Transition metals as electron traps. II. Structures, energetics and electron transfer dissociations of ternary Co, Ni and Zn-peptide complexes in the gas phase

Native electrospray mass spectrometry approaches to probe the interaction between zinc and an anti-angiogenic peptide from histidine-rich glycoprotein

Zinc modulates the biological function of histidine-rich glycoprotein (HRG) through binding to its His-rich region (HRR). The Zn2+-binding properties of a 35 amino-acid biologically-active peptide mimic of the HRR, HRGP330, were investigated using dissociative mass spectrometry approaches in addition to travelling-wave ion mobility mass spectrometry (TWIM-MS). Native mass spectrometry confirmed zinc binding to HRGP330; however, broadening of the 1H NMR resonances upon addition of Zn2+ ions precluded the attainment of structural information. A complementary approach employing TWIM-MS indicated that HRGP330 has a more compact structure in the presence of Zn2+ ions. Top-down MS/MS data supported a metal-binding-induced conformational change, as fewer fragments were observed for Zn2+-bound HRGP330. Zn2+-bound fragments of both N-terminal and C-terminal ends of the peptide were identified from collision-induced dissociation (CID) and electron transfer dissociation/proton transfer reaction (ETD/PTR) experiments, suggesting that multiple binding sites exist within this region of HRG. The combination of mass spectrometry and NMR approaches provides new insight into the highly dynamic interaction between zinc and this His-rich peptide.

Preparation of Labile Ni+(cyclam) Cations in the Gas Phase Using Electron-Transfer Reduction through Ion-Ion Recombination in an Ion Trap and Structural Characterization with Vibrational Spectroscopy

Gas-phase ion chemistry methods that capture and characterize the degree of activation of small molecules in the active sites of homogeneous catalysts form a powerful new tool to unravel how ligand environments affect reactivity. A key roadblock in this development, however, is the ability to generate the fragile metal oxidation states that are essential for catalytic activity. Here we demonstrate the preparation of the key Ni(I) center in the widely used cyclam scaffold using ion-ion recombination as a gas-phase alternative to electrochemical reduction. The singly charged Ni⁺(cyclam) coordination complex is generated by electron transfer from fluoranthene and azobenzene anions to doubly charged Ni²⁺(cyclam), using the electron-transfer dissociation protocol in a commercial quadrupole ion trap instrument and in a custom-built octopole RF ion trap. The successful preparation of the Ni⁺(cyclam) cation is verified through analysis of its vibrational spectrum obtained using the infrared free electron laser FELIX.

Difference of Electron Capture and Transfer Dissociation Mass Spectrometry on Ni(2+)-, Cu(2+)-, and Zn(2+)-Polyhistidine Complexes in the Absence of Remote Protons

Electron capture dissociation (ECD) and electron transfer dissociation (ETD) in metal-peptide complexes are dependent on the metal cation in the complex. The divalent transition metals Ni(2+), Cu(2+), and Zn(2+) were used as charge carriers to produce metal-polyhistidine complexes in the absence of remote protons, since these metal cations strongly bind to neutral histidine residues in peptides. In the case of the ECD and ETD of Cu(2+)-polyhistidine complexes, the metal cation in the complex was reduced and the recombination energy was redistributed throughout the peptide to lead a zwitterionic peptide form having a protonated histidine residue and a deprotonated amide nitrogen. The zwitterion then underwent peptide bond cleavage, producing a and b fragment ions. In contrast, ECD and ETD induced different fragmentation processes in Zn(2+)-polyhistidