Formation of molecular radical cations of enkephalin derivatives via collision-induced dissociation of electrospray-generated copper (II) complex ions of amines and peptides

Structure and fragmentation chemistry of the peptide radical cations of glycylphenylalanylglycine (GFG)

Herein, we explore the generation and characterization of the radical cations of glycylphenylalanylglycine, or [GFG]•+, formed via dissociative electron-transfer reaction from the tripeptide to copper(II) within a ternary complex. A comprehensive investigation employing isotopic labeling, infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations elucidated the details and energetics in formation of the peptide radical cations as well as their dissociation products. Unlike conventional aromatic-containing peptide radical cations that primarily form canonical π-radicals, our findings reveal that 75% of the population of the experimentally produced [GFG]•+ precursors are [GFα•G]+, where the radical resides on the middle α-carbon of the phenylalanyl residue. This unexpected isomeric ion has an enthalpy of 6.8 kcal/mol above the global minimum, which has an N-terminal captodative structure, [Gα•FG]+, comprising 25% of the population. The [b₂-H]•+ product ions are also present in a ratio of 75/25 from [GFα•G]+/ [Gα•FG]+, the results of which are obtained from matches between the IRMPD action spectrum and predicted IR absorption spectra of the [b₂-H]•+ candidate structures, as well as from IRMPD isomer population analyses.

New free radical-initiated peptide sequencing (FRIPS) mass spectrometry reagent with high conjugation efficiency enabling single-step peptide sequencing

A newly designed TEMPO-FRIPS reagent, 4-(2,2,6,6-tetramethylpiperidine-1-oxyl) methyl benzyl succinic acid N-hydroxysuccinimide ester or p-TEMPO–Bn–Sc–NHS, was synthesized to achieve single-step free radical-initiated peptide sequencing mass spectrometry (FRIPS MS) for a number of model peptides, including phosphopeptides. The p-TEMPO–Bn–Sc–NHS reagent was conjugated to target peptides, and the resulting peptides were subjected to collisional activation. The peptide backbone dissociation behaviors of the MS/MS and MS³ experiments were monitored in positive ion mode. Fragment ions were observed even at the single-step thermal activation of the p-TEMPO–Bn–Sc–peptides, showing mainly a-/x- and c-/z-type fragments and neutral loss ions. This confirms that radical-driven peptide backbone dissociations occurred with the p-TEMPO–Bn–Sc–peptides. Compared to the previous version of the TEMPO reagent, i.e., o-TEMPO–Bz–C(O)–NHS, the newly designed p-TEMPO–Bn–Sc–NHS has better conjugation efficiency for the target peptides owing to its improved structural flexibility and solubility in the experimental reagents. An energetic interpretation using the survival fraction as a function of applied normalized collision energy (NCE) ascertained the difference in the thermal activation between p-TEMPO–Bn–Sc– and o-TEMPO–Bz–C(O)– radical initiators. This study clearly demonstrates that the application of the p-TEMPO–Bn–Sc– radical initiator can improve the duty cycle, and this FRIPS MS approach has the potential to be implemented in proteomics studies, including phosphoproteomics.

First Steps to Rationalize Host-Guest Interaction between α-, β-, and γ-Cyclodextrin and Divalent First-Row Transition and Post-transition Metals (Subgroups VIIB, VIIIB, and IIB)

Cyclodextrins (CDs) are cyclic oligosaccharides mainly composed of six, seven, and eight glucose units, so-called α-, β-, and γ-CDs, respectively. They own a very particular molecular structure exhibiting hydrophilic features thanks to primary and secondary rims and delimiting a hydrophobic internal cavity. The latter can encapsulate organic compounds, but the former can form supramolecular complexes by hydrogen-bonding or electrostatic interactions. CDs have been used in catalytic processes to increase mass transfer in aqueous-organic two-phase systems or to prepare catalysts. In the last case, interaction between CDs and metal salts was considered to be a key point in obtaining highly active catalysts. Up to now, no work was reported on the investigation of factors affecting the binding of metal to CD. In the study herein, we present the favorable combination of electrospray ionization coupled to mass spectrometry [ESI-MS(/MS)] and density functional theory molecular modeling [B3LYP/Def2-SV(P)] to delineate some determinants governing the coordination of first-row divalent transition metals (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Fe²⁺) and one post-transition metal (Zn²⁺) with α-, β-, and γ-CDs. A large set of features concerning the metal itself (ionic radius, electron configuration, and spin state) as well as the complexes formed (the most stable conformer, relative abundance in MS, CE₅₀ value in MS/MS, binding energy, effective coordination number, average bond lengths, binding site localization, bond dissociation energies, and natural bond orbital distribution) were screened. Taking into account all of these properties, various selectivity rankings have been delineated, portraying differential association/dissociation behaviors. Nonetheless, unique 3D topologies for each CD-metal complex were emphasized. The combination of these approaches brings a stone for building a compendium of molecular features to serve as a suitable descriptor or predictor for a better first round rationalization of catalytic activities.

Mechanistic examination of Cα -Cβ tyrosyl bond cleavage: Spectroscopic investigation of the generation of α-glycyl radical cations from tyrosyl (glycyl/alanyl)tryptophan

In this study, dissociative one-electron transfer dissociation of [CuII (dien)Y(G/A)W]•2+ [dien = diethylenetriamine; Y(G/A)W = tyrosyl (glycyl/alanyl)tryptophan] was used to generate the tripeptide radical cations [Y(G/A)W]•+ ; subsequent loss of the Tyr side chain formed [Gα • (G/A)W]+ . The π-centered species [YGWπ • ]+ generated the α-centered species [Gα • GW]+ through Cα -Cβ bond cleavage, as revealed using infrared multiple photon dissociation (IRMPD) measurements and density functional theory (DFT) calculations. Comparisons of experimental and theoretical IR spectra confirmed that both the charge and spin densities of [Y(G/A)Wπ • ]+ were delocalized initially at the tryptophan indolyl ring; subsequent formation of the final [Gα • (G/A)W]+ structure gave the highest spin density at the α-carbon atom of the N-terminal glycine residue, with a proton solvated by the first amide oxygen atom. The IRMPD mass spectra and action spectra of the [Gα • (G/A)W]+ species were all distinctly different from those of their isomeric [G(G/A)Wπ • ]+ species. The mechanism of formation of the captodative [Gα • (G/A)W]+ species-with the charge site separated from the radical site-from [Y(G/A)Wπ • ]+ has been elucidated. DFT calculations suggested that the Cα -Cβ bond cleavage of the tyrosine residue in the radical cationic [Y(G/A)Wπ • ]+ precursor involves (a) through-space electron transfer between the indolyl and phenolic groups; (b) formation of proton-bound dimers through Cα -Cβ cleavage of the tyrosine residue; and (c) a concerted proton rearrangement from the phenolic OH group to the carboxyl group and formation of the α-carbon-centered product [Gα • (G/A)W]+ through hydrogen bond cleavage. The barriers for the electron transfer (a), the Cα -Cβ cleavage (b), and the protonation rearrangement (c) were 12.8, 26.5, and 10.3 kcal mol-1 , respectively.

Dissociative electron transfer of copper(ii) complexes of glycyl(glycyl/alanyl)tryptophan in vacuo: IRMPD action spectroscopy provides evidence of transition from zwitterionic to non-zwitterionic peptide structures

We report herein the first detailed study of the mechanism of redox reactions occurring during the gas-phase dissociative electron transfer of prototypical ternary [Cu^II(dien)M]˙²⁺ complexes (M, peptide). The two final products are (i) the oxidized non-zwitterionic π-centered [M]˙⁺ species with both the charge and spin densities delocalized over the indole ring of the tryptophan residue and with a C-terminal COOH group intact, and (ii) the complementary ion [Cu^I(dien)]⁺. Infrared multiple photon dissociation (IRMPD) action spectroscopy and low-energy collision-induced dissociation (CID) experiments, in conjunction with density functional theory (DFT) calculations, revealed the structural details of the mass-isolated precursor and product cations. Our experimental and theoretical results indicate that the doubly positively charged precursor [Cu^II(dien)M]˙²⁺ features electrostatic coordination through the anionic carboxylate end of the zwitterionic M moiety. An additional interaction exists between the indole ring of the tryptophan residue and one of the primary amino groups of the dien ligand; the DFT calculations provided the structures of the precursor ion, intermediates, and products, and enabled us to keep track of the locations of the charge and unpaired electron. The dissociative one-electron transfer reaction is initiated by a gradual transition of the M tripeptide from the zwitterionic form in [Cu^II(dien)M]˙²⁺ to the non-zwitterionic M intermediate, through a cascade of conformational changes and proton transfers. In the next step, the highest energy intermediate is formed; here, the copper center is 5-coordinate with coordination from both the carboxylic acid group and the indole ring. A subsequent switch back to 4-coordination to an intermediate IM1, where attachment to GGW occurs through the indole ring only, creates the structure that ultimately undergoes dissociation.

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 ^. NO₂ 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. This first direct experimental evidence for the elementary steps of the radical-mediated tyrosine nitration mechanism in the gas phase provides a fundamental insight into the regioselectivity of biological tyrosine ortho-nitration.

Free radical-initiated peptide sequencing (FRIPS)-based cross-linkers for improved peptide and protein structure analysis

Free radical-initiated peptide sequencing (FRIPS) has recently been introduced as an analytical strategy to create peptide radical ions in a predictable and effective way by collisional activation of specifically modified peptides ions. FRIPS is based on the unimolecular dissociation of open-shell ions and yields fragments that resemble those obtained by electron capture dissociation (ECD) or electron transfer dissociation (ETD). In this review article, we describe the fundamentals of FRIPS and highlight its fruitful combination with chemical cross-linking/mass spectrometry (MS) as a highly promising option to derive complementary structural information of peptides and proteins. FRIPS does not only yield exhaustive sequence information of cross-linked peptides, but also defines the exact cross-linking sites of the connected peptides. The development of more advanced FRIPS cross-linkers that extend the FRIPS-based cross-linking/MS approach to the study of large protein assemblies and protein interaction networks can be eagerly anticipated.

