Formation of peptide radical cations (m+·) in electron capture dissociation of peptides adducted with group IIB metal ions

Dissociation of Mannose-Rich Glycans Using Collision-Based and Electron-Based Ion Activation Methods

Three dissociation methods, including collision-induced dissociation (CID), electron capture dissociation (ECD), and electronic excitation dissociation (EED), were evaluated for the dissociation of doubly charged glycans using sodium or magnesium ions as charge carriers. CID produced mainly glycosidic cleavages, although more cross-ring fragment ions could be obtained at higher intensities when magnesium ions were used as charge carriers [M + Mg]²⁺. The ^0,2A₃, ^0,3A₃, and ^0,4A₃ ions provided structural information on the 3 → 1 and 6 → 1 linkages of the mannoses. Some internal fragment ions, such as ^2,4A₅_Y_3β, were also produced in high abundance, thus providing additional information on the glycan structure. ECD produced limited fragments compared to other dissociation methods when either of the metal ions were used as charge carriers. Cross-ring fragments were obtained in relatively high abundance, with the charge mainly retained on the nonreducing end. EED produced extensive glycosidic and cross-ring cleavages when either metal charge carrier was used. A higher fragmentation efficiency was achieved and more structural-specific fragments were produced when Na⁺ was used as the charge carrier. Of the 31 possible cross-ring cleavages, including 0,2-, 0,4-, 1,5-, 2,4-, and 3,5-cleavages, 25 were found, thus providing extensive linkage information. A wide range of fragment ions could be obtained in all dissociation methods when Mg²⁺ was used as the charge carrier. Two specific analytical approaches were found to produce extensively structural-specific information on the glycans studied, namely CID of magnesiated glycans and EED of sodiated glycans. These two methods were selected to further analyze the larger mannose-rich glycans Man₆GlcNAc₂ and Man₈GlcNAc₂ and generated extensive structural information.

Electron transfer dissociation mass spectrometry of acidic phosphorylated peptides cationized with trivalent praseodymium

The lanthanide ion praseodymium, Pr(III), was employed to study metallated ion formation and electron transfer dissociation (ETD) of 27 biological and model highly acidic phosphopeptides. All phosphopeptides investigated form metallated ions by electrospray ionization (ESI) that can be studied by ETD to yield abundant sequence information. The ions formed are [M + Pr - H]²⁺ , [M + Pr]³⁺ , and [M + Pr + H]⁴⁺ . All biological phosphopeptides with a chain length of seven or more residues generate [M + Pr]³⁺ . For biological phosphopeptides, [M + Pr]³⁺ undergoes more backbone cleavage by ETD than [M + Pr - H]²⁺ and, in some cases, full sequence coverage occurs. Acidic model phosphorylated hexa-peptides and octa-peptides, composed of alanine residues and one phosphorylated residue, form exclusively [M + Pr - H]²⁺ by ESI. Limited sequence information is obtained by ETD of [M + Pr - H]²⁺ with only metallated product ions being generated. For two biological phosphopeptides, [M + Pr + H]⁴⁺ is observed and may be due to the presence of at least one residue with a highly basic side chain that facilitates the addition of an extra proton. For the model phosphopeptides, more sequence coverage occurs when the phosphorylated residue is in the middle of the sequence than at either the N- or C-terminus. ETD of the metallated precursor ions formed by ESI generates exclusively metallated and nonmetallated c- and z-ions for the biological phosphopeptides, while metallated c-ions, z-ions, and a few y-ions form for the model phosphopeptides. Most of the product ions contain the phosphorylated residue indicating that the metal ion binds predominantly at the deprotonated phosphate group. The results of this study indicate that ETD is a promising tool for sequencing highly acidic phosphorylated peptides by metal adduction with Pr (III) and, by extension, all nonradioactive lanthanide metal ions.

Influence of the metals and ligands in dinuclear complexes on phosphopeptide sequencing by electron-transfer dissociation tandem mass spectrometry

Phosphorylation is one of the most important protein modifications, and electron-transfer dissociation tandem mass spectrometry (ETD-MS/MS) is a potentially useful method for the sequencing of phosphopeptides, including determination of the phosphorylation site. Notably, ETD-MS/MS typically provides useful information when the precursor contains more than three positive charges. It is not yet used as an analysis method for large-scale phosphopeptide production due to difficulties occurring in the production of acidic phosphopeptides having more than three positive charges. To increase the charge state of phosphopeptides, we used dinuclear metal complexes, which selectively bind to the phosphate group in phosphopeptides with the addition of positive charge(s). Dinuclear copper, zinc, and gallium complexes were tested and it was found that the type of metal present in the complex strongly affected the affinity of the phosphorylated compounds and their ETD fragmentation. The dinuclear copper complex interacted weakly with the phosphate groups and ETD-induced peptide fragmentation was largely suppressed by the presence of Cu²⁺, which worked as an electron trap. The dinuclear gallium complex was strongly bound to a phosphate group. However, the ligand binding to gallium acted as an electron trap and the presence of dinuclear gallium complex in the precursor for ETD-MS/MS hampered the sequencing of the phosphopeptides, as in the case of dinuclear copper complexes. In contrast, dinuclear zinc complexes efficiently bind to phosphopeptides with an increase in the charge state, facilitating phosphopeptide sequencing by ETD-MS/MS. The fragmentation of the ligand and peptide backbone in the dinuclear zinc-phosphopeptide complex were competitively induced by ETD. These processes are influenced by the ligand structure and so the detailed ETD fragmentation pathways were investigated using density functional theory calculations.

A Database of Transition-Metal-Coordinated Peptide Cross-Sections: Selective Interaction with Specific Amino Acid Residues

Ion mobility mass spectrometry (IMS-MS) techniques were used to generate a database of 2288 collision cross sections of transition-metal-coordinated tryptic peptide ions. This database consists of cross sections for 1253 [Pep + X]²⁺ and 1035 [Pep + X + H]³⁺, where X²⁺ corresponds to Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺. This number of measurements enables the extraction of structural trends for transition-metal-coordinated peptide ions. The range of structures and changes in collision cross sections for X²⁺-coordinated species (compared with protonated species of the same charge state) is similar to Mg²⁺-coordinated species. This suggests that the structures are largely determined by similarities in cation size with differences among the cross section distributions presumably caused by X²⁺ interactions with specific functional groups offered by the residue R-groups or the peptide backbone. Cross section contributions for individual residues upon X²⁺ solvation are assessed with the derivation of intrinsic size parameters (ISPs). The comparison of the [Pep + X]²⁺ ISPs with those previously reported for [Pep + Mg]²⁺ ions displays a lower contribution to the cross section for His, carboxyamidomethylated Cys, and Met, and is consistent with specific metal-residue interactions identified within protein X-ray crystallography databases. Graphical Abstract ᅟ.

Effects of acidic peptide size and sequence on trivalent praseodymium adduction and electron transfer dissociation mass spectrometry

Using the lanthanide ion praseodymium, Pr(III), metallated ion formation and electron transfer dissociation (ETD) were studied for 25 biological and model acidic peptides. For chain lengths of seven or more residues, even highly acidic peptides that can be difficult to protonate by electrospray ionization will metallate and undergo abundant ETD fragmentation. Peptides composed of predominantly acidic residues form only the deprotonated ion, [M + Pr - H]2+ ; this ion yields near complete ETD sequence coverage for larger peptides. Peptides with a mixture of acidic and neutral residues generate [M + Pr]3+ , which cleaves between every residue for many peptides. Acidic peptides that contain at least one residue with a basic side chain also produce the protonated ion, [M + Pr + H]4+ ; this ion undergoes the most extensive sequence coverage by ETD. Primarily metallated and non-metallated c- and z-ions form for all peptides investigated. Metal adducted product ions are only present when at least half of the peptide sequence can be incorporated into the ion; this suggests that the metal ion simultaneously attaches to more than one acidic site. The only site consistently lacking dissociation is at the N-terminal side of a proline residue. Increasing peptide chain length generates more backbone cleavage for metal-peptide complexes with the same charge state. For acidic peptides with the same length, increasing the precursor ion charge state from 2+ to 3+ also leads to more cleavage. The results of this study indicate that highly acidic peptides can be sequenced by ETD of complexes formed with Pr(III). Copyright © 2017 John Wiley & Sons, Ltd.

