubly Charged Small Organic Fragments Derived from [Ce(tripeptide)(CHCN)] Complexes: Observation of the Elusive [b + H] Ions.

[a + H] ions generated from Ln/tripeptide complexes, where Ln = La or Ce, have similar structures to the linear [a] ions but with protonation at both the terminal NH and N═CH groups. Ion stability is favored by having the basic secondary amine of the proline residue at the N-terminus and by an amino acid residue accommodating one of the protons on the side chain. Dissociation of [a + H] ions derived from peptides containing only aliphatic residues is by cleavage of the second amide bond to give [b] or [a] ions along with internal [a] ions.

Evidence for the Prerequisite Formation of Phenoxyl Radicals in Radical-Mediated Peptide Tyrosine Nitration In Vacuo.

The elementary mechanism of radical-mediated peptide tyrosine nitration, which is a hallmark of post-translational modification of proteins under nitrative stress in vivo, has been elucidated in detail by using an integrated approach that combines the gas-phase synthesis of prototypical molecular tyrosine-containing peptide radical cations, ion-molecule reactions, and isotopic labeling experiments with DFT calculations. This reaction first involves the radical recombination of NO towards the prerequisite phenoxyl radical tautomer of a tyrosine residue, followed by proton rearrangements, finally yielding the stable and regioselective 3-nitrotyrosyl residue product. In contrast, nitration with the π-phenolic radical cation tautomer is inefficient.

Structures and Dissociation Products of Ce/Peptide Complexes: Competition between Coordination and Charge Delocalization.

Structures of [Ce(GGG)] and [Ce(GGG ? H)] have been investigated by DFT calculations. The two lowest-energy structures of the triply charged metal complex have the peptide in either the iminol or conventional zwitterionic form, and these ions have almost identical energies. In the doubly charged complex, the iminol and charge-solvated structures are the best structures on the potential energy surface, but the latter is favored.

Loss of water from protonated polyglycines: interconversion and dissociation of the product imidazolone ions.

Collision-induced dissociation of isotopically labelled protonated pentaglycine produced two abundant [b5]+ ions, the products of the loss of water from the first and second amide groups, labelled [b5]+I and [b5]+II. IRMPD spectroscopy and DFT calculations show that these two [b5]+ ions feature N1-protonated 3,5-dihydro-4H-imidazol-4-one structures. 15N-Labelling established that some interconversion occurs between these two ions but dissociations are preferred.

Interconversion between 4-Imidazolone Ions; Isomers of [b] Derived from Protonated Tetraglycine.

Collision-induced dissociations of isotopically labeled protonated tetraglycines establish that the [b] ion formed by loss of water from the second amide bond (structure II) rearranges to form N-protonated 3,5-dihydro-4H-imidazol-4-one (structure I), the product of water loss from the first amide bond. Structure II is slightly higher in energy than I (ΔH at 0 K is 5.1 kJ mol, as calculated at M06-2X/6-311++G-(d,p)), and the barrier to interconversion is 139.

Isomerization versus dissociation of phenylalanylglycyltryptophan radical cations.

Four isomers of the radical cation of tripeptide phenylalanylglycyltryptophan, in which the initial location of the radical center is well defined, have been isolated and their collision-induced dissociation (CID) spectra examined. These ions, the π-centered [FGW˙], α-carbon- [FG˙W], N-centered [FGW˙] and ζ-carbon- [F˙GW] radical cations, were generated via collision-induced dissociation (CID) of transition metal-ligand-peptide complexes, side chain fragmentation of a π-centered radical cation, homolytic cleavage of a labile nitrogen-nitrogen single bond, and laser induced dissociation of an iodinated peptide, respectively. The π-centered and tryptophan N-centered peptide radical cations produced almost identical CID spectra, despite the different locations of their initial radical sites, which indicated that interconversion between the π-centered and tryptophan N-centered radical cations is facile.

Ground and Excited-Electronic-State Dissociations of Hydrogen-Rich and Hydrogen-Deficient Tyrosine Peptide Cation Radicals.

We report a comprehensive study of collision-induced dissociation (CID) and near-UV photodissociation (UVPD) of a series of tyrosine-containing peptide cation radicals of the hydrogen-rich and hydrogen-deficient types. Stable, long-lived, hydrogen-rich peptide cation radicals, such as [AAAYR + 2H](+●) and several of its sequence and homology variants, were generated by electron transfer dissociation (ETD) of peptide-crown-ether complexes, and their CID-MS(3) dissociations were found to be dramatically different from those upon ETD of the respective peptide dications. All of the hydrogen-rich peptide cation radicals contained major (77%-94%) fractions of species having radical chromophores created by ETD that underwent photodissociation at 355 nm.