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_π˙]⁺, α-carbon- [FG_α˙W]⁺, N-centered [FGW_N˙]⁺ and ζ-carbon- [F_ζ˙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. By contrast, the α-carbon-glycyl radical [FG_α˙W]⁺, and ζ-phenyl radical [F_ζ˙GW]⁺, featured different dissociation product ions, suggesting that the interconversions among α-carbon, π-centered (or tryptophan N-centered) and ζ-carbon-radical cations have higher barriers than those to dissociation. Density functional theory calculations have been used to perform systematic mechanistic investigations on the interconversions between these isomers and to study selected fragmentation pathways for these isomeric peptide radical cations. The results showed that the energy barrier for interconversion between [FGW_π˙]⁺ and [FGW_N˙]⁺ is only 31.1 kcal mol^-1, much lower than the barriers to their dissociation (40.3 kcal mol^-1). For the [FGW_π˙]⁺, [FG_α˙W]⁺, and [F_ζ˙GW]⁺, the barriers to interconversion are higher than those to dissociation, suggesting that interconversions among these isomers are not competitive with dissociations. The [z₃ - H]˙⁺ ions isolated from [FG_α˙W]⁺ and [F_ζ˙GW]⁺ show distinctly different fragmentation patterns, indicating that the structures of these ions are different and this result is supported by the DFT calculations.

Analysis of the lithiated leucosceptroids from Leucosceptrum canum to facilitate their identification and differentiation by electrospray ionization tandem mass spectrometry

RATIONALE

Leucosceptroids, isolated from Leucosceptrum canum (L. canum), are sesterterpenoids with novel molecular scaffolds which possess potent antifeedant activities. Their molecular scaffolds comprise a 5,6,5-framework with a great diversity due to different hydroxylation and substituent positions. The biological activities of leucosceptroids are affected by the subtle structural differences. The structural characterization and differentiation of the leucosceptroid isomers are of great importance.

METHODS

Firstly, different kinds of cation adducts of leucosceptroids were examined by adding alkali metal ions to the solution, and the lithiated adducts of leucosceptroid were found to be readily formed and yielded characteristic fragment ions under collision-induced dissociation (CID). Then, a systematic mass spectrometric investigation of the [M + Li](+) ions was carried out to clarify their characteristic fragment pathways by electrospray ionization quadrupole time-of-flight-type tandem mass spectrometry (ESI-QTOF-MS/MS). The proposed fragmentation pathways were confirmed through ion trap ESI-MS(n) (n ≥ 3) spectra. Finally, the proposed MS/MS method was applied to investigate the extracts of L. canum.

RESULTS

A specific fragmentation pathway of the lithiated adduct, which leads to the production of diagnostic ions of leucosceptroids, was observed. This fragmentation is initiated by cleavage of the C-ring and leads to formation of two types of ions by further dissociation. Both pathways could yield characteristic fragment ions, which could be used to define the substituents at the skeletal structure and at the C-ring. The representative characteristic fragmentations of [M + Li](+) ions and the proposed fragmentation pathways were successfully adopted to investigate the L. canum extracts, and a total of eleven leucosceptroids were identified or tentatively characterized.

CONCLUSIONS

The characteristic fragmentation pathways of [M + Li](+) species of leucosceptroid isomers were proposed. Three types of leucosceptroid isomers were successfully differentiated. Eleven leucosceptroids were characterized from L. canum extracts. The fragmentation knowledge will facilitate the analysis of leucosceptroids and other sesterterpenoids in future research. Copyright © 2016 John Wiley & Sons, Ltd.

Radical-driven peptide backbone dissociation tandem mass spectrometry

In recent years, a number of novel tandem mass spectrometry approaches utilizing radical-driven peptide gas-phase fragmentation chemistry have been developed. These approaches show a peptide fragmentation pattern quite different from that of collision-induced dissociation (CID). The peptide fragmentation features of these approaches share some in common with electron capture dissociation (ECD) or electron transfer dissociation (ETD) without the use of sophisticated equipment such as a Fourier-transform mass spectrometer. For example, Siu and coworkers showed that CID of transition metal (ligand)-peptide ternary complexes led to the formation of peptide radical ions through dissociative electron transfer (Chu et al., 2000. J Phys Chem B 104:3393-3397). The subsequent collisional activation of the generated radical ions resulted in a number of characteristic product ions, including a, c, x, z-type fragments and notable side-chain losses. Another example is the free radical initiated peptide sequencing (FRIPS) approach, in which Porter et al. and Beauchamp et al. independently introduced a free radical initiator to the primary amine group of the lysine side chain or N-terminus of peptides (Masterson et al., 2004. J Am Chem Soc 126:720-721; Hodyss et al., 2005 J Am Chem Soc 127: 12436-12437). Photodetachment of gaseous multiply charged peptide anions (Joly et al., 2008. J Am Chem Soc 130:13832-13833) and UV photodissociation of photolabile radical precursors including a C-I bond (Ly & Julian, 2008. J Am Chem Soc 130:351-358; Ly & Julian, 2009. J Am Soc Mass Spectrom 20:1148-1158) also provide another route to generate radical ions. In this review, we provide a brief summary of recent results obtained through the radical-driven peptide backbone dissociation tandem mass spectrometry approach.

Gas-phase tyrosine-to-cysteine radical migration in model systems

Radical migration, both intramolecular and intermolecular, from the tyrosine phenoxyl radical Tyr(O(∙)) to the cysteine radical Cys(S(∙)) in model peptide systems was observed in the gas phase. Ion-molecule reactions (IMRs) between the radical cation of homotyrosine and propyl thiol resulted in a fast hydrogen atom transfer. In addition, radical cations of the peptide LysTyrCys were formed via two different methods, affording regiospecific production of Tyr(O(∙)) or Cys(S(∙)) radicals. Collision-induced dissociation of these isomeric species displayed evidence of radical migration from the oxygen to sulfur, but not for the reverse process. This was supported by theoretical calculations, which showed the Cys(S(∙)) radical slightly lower in energy than the Tyr(O(∙)) isomer. IMRs of the LysTyrCys radical cation with allyl iodide further confirmed these findings. A mechanism for radical migration involving a proton shuttle by the C-terminal carboxylic group is proposed.

Formation of curcumin molecular ion under electrospray ionisation conditions in the presence of metal cations

Electrospray ionisation (ESI) mass spectra obtained for solutions containing curcumin, copper cation and other metal cations, namely Co2+, Ni2+, Mn2+ and Zn2+, have shown an abundant curcumin molecular ion at m/z 368. This ion was not formed for solutions containing curcumin and copper cations or for those containing curcumin and other metal cations. To the best of our knowledge, it is the first example of a system in which copper cations and other metal cations promote formation of organic radical cation under ESI conditions.

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

The CID spectra of [Y^ε˙GG]⁺ and [YGG^α˙]⁺ are identical, showing that interconversion occurs prior to dissociation. For [Y^ε˙GG]⁺, [Y^π˙GG]⁺ and [YG^α˙G]⁺, the dissociation products are all distinctly different, indicating that dissociation occurs more readily than isomerization.

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. The LID spectra of [M+H](+) featured product ions that were very similar to the observed radical-induced fragmentations in the CID spectra of analogous odd-electron radical cations generated through dissociative electron-transfer in metal-ligand-peptide complexes or through laser photolysis of iodopeptides.

CONCLUSIONS

LID of [M+H](+) ions results in fragmentation channels that are comparable with those observed upon the CID of M(•+) ions, with a range of fascinating radical-induced fragmentations.

Fragmentation chemistry of [Met-Gly]•+, [Gly-Met]•+, and [Met-Met]•+ radical cations

Radical cations [Met-Gly](•+), [Gly-Met](•+), and [Met-Met](•+) have been generated through collision-induced dissociation (CID) of [Cu(II)(CH3CN)2(peptide)](•2+) complexes. Their fragmentation patterns and dissociation mechanisms have been studied both experimentally and theoretically using density functional theory at the UB3LYP/6-311++G(d,p) level. The captodative structure, in which the radical is located at the α-carbon of the N-terminal residue and the proton is on the amide oxygen, is the lowest energy structure on each potential energy surface. The canonical structure, with the charge and spin both located on the sulfur, and the distonic ion with the proton on the terminal amino group, and the radical on the α-carbon of the C-terminal residue have similar energies. Interconversion between the canonical structures and the captodative isomers is facile and occurs prior to fragmentation. However, isomerization to produce the distonic structure is energetically less favorable and cannot compete with dissociation except in the case of [Gly-Met](•+). Charge-driven dissociations result in formation of [b(n) - H](•+) and a(1) ions. Radical-driven dissociation leads to the loss of the side chain of methionine as CH3-S-CH=CH2 producing α-glycyl radicals from both [Gly-Met](•+) and [Met-Met](•+). For [Met-Met](•+), loss of the side chain occurs at the C-terminal as shown by both labeling experiments and computations. The product, the distonic ion of [Met-Gly](•+), NH3 (+)CH(CH2CH2SCH3)CONHCH(•)COOH dissociates by loss of CH3S(•). The isomeric distonic ion NH3 (+)CH2CONHC(•)(CH2CH2SCH3)COOH is accessible directly from the canonical [Gly-Met](•+) ion. A fragmentation pathway that characterizes this ion (and the distonic ion of [Met-Met](•+)) is homolytic fission of the Cβ-Cγ bond to lose CH3SCH2(•).

Gas-phase peptide sulfinyl radical ions: formation and unimolecular dissociation

A variety of peptide sulfinyl radical (RSO•) ions with a well-defined radical site at the cysteine side chain were formed at atmospheric pressure (AP), sampled into a mass spectrometer, and investigated via collision-induced dissociation (CID). The radical ion formation was based on AP reactions between oxidative radicals and peptide ions containing single inter-chain disulfide bond or free thiol group generated from nanoelectrospray ionization (nanoESI). The radical induced reactions allowed large flexibility in forming peptide radical ions independent of ion polarity (protonated or deprotonated) or charge state (singly or multiply charged). More than 20 peptide sulfinyl radical ions in either positive or negative ion mode were subjected to low energy collisional activation on a triple-quadrupole/linear ion trap mass spectrometer. The competition between radical- and charge-directed fragmentation pathways was largely affected by the presence of mobile protons. For peptide sulfinyl radical ions with reduced proton mobility (i.e., singly protonated, containing basic amino acid residues), loss of 62 Da (CH(2)SO), a radical-initiated dissociation channel, was dominant. For systems with mobile protons, this channel was suppressed, while charge-directed amide bond cleavages were preferred. The polarity of charge was found to significantly alter the radical-initiated dissociation channels, which might be related to the difference in stability of the product ions in different ion charge polarities.