The Effects of Trivalent Lanthanide Cationization on the Electron Transfer Dissociation of Acidic Fibrinopeptide B and its Analogs

Electrospray ionization (ESI) on mixtures of acidic fibrinopeptide B and two peptide analogs with trivalent lanthanide salts generates [M + Met + H](4+), [M + Met](3+), and [M + Met -H](2+), where M = peptide and Met = metal (except radioactive promethium). These ions undergo extensive and highly efficient electron transfer dissociation (ETD) to form metallated and non-metallated c- and z-ions. All metal adducted product ions contain at least two acidic sites, which suggest attachment of the lanthanide cation at the side chains of one or more acidic residues. The three peptides undergo similar fragmentation. ETD on [M + Met + H](4+) leads to cleavage at every residue; the presence of both a metal ion and an extra proton is very effective in promoting sequence-informative fragmentation. Backbone dissociation of [M + Met](3+) is also extensive, although cleavage does not always occur between adjacent glutamic acid residues. For [M + Met - H ](2+), a more limited range of product ions form. All lanthanide metal peptide complexes display similar fragmentation except for europium (Eu). ETD on [M + Eu - H](2+) and [M + Eu](3+) yields a limited amount of peptide backbone cleavage; however, [M + Eu + H](4+) dissociates extensively with cleavage at every residue. With the exception of the results for Eu(III), metallated peptide ion formation by ESI, ETD fragmentation efficiencies, and product ion formation are unaffected by the identity of the lanthanide cation. Adduction with trivalent lanthanide metal ions is a promising tool for sequence analysis of acidic peptides by ETD. Graphical Abstract ᅟ.

Dissociation of trivalent metal ion (Al(3+), Ga(3+), In(3+) and Rh(3+))--peptide complexes under electron capture dissociation conditions

RATIONALE

The electron capture dissociation (ECD) of proteins/peptides is affected by the nature of charge carrier. It has been reported that transition metal ions could tune the ECD pathway of peptides. To further explore the charge carrier effect of metal ions, ECD of peptides adducted with trivalent transition metal ions, including group IIIB (Al(3+), Ga(3+), and In(3+) ) and Rh(3+), were investigated and compared with that of the lanthanide ion (Ln(3+)).

METHODS

Bradykinin-derived peptides were used as model peptides to probe the dissociation pathways. The ECD experiments were performed on a Bruker APEX III 4.7T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer.

RESULTS

Typical c-/z-ions with and without metal ions were observed in the ECD of peptides adducted with Group IIIB metal ions as charge carriers. Connection of non-metalated c-ions and metalated z-ions at the position of the serine residue indicated that serine is one of the binding sites of the metal ion on the model peptides. Typical slow heating ions, including metalated a-/b-ions and non-metalated y-ions, were generated in ECD of Rh(3+) -adducted peptides.

CONCLUSIONS

Based on the experimental results, it is proposed that (i) for Group IIIB metal ion-peptide complexes, the incoming electron is captured by the proton in the salt-bridge structures of precursor ions; (ii) for Rh(3+) -peptide complexes, the incoming electron is captured by the metal ion due to the formation of charge-solvated precursor ions formed through arginine residue-metal coordination. Our results indicate that the heterogeneity of precursor ions plays an important role for the ECD of metalated peptides.

N-Cα Bond Cleavage of Zinc-Polyhistidine Complexes in Electron Transfer Dissociation Mediated by Zwitterion Formation: Experimental Evidence and Theoretical Analysis of the Utah-Washington Model

Electron capture dissociation (ECD) and electron transfer dissociation (ETD) of gas-phase ions are widely used for peptide/protein sequencing by mass spectrometry. To understand the general mechanism of ECD/ETD of peptides, we focused on the ETD fragmentation of metal-peptide complexes in the absence of remote protons. Since Zn(2+) strongly binds to neutral histidine residues in peptides, Zn(2+)-polyhistidine complexation does not generate any remote protons. However, in the absence of remote protons, electron transfer to the Zn(2+)-polyhistidine complex induced the N-Cα bond cleavage. The formation pathway for the ETD products was investigated by density functional theory calculations. The calculations showed that the charge-reduced zinc-peptide radical, [M + Zn](•+), can exist in the low-energy zwitterionic amide π* states, which underwent homolytic N-Cα bond dissociation. The homolytic cleavage resulted in the donation of an electron from the N-Cα bond to the nitrogen atom, producing an iminoenol c' anion. The counterpart z(•) radical contained a radical site on the α-carbon atom. The iminoenol c' anion then abstracted a proton to presumably form the more stable amide c' fragment. The current experimental and computational joint study strongly suggested that the N-Cα bond cleavage occurred through the aminoketyl radical-anion formation for Zn(2+)-polyhistidine complexes in ETD.