Nucleophilic substitution by amide nitrogen in the aromatic rings of [zn - H]˙⁺ ions; the structures of the [b₂ - H - 17]˙⁺ and [c1 - 17]⁺ ions.

Peptide radical cations that contain an aromatic amino acid residue cleave to give [zn - H]˙⁺ ions with [b2 - H - 17]˙⁺ and [c1 - 17](+) ions, the dominant products in the dissociation of [zn - H]˙⁺, also present in lower abundance in the CID spectra. Isotopic labeling in the aromatic ring of [Yπ˙GG](+) establishes that in the formation of [b2 - H - 17]˙⁺ ions a hydrogen from the δ-position of the Y residue is lost, indicating that nucleophilic substitution on the aromatic ring has occurred. A preliminary DFT investigation of nine plausible structures for the [c1 - 17](+) ion derived from [Y(π)˙GG](+) shows that two structures resulting from attack on the aromatic ring by oxygen and nitrogen atoms from the peptide backbone have significantly better energies than other isomers.

Formation, isomerization, and dissociation of ε- and α-carbon-centered tyrosylglycylglycine radical cations.

The fragmentation products of the ε-carbon-centered radical cations [Y(ε)˙LG](+) and [Y(ε)˙GL](+), made by 266 nm laser photolysis of protonated 3-iodotyrosine-containing peptides, are substantially different from those of their π-centered isomers [Y(π)˙LG](+) and [Y(π)˙GL](+), made by dissociative electron transfer from ternary metal-ligand-peptide complexes. For leucine-containing peptides the major pathway for the ε-carbon-centered radical cations is loss of the side chain of the leucine residue forming [YG(α)˙G](+) and [YGG(α)˙](+), whereas for the π-radicals it is the side chain of the tyrosine residue that is lost, giving [G(α)˙LG](+) and [G(α)˙GL](+). The fragmentations of the product ions [YG(α)˙G](+) and [YGG(α)˙](+) are compared with those of the isomeric [Y(ε)˙GG](+) and [Y(π)˙GG](+) ions.

Discovery and mechanistic studies of facile N-terminal Cα-C bond cleavages in the dissociation of tyrosine-containing peptide radical cations.

Fascinating N-terminal Cα-C bond cleavages in a series of nonbasic tyrosine-containing peptide radical cations have been observed under low-energy collision-induced dissociation (CID), leading to the generation of rarely observed x-type radical fragments, with significant abundances. CID experiments of the radical cations of the alanyltyrosylglycine tripeptide and its analogues suggested that the N-terminal Cα-C bond cleavage, yielding its [x2 + H](•+) radical cation, does not involve an N-terminal α-carbon-centered radical. Theoretical examination of a prototypical radical cation of the alanyltyrosine dipeptide, using density functional theory calculations, suggested that direct N-terminal Cα-C bond cleavage could produce an ion-molecule complex formed between the incipient a1(+) and x1(•) fragments.

Laser-induced dissociation of singly protonated peptides at 193 and 266 nm within a hybrid linear ion trap mass spectrometer.

Rationale: We implemented, for the first time, laser-induced dissociation (LID) within a modified hybrid linear ion trap mass spectrometer, QTrap, while preserving the original scanning capabilities and routine performance of the instrument.

Methods: Precursor ions of interest were mass-selected in the first quadrupole (Q1), trapped in the radiofrequency-only quadrupole (q2), photodissociated under irradiation with a 193- or 266-nm laser beam in the third quadrupole (q3), and mass-analyzed using the linear ion trap.

Results: LID of singly charged protonated peptides revealed, in addition to conventional amide-bond cleavages, preferential fragmentation at Cα -C/N-Cα bonds of the backbone as well as at the Cα -Cβ /Cβ -Cγ bonds of the side-chains.

Formation and dissociation of phosphorylated peptide radical cations.

In this study, we generated phosphoserine- and phosphothreonine-containing peptide radical cations through low-energy collision-induced dissociation (CID) of the ternary metal-ligand phosphorylated peptide complexes [Cu(II)(terpy)(p)M](·2+) and [Co(III)(salen)(p)M](·+) [(p)M: phosphorylated angiotensin III derivative; terpy: 2,2':6',2''-terpyridine; salen: N,N'-ethylenebis(salicylideneiminato)]. Subsequent CID of the phosphorylated peptide radical cations ((p)M(·+)) revealed fascinating gas-phase radical chemistry, yielding (1) charge-directed b- and y-type product ions, (2) radical-driven product ions through cleavages of peptide backbones and side chains, and (3) different degrees of formation of [M - H(3)PO(4)](·+) species through phosphate ester bond cleavage. The CID spectra of the (p)M(·+) species and their non-phosphorylated analogues featured fragment ions of similar sequence, suggesting that the phosphoryl group did not play a significant role in the fragmentation of the peptide backbone or side chain.