Dissociation chemistry of hydrogen-deficient radical peptide anions

The fragmentation chemistry of anionic deprotonated hydrogen-deficient radical peptides is investigated. Homolytic photodissociation of carbon-iodine bonds with 266 nm light is used to generate the radical species, which are subsequently subjected to collisional activation to induce further dissociation. The charges do not play a central role in the fragmentation chemistry; hence deprotonated peptides that fragment via radical directed dissociation do so via mechanisms which have been reported previously for protonated peptides. However, charge polarity does influence the overall fragmentation of the peptide. For example, the absence of mobile protons favors radical directed dissociation for singly deprotonated peptides. Similarly, a favorable dissociation mechanism initiated at the N-terminus is more notable for anionic peptides where the N-terminus is not protonated (which inhibits the mechanism). In addition, collisional activation of the anionic peptides containing carbon-iodine bonds leads to homolytic cleavage and generation of the radical species, which is not observed for protonated peptides presumably due to competition from lower energy dissociation channels. Finally, for multiply deprotonated radical peptides, electron detachment becomes a competitive channel both during the initial photoactivation and following subsequent collisional activation of the radical. Possible mechanisms that might account for this novel collision-induced electron detachment are discussed.

Fragmentation of singly, doubly, and triply charged hydrogen deficient peptide radical cations in infrared multiphoton dissociation and electron induced dissociation

Gas phase fragmentation of hydrogen deficient peptide radical cations continues to be an active area of research. While collision induced dissociation (CID) of singly charged species is widely examined, dissociation channels of singly and multiply charged radical cations in infrared multiphoton dissociation (IRMPD) and electron induced dissociation (EID) have not been, so far, investigated. Here, we report on the gas phase dissociation of singly, doubly and triply charged hydrogen deficient peptide radicals, [M + nH]((n+1)+·) (n=0, 1, 2), in MS(3) IRMPD and EID and compare the observed fragmentation pathways to those obtained in MS(3) CID. Backbone fragmentation in MS(3) IRMPD and EID was highly dependent on the charge state of the radical precursor ions, whereas amino acid side chain cleavages were largely independent of the charge state selected for fragmentation. Cleavages at aromatic amino acids, either through side chain loss or backbone fragmentation, were significantly enhanced over other dissociation channels. For singly charged species, the MS(3) IRMPD and EID spectra were mainly governed by radical-driven dissociation. Fragmentation of doubly and triply charged radical cations proceeded through both radical- and charge-driven processes, resulting in the formation of a wide range of backbone product ions including, a-, b-, c-, y-, x-, and z-type. While similarities existed between MS(3) CID, IRMPD, and EID of the same species, several backbone product ions and side chain losses were unique for each activation method. Furthermore, dominant dissociation pathways in each spectrum were dependent on ion activation method, amino acid composition, and charge state selected for fragmentation.

Formation of diclofenac molecular ions as the effect of Cu(2+)-π interaction under electrospray ionization mass spectrometry conditions

Solutions containing diclofenac (M) and a copper salt [CuCl2, Cu(ClO4)2, Cu(NO3)2, CuSO4) were analysed by electrospray ionisation mass spectrometry (ESI-MS). Because of the cation-pi interactions in diclofenac-Cu(II) complexes, the diclofenac molecular ion M+* at m/z 295 was formed. It was found that the solvent composition (methanol versus water/methanol) and counter ion strongly affect the M+* ion formation. Formation pathways of ion M+* are discussed.

Facile identification of phosphorylation sites in peptides by radical directed dissociation

Identification of phosphorylation sites is of interest due to their importance in protein regulation; however, the identification of the exact sites of this modification is not always easily obtained due to the dynamic nature of phosphorylation and the challenges faced during mass spectrometric analysis. Herein we elaborate on our previous communication (Diedrich, J. K.; Julian, R. R. J. Am. Chem. Soc. 2008, 130, 12212-12213) describing a novel technique for assignment of phosphorylation in a site-specific and facile manner. Phosphorylation sites are selectively modified through β elimination followed by Michael addition chemistry to install a photolabile group. Photodissociation with 266 nm light yields homolytic cleavage at the modification site, generating a β radical which is poised to fragment the peptide backbone. Dissociation primarily yields d-type ions at the previously phosphorylated residue, allowing facile identification. Radical directed fragmentation also occurs in smaller abundances at neighboring residues. The mechanisms behind this selective radical fragmentation are presented and the utility is discussed. Fragmentation is shown to be independent of charge state allowing analysis of a wide variety of peptide sequences including peptides with multiple phosphorylation sites. A comparison of this technique is made with collision induced dissociation (CID) and electron capture dissociation (ECD) for representative peptides.

Gas phase photo-formation and vacuum UV photofragmentation spectroscopy of tryptophan and tyrosine radical-containing peptides

Tryptophan (Trp(•)) and tyrosyl (Tyr(•)) radical containing peptides were produced by UV laser-induced electron detachment from a suitable precursor. Vacuum ultraviolet (VUV) action spectra of these radical peptides were recorded with synchrotron radiation in the 4.5-16 eV range, from which fragmentation pathways and yields are measured as a function of the VUV photon energy. An enhancement in photofragmentation yields of radical species by 1 order of magnitude with respect to nonradical peptides is demonstrated here for the first time. Photofragmentation spectra are compared with absorption spectra for model chromophores calculated in the frame of the time-dependent density functional theory (TDDFT). A qualitative agreement in the position of bands in the 6-8 eV region is observed between experimental photofragmentation and calculated absorption spectra. Photofragmentation spectra of peptide radicals can be useful to better assess the complex deactivation pathways that occur following the absorption of a VUV photon in biomolecular radical anions.