Effect of structural parameters on the electron capture dissociation and collision-induced dissociation pathways of copper(II)-peptide complexes

The gas-phase dissociation pathways of proteins/peptides are usually affected by the nature of the charge carrier and the sequence of amino acid residues. The effects of peptide structural parameters, including peptide composition, chain length and amide hydrogen, on the gas-phase dissociation of Cu(II)-model peptide complexes were explored in this study. Polyglycine peptides with flexible frames were used as probes to reduce the complexity of the system and illustrate the mechanism. Results revealed that the types of fragment ions generated in the electron capture dissociation (ECD) of Cu(II)-adducted peptides changed according to the basic amino acid residue composition. Charged or neutral tryptophan side-chain losses were observed in the collision-induced dissociation (CID) of Cu(II)-peptide complexes. Internal electron transfer between tryptophan and metal ion within the complex occurred during the CID reaction, leaving the charge-reduced Cu(+) as a closed d-shell stable electron configuration. The choice of the reaction channel was then determined by the gas-phase basicity of the peptide. Amide hydrogen was critical in the formation of metalated b-/y-ions in the ECD process as determined through mutation of the backbone amide group. Increasing the chain length suppressed the ECD of Cu-metalated peptide species. Our results indicate that the structural parameters of peptides play important roles in the gas-phase dissociation processes of Cu-peptide complexes.

Combining gas phase electron capture and IRMPD action spectroscopy to probe the electronic structure of a metastable reduced organometallic complex containing a non-innocent ligand

Gas-phase reduction of a Zn(ii) complex followed by IR spectroscopy shows that the incoming electron is localized on the metal rather than on the ligand.

Electron transfer dissociation mass spectrometry of peptides containing free cysteine using group XII metals as a charge carrier

Electron transfer dissociation (ETD) has been used for peptide sequencing. Since ETD preferentially produces the c'/z(•) fragment pair, peptide sequencing is generally performed by interpretation of mass differences between series of consecutive c' and z(•) ions. However, the presence of free cysteine residues in a precursor promotes peptide bond cleavage, hindering interpretation of the ETD spectrum. In the present study, the divalent group XII metals, such as Zn(2+), Cd(2+) and Hg(2+), were used as charge carriers to produce metal-peptide complexes. The thiol group is deprotonated by complexation with the group XII metal. The formation of b and y' ions was successfully suppressed by using the zinc-peptide complex as a precursor, indicating Zn(2+)-aided ETD to be a useful method for sequencing of cysteine-containing peptides. By contrast, ETD of Cd(2+)- and Hg(2+)-peptide complexes mainly led to SH2 loss and radical cation formation, respectively. These processes were mediated by recombination energy between the metal cation and an electron. The presence of monovalent cadmium and neutral mercury in ETD products was confirmed by MS(3) analysis with collision-induced dissociation.

Mechanistic study on electron capture dissociation of the oligosaccharide-Mg²⁺ complex

Electron capture dissociation (ECD) has shown great potential in structural characterization of glycans. However, our current understanding of the glycan ECD process is inadequate for accurate interpretation of the complex glycan ECD spectra. Here, we present the first comprehensive theoretical investigation on the ECD fragmentation behavior of metal-adducted glycans, using the cellobiose-Mg²⁺ complex as the model system. Molecular dynamics simulation was carried out to determine the typical glycan-Mg²⁺ binding patterns and the lowest-energy conformer identified was used as the initial geometry for density functional theory-based theoretical modeling. It was found that the electron is preferentially captured by Mg²⁺ and the resultant Mg⁺• can abstract a hydroxyl group from the glycan moiety to form a carbon radical. Subsequent radical migration and α-cleavage(s) result in the formation of a variety of product ions. The proposed hydroxyl abstraction mechanism correlates well with the major features in the ECD spectrum of the Mg²⁺-adducted cellohexaose. The mechanism presented here also predicts the presence of secondary, radical-induced fragmentation pathways. These secondary fragment ions could be misinterpreted, leading to erroneous structural determination. The present study highlights an urgent need for continuing investigation of the glycan ECD mechanism, which is imperative for successful development of bioinformatics tools that can take advantage of the rich structural information provided by ECD of metal-adducted glycans.