Formation of peptide radical ions through dissociative electron transfer in ternary metal-ligand-peptide complexes

The formation and fragmentation of odd-electron ions of peptides and proteins is of interest to applications in biological mass spectrometry. Gas-phase redox chemistry occurring during collision-induced dissociation of ternary metal-ligand-peptide complexes enables the formation of a variety of peptide radicals, including the canonical radical cations, M(+•), radical dications, [M+H](2+•), radical anions, [M-2H](-•) and phosphorylated radical cations. In addition, odd-electron peptide ions with well-defined initial location of the radical site are produced through side-chain losses from the radical ions. Subsequent fragmentation of these species provides information regarding the role of charge and location of the radical site on the competition between radical-induced and proton-driven fragmentation of odd-electron peptide ions. This account summarizes current understanding of the factors that control the efficiency of the intramolecular electron transfer (ET) in ternary metal-ligand-peptide complexes resulting in formation of odd-electron peptide ions. Specifically, we discuss the effect of the metal center, the ligand and the peptide structure on the competition between the ET, proton transfer (PT) and loss of neutral peptide and neutral peptide fragments from the complex. Fundamental studies of the structures, stabilities and the energetics and dynamics of fragmentation of these complexes are also important for detailed molecular-level understanding of photosynthesis and respiration in biological systems.

A comparison of the fragmentation pathways of [Cu(II)(Ma)(Mb)]•2+ complexes where Ma and Mb are peptides containing either a tryptophan or a tyrosine residue

[Cu(II)(M(a))(M(b))](•2+) complexes, where M(a) and M(b) are dipeptides or tripeptides each containing either a tryptophan (W) or tyrosine (Y) residue, have been examined by means of electrospray tandem mass spectrometry. Collision-induced dissociations (CIDs) of complexes containing identical peptides having a tryptophan residue generated abundant radical cations of the peptides; by contrast, for complexes containing peptides having a tyrosine residue, the main fragmentation channel is dissociative proton transfer to give [M(a) + H](+) and [Cu(II)(M(b)-H)](•+). When there are two different peptides in the complex, each containing a tryptophan residue, radical cations are again the major products, with their relative abundances depending on the locations of the tryptophan residue in the peptides. In the CIDs of mixed complexes, where one peptide contains a tryptophan residue and the other a tyrosine residue, the main fragmentation channel is formation of the radical cation of the tryptophan-containing peptide and not proton transfer from the tyrosine-containing peptide to give a protonated peptide.

Peptide fragmentation by corona discharge induced electrochemical ionization

Fundamental studies have greatly improved our understanding of electrospray, including the underlying electrochemical reactions. Generally regarded as disadvantageous, we have recently shown that corona discharge (CD) can be used as an effective method to create a radical cation species [M](+·), thus optimizing the electrochemical reactions that occur on the surface of the stainless steel (SS) electrospray capillary tip. This technique is known as CD initiated electrochemical ionization (CD-ECI). Here, we report on the fundamental studies using CD-ECI to induce analytically useful in-source fragmentation of a range of molecules that complex transition metals. Compounds that have been selectively fragmented using CD-ECI include enolate forming phenylglycine containing peptides, glycopeptides, nucleosides, and phosphopeptides. Collision induced dissociation (CID) or other activation techniques were not necessary for CD-ECI fragmentation. A four step mechanism was proposed: (1) complexation using either Fe in the SS capillary tip material or Cu(II) as an offline complexation reagent; (2) electrochemical oxidation of the complexed metal and thus formation of a radical cation (e.g.; Fe - e(-) → Fe(+·)); (3) radical fragmentation of the complexed compound; (4) electrospray ionization of the fragmented neutrals. Fragmentation patterns resembling b- and y-type ions were observed and allowed the localization of the phosphorylation sites.

Fragmentation of alpha-radical cations of arginine-containing peptides

Fragmentation pathways of peptide radical cations, M(+*), with well-defined initial location of the radical site were explored using collision-induced dissociation (CID) experiments. Peptide radical cations were produced by gas-phase fragmentation of Co(III)(salen)-peptide complexes [salen = N,N'-ethylenebis (salicylideneiminato)]. Subsequent hydrogen abstraction from the beta-carbon of the side-chain followed by C(alpha)-C(beta) bond cleavage results in the loss of a neutral side chain and formation of an alpha-radical cation with the radical site localized on the alpha-carbon of the backbone. Similar CID spectra dominated by radical-driven dissociation products were obtained for a number of arginine-containing alpha-radicals, suggesting that for these systems radical migration precedes fragmentation. In contrast, proton-driven fragmentation dominates CID spectra of alpha-radicals produced via the loss of the arginine side chain. Radical-driven fragmentation of large M(+*) peptide radical cations is dominated by side-chain losses, formation of even-electron a-ions and odd-electron x-ions resulting from C(alpha)-C bond cleavages, formation of odd-electron z-ions, and loss of the N-terminal residue. In contrast, charge-driven fragmentation produces even-electron y-ions and odd-electron b-ions.

Activated-electron photodetachment dissociation for the structural characterization of protein polyanions

Multiply deprotonated anions [M - nH](n-) of large peptide mellitin, ubiquitin, and beta-casein proteins were subjected to laser irradiation at 260 nm in a quadrupole ion trap. For all compounds, the predominant event consecutive to laser irradiation was the detachment of an electron. The subsequent isolation and collisional activation of the oxidized [M - nH]((n-1)-*) resulted in extensive fragmentation of the peptide backbone. For mellitin peptide, nearly a complete series of c(*), z, and a(*), x product ions were observed. Applied to proteins, this technique, coined as activated-electron photodetachment dissociation (activated-EPD), achieved much more extensive sequence coverage than regular collision activated dissociation (CAD) on the even-electron components. Furthermore, the activated-EPD spectrum of beta-casein displayed phosphorylated fragment ions which suggest that the method is able to preserve part of the labile bonds of post-translational modifications. Activated-EPD is, therefore, a promising complementary technique to other dissociation techniques governed by radicals, i.e., electron capture dissociation (ECD), electron transfer dissociation (ETD), and electron detachment dissociation (EDD), for the structural characterization of large peptides and small proteins.

Radical-driven dissociation of odd-electron peptide radical ions produced in 157 nm photodissociation

Odd-electron a + 1 radical ions generated in the 157 nm photodissociation of peptide ions were investigated in an ion trap mass spectrometer. To localize the radical, peptide backbone amide hydrogens were replaced with deuterium. When the resulting radical ions underwent hydrogen elimination, no H/D scrambling was obvious, suggesting that without collisional activation, the radical resides on the terminal alpha-carbon. Upon collisional excitation, odd-electron radical ions dissociate through two favored pathways: the production of a-type ions at aromatic amino acids via homolytic cleavage of backbone C(alpha)-C(O) bonds and side-chain losses at nonaromatic amino acids. When aromatic residues are not present, nonaromatic residues can also lead to a-type ions. In addition to a-type ions, serine and threonine yield c(n-1) and a(n-1) + 1 ions where n denotes the position of the serine or threonine. All of these fragments appear to be directed by the radical and they strongly depend on the amino acid side-chain structure. In addition, thermal fragments are also occasionally observed following cleavage of labile Xxx-Pro bonds and their formation appears to be kinetically competitive with radical migration.

Radical cations of amino acids and peptides: structures and stabilities

Amino acid and peptide radical cations, M*+, are formed by oxidative dissociations of [Cu(auxiliary ligand)(M)]*2+ and [Metal(III)(salen)(M)]+ complexes. The most easily formed radicals contain either an aromatic or basic amino acid residue. Aromatic amino acids have low ionization energies, are easily oxidized and delocalize the charge and spin over the ring systems; basic amino acids facilitate formation of alpha-radicals that have captodative structures in which the charge and spin are formally separated, although feeding back some of the charge onto the amide or carboxyl group adjacent to the radical center through hydrogen bonding enriches the electron-withdrawing properties and is highly stabilizing. DFT calculations located five isomers of His*+ with an alpha-radical with a captodative structure at the global minimum in a deep potential well. An IRMPD spectrum confirmed that this isomer is the experimentally observed "long-lived" isomer. When both charge and spin are on the peptide backbone, as in [GGG]*+, captodative structures have the lowest energies; the barriers to interconversion between the three isomeric alpha-radicals of [GGG]*+ are high as the charge impedes migration of a hydrogen atom. Dissociation of [GGG]*+ is charge-driven. In peptide radical cations containing a basic amino acid residue the charge is sequestered on the side chain and the radical center, either on the backbone or on another side chain, initiates the fragmentation.

Kinetics for tautomerizations and dissociations of triglycine radical cations

Fragmentations of tautomers of the alpha-centered radical triglycine radical cation, [GGG(*)](+), [GG(*)G](+), and [G(*)GG](+), are charge-driven, giving b-type ions; these are processes that are facilitated by a mobile proton, as in the fragmentation of protonated triglycine (Rodriquez, C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012). By contrast, radical centers are less mobile. Two mechanisms have been examined theoretically utilizing density functional theory and Rice-Ramsperger-Kassel-Marcus modeling: (1) a direct hydrogen-atom migration between two alpha-carbons, and (2) a two-step proton migration involving canonical [GGG](*+) as an intermediate. Predictions employing the latter mechanism are in good agreement with results of recent CID experiments (Chu, I. K. et al. J. Am. Chem. Soc. 2008, 130, 7862-7872).

Experimental and computational studies of the macrocyclic effect of an auxiliary ligand on electron and proton transfers within ternary copper(II)-histidine complexes

The dissociation of [Cu(II)(L)His](*2+) complexes [L = diethylenetriamine (dien) or 1,4,7-triazacyclononane (9-aneN(3))] bears a strong resemblance to the previously reported behavior of [Cu(II)(L)GGH](*2+) complexes. We have used low-energy collision-induced dissociation experiments and density functional theory (DFT) calculations at the B3LYP/6-31+G(d) level to study the macrocyclic effect of the auxiliary ligands on the formation of His(*+) from prototypical [Cu(II)(L)His](*2+) systems. DFT revealed that the relative energy barriers of the same electron-transfer (ET) dissociation pathways of [Cu(II)(9-aneN(3))His](*2+) and [Cu(II)(dien)His](*2+) are very similar, with the ET reactions of [Cu(II)(9-aneN(3))His](*2+) leading to the generation of two distinct His(*+) species; in contrast, the proton transfer (PT) dissociation pathways of [Cu(II)(9-aneN(3))His](*2+) and [Cu(II)(dien)His](*2+) differ considerably. The PT reactions of [Cu(II)(9-aneN(3))His](*2+) are associated with substantially higher barriers (>13 kcal/mol) than those of [Cu(II)(dien)His](*2+). Thus, the sterically encumbered auxiliary 9-aneN(3) ligand facilitates ET reactions while moderating PT reactions, allowing the formation of hitherto nonobservable histidine radical cations.