Influence of metal-peptide complexation on fragmentation and inter-fragment hydrogen migration in electron transfer dissociation

The use of metal salts in electrospray ionization (ESI) of peptides increases the charge state of peptide ions, facilitating electron transfer dissociation (ETD) in tandem mass spectrometry. In the present study, K(+) and Ca(2+) were used as charge carriers to form multiply-charged metal-peptide complexes. ETD of the potassium- or calcium-peptide complex was initiated by transfer of an electron to a proton remote from the metal cation, and a c'-z• fragment complex, in which the c' and z• fragments were linked together via a metal cation coordinating with several amino acid residues, was formed. The presence of a metal cation in the precursor for ETD increased the lifetime of the c'-z• fragment complex, eventually generating c• and z' fragments through inter-fragment hydrogen migration. The degree of hydrogen migration was dependent on the location of the metal cation in the metal-peptide complex, but was not reconciled with conformation of the precursor ion obtained by molecular mechanics simulation. In contrast, the location of the metal cation in the intermediate suggested by the ETD spectrum was in agreement with the conformation of "proton-removed" precursors, indicating that the charge reduction of precursor ions by ETD induces conformational rearrangement during the fragmentation process.

Insights into the binding sites of organometallic ruthenium anticancer compounds on peptides using ultra-high resolution mass spectrometry

The binding sites of two ruthenium(II) organometallic complexes of the form [(η(6)-arene)Ru(N,N)Cl](+), where arene/N,N = biphenyl (bip)/bipyridine (bipy) for complex AH076, and biphenyl (bip)/o-phenylenediamine (o-pda) for complex AH078, on the peptides angiotensin and bombesin have been investigated using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Fragmentation was performed using collisionally activated dissociation (CAD), with, in some cases, additional data being provided by electron capture dissociation (ECD). The primary binding sites were identified as methionine and histidine, with further coordination to phenylalanine, potentially through a π-stacking interaction, which has been observed here for the first time. This initial peptide study was expanded to investigate protein binding through reaction with insulin, on which the binding sites proposed are histidine, glutamic acid, and tyrosine. Further reaction of the ruthenium complexes with the oxidized B chain of insulin, in which two cysteine residues are oxidized to cysteine sulfonic acid (Cys-SO3H), and glutathione, which had been oxidized with hydrogen peroxide to convert the cysteine to cysteine sulfonic acid, provided further support for histidine and glutamic acid binding, respectively.

Electron capture dissociation of trivalent metal ion-peptide complexes

With electrospray ionization from aqueous solutions, trivalent metal ions readily adduct to small peptides resulting in formation of predominantly (peptide + M(T) - H)(2+), where M(T) = La, Tm, Lu, Sm, Ho, Yb, Pm, Tb, or Eu, for peptides with molecular weights below ~1000 Da, and predominantly (peptide + M(T))(3+) for larger peptides. ECD of (peptide + M(T) - H)(2+) results in extensive fragmentation from which nearly complete sequence information can be obtained, even for peptides for which only singly protonated ions are formed in the absence of the metal ions. ECD of these doubly charged complexes containing M(T) results in significantly higher electron capture efficiency and sequence coverage than peptide-divalent metal ion complexes that have the same net charge. Formation of salt-bridge structures in which the metal ion coordinates to a carboxylate group are favored even for (peptide + M(T))(3+). ECD of these latter complexes for large peptides results in electron capture by the protonation site located remotely from the metal ion and predominantly c/z fragments for all metals, except Eu(3+), which undergoes a one electron reduction and only loss of small neutral molecules and b/y fragments are formed. These results indicate that solvation of the metal ion in these complexes is extensive, which results in the electrochemical properties of these metal ions being similar in both the peptide environment and in bulk water.

Reaction pathways of Sc+ (3D, 1D) and Fe+ (6D, 4F) with acetone in the gas phase: metal ion oxidation and acetone deethanization

The reactions of Sc(+) ((3)D, (1)D) and Fe(+) ((6)D, (4)F) with acetone have been investigated in both high- and low-spin states using density functional theory. Our calculations have indicated that oxidation of Sc(+) by acetone can take place by (1) metal-mediated H migration, (2) direct methyl-H shift and/or (3) C=O insertion. The most energetically favorable pathway is metal-mediated H migration followed by intramolecular ScO(+) rotation and dissociation. For the deethanization of acetone mediated by Fe(+), the reaction occurs on either the quartet or sextet surfaces through five elementary steps, i.e. encounter complexation, C-C bond activation, methyl migration, C-C coupling and non-reactive dissociation. The rate-determining step along the quartet-state potential-energy surface (PES) is similar to that in the case of Ni(+) ((2)F, 3d(9)), namely the methyl-migration step. For the sextet-state PES, however, the energy barrier for methyl migration is lower than that for C-C bond activation, and the rate-determining step is C-C coupling. In general, the low-spin-state pathways are lower in energy than the high-spin-state pathways; therefore, the reaction pathways for the oxidation of Sc(+) and the Fe(+)-mediated deethanization of acetone mostly involve the low-spin states.