Anion-pi interactions-interactions between benzo-crown ether metal cation complexes and counter ions

The loss of X(.) radical from [M + Cu + X](+) ions (copper reduction) has been studied by the so called in-source fragmentation at higher cone voltage (M = crown ether molecule, X(-) = counter ion, ClO(4)(-), NO(3)(-), Cl(-)). The loss of X(.) has been found to be affected by the presence/lack of aromatic ring poor/rich in electrons. Namely, the loss of X(.) occurs with lower efficiency for the [NO(2)-B15C5 + Cu + X](+) ions than for the [B15C5 + Cu + X](+) ions, where NO(2)-B15C5 = 3-nitro-benzo-15-crown-5, B15C5 = benzo-15-crown-5. A reasonable explanation is that Anion-pi interactions prevent the loss of X(.) from the [NO(2)-B15C5 + Cu + X](+) ions. The presence of the electron-withdrawing NO(2) group causes the aromatic ring to be poor in electrons and thus its enhances its interactions with anions. For the ion containing the aromatic ring enriched in electrons, namely [NH(2)-B15C5 + Cu + ClO(4)](+) where NH(2)-B15C5 = 3-amino-benzo-15-crown-5, the opposite situation has been observed. Because of Anion-pi repulsion the loss of X(.) radical proceeds more readily for [NH(2)-B15C5 + Cu + X](+) than for [B15C5 + Cu + X](+). Iron reduction has also been found to be affected by Anion-pi interactions. Namely, the loss of CH(3)O(.) radical from the ion [B15C5 + Fe + NO(3) + CH(3)O](+) proceeds more readily than from [NO(2)(-)B15C5 + Fe + NO(3) + CH(3)O](+).

Cu(II)-catalyzed reactions in ternary [Cu(AA)(AA - H)]+ complexes (AA = Gly, Ala, Val, Leu, Ile, t-Leu, Phe)

The unimolecular chemistry of [Cu(II)AA(AA - H)](+) complexes, composed of an intact and a deprotonated amino acid (AA) ligand, have been probed in the gas phase by tandem and multistage mass spectrometry in an electrospray ionization quadrupole ion trap mass spectrometer. The amino acids examined include Gly, Ala, Val, Leu, Ile, t-Leu and Phe. Upon collisionally-activated dissociation (CAD), the [Cu(II)AA(AA - H)](+) complexes undergo decarboxylation with simultaneous reduction of Cu(II) to Cu(I); during this process, a radical site is created at the alpha-carbon of the decarboxylated ligand (H(2)N(1) - (*)C(alpha)H - C(beta)H(2) - R; R = side chain substituent). The radical site is able to move along the backbone of the decarboxylated amino acid to form two new radicals (HN(1)(*) - C(alpha)H(2) - C(beta)H(2) - R and H(2)N(1) - C(alpha)H(2) - (*)C(beta)H - R). From the complexes of Gly and t-Leu, only C(alpha) and N(1) radicals can be formed. The whole radical ligand can be lost to form [Cu(I)AA](+) from these three isomeric radicals. Alternatively, further radical induced dissociations can take place along the backbone of the decarboxylated amino acid ligand to yield [Cu(II)AA(AA - 2H - CO(2))](+), [Cu(I)AA((*)NH(2))](+), [Cu(I)AA(HN = C(alpha)H(2))](+), or [Cu(I)AA(H(2)N - C(alpha)H = C(beta)H - R'](+) (R' = partial side chain substituent). The sodiated copper complexes, [Cu(II)(AA - H + Na)(AA - H)](+), show the same fragmentation patterns as their non-sodiated counterparts; sodium ion is retained on the intact amino acid ligand and is not involved in the CAD pathways. The amino groups of both AA units, the carbonyl group of the intact amino acid, and the deprotonated hydroxyl oxygen coordinate Cu(II) in square-planar fashion. Ab initio calculations indicate that the metal ion facilitates hydrogen atom shuttling between the N(1), C(alpha) and C(beta) atoms of the decarboxylated amino acid ligand. The dissociations of the decarboxylated radical ions unveil important insight about the so far largely unknown intrinsic chemistry of alpha-amino acid and peptide radicals, which are implicated as intermediates in numerous pathogenic biological processes.

Formation of organometallic species, [M - H](+) ions and radical cations upon mass spectrometric fragmentation of mercury-crown ether complexes

Mass spectrometric fragmentation pathways of [M + HgClO(4)](+) (M - crown ether molecule), determined by tandem mass spectrometry experiments, are discussed in detail. The decomposition of [M + HgClO(4)](+) proceeds along three fragmentation pathways: formation of [M - H](+) ions, formation of organometallic species, namely [M - H + Hg](+) ions, and formation of radical cations [M](+*). The factors influencing these processes, namely crown ether cavities and the presence of electron withdrawing/electron donor groups, have been discussed.