Coordination sphere tuning of the electron transfer dissociation behavior of Cu(II)-peptide complexes

In contrast to previous electron capture dissociation (ECD) studies, we find that electron transfer dissociation (ETD) of Cu(II)-peptide complexes can generate c- and z-type product ions when the peptide has a sufficient number of strongly coordinating residues. Double-resonance experiments, ion-molecule reactions, and collision-induced dissociation (CID) prove that the c and z product ions are formed via typical radical pathways without the associated reduction of Cu(II), despite the high second ionization energy of Cu. A positive correlation between the number of Cu(II) binding groups in the peptide sequence and the extent of c and z ion formation was also observed. This trend is rationalized by considering that the recombination energy of Cu(II) can be lowered by strong binding ligands to an extent that enables electron transfer to non-Cu sites (e.g., protonation sites) to compete with Cu(II) reduction, thereby generating c/z ions in a manner similar to that observed for protonated (i.e., nonmetalated) peptides.

Transition metal ions: charge carriers that mediate the electron capture dissociation pathways of peptides

Electron capture dissociation (ECD) of model peptides adducted with first row divalent transition metal ions, including Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+), were investigated. Model peptides with general sequence of ZGGGXGGGZ were used as probes to unveil the ECD mechanism of metalated peptides, where X is either V or W; and Z is either R or N. Peptides metalated with different divalent transition metal ions were found to generate different ECD tandem mass spectra. ECD spectra of peptides metalated by Mn(2+) and Zn(2+) were similar to those generated by ECD of peptides adducted with alkaline earth metal ions. Series of c-/z-type fragment ions with and without metal ions were observed. ECD of Fe(2+), Co(2+), and Ni(2+) adducted peptides yielded abundant metalated a-/y-type fragment ions; whereas ECD of Cu(2+) adducted peptides generated predominantly metalated b-/y-type fragment ions. From the present experimental results, it was postulated that electronic configuration of metal ions is an important factor in determining the ECD behavior of the metalated peptides. Due presumably to the stability of the electronic configuration, metal ions with fully-filled (i.e., Zn(2+)) and half filled (i.e., Mn(2+)) d-orbitals might not capture the incoming electron. Dissociation of the metal ions adducted peptides would proceed through the usual ECD channel(s) via "hot-hydrogen" or "superbase" intermediates, to form series of c-/z(•)- fragments. For other transition metal ions studied, reduction of the metal ions might occur preferentially. The energy liberated by the metal ion reduction would provide enough internal energy to generate the "slow-heating" type of fragment ions, i.e., metalated a-/y- fragments and metalated b-/y- fragments.

Electron transfer dissociation of dipositive uranyl and plutonyl coordination complexes

Reported here is a comparison of electron transfer dissociation (ETD) and collision-induced dissociation (CID) of solvent-coordinated dipositive uranyl and plutonyl ions generated by electrospray ionization. Fundamental differences between the ETD and CID processes are apparent, as are differences between the intrinsic chemistries of uranyl and plutonyl. Reduction of both charge and oxidation state, which is inherent in ETD activation of [An(VI) O(2) (CH(3) COCH(3) )(4) ](2+) , [An(VI) O(2) (CH(3) CN)(4) ](2) , [U(VI) O(2) (CH(3) COCH(3) )(5) ](2+) and [U(VI) O(2) (CH(3) CN)(5) ](2+) (An = U or Pu), is accompanied by ligand loss. Resulting low-coordinate uranyl(V) complexes add O(2) , whereas plutonyl(V) complexes do not. In contrast, CID of the same complexes generates predominantly doubly-charged products through loss of coordinating ligands. Singly-charged CID products of [U(VI) O(2) (CH(3) COCH(3) )(4,5) ](2+) , [U(VI) O(2) (CH(3) CN)(4,5) ](2+) and [Pu(VI) O(2) (CH(3) CN)(4) ](2+) retain the hexavalent metal oxidation state with the addition of hydroxide or acetone enolate anion ligands. However, CID of [Pu(VI) O(2) (CH(3) COCH(3) )(4) ](2+) generates monopositive plutonyl(V) complexes, reflecting relatively more facile reduction of Pu(VI) to Pu(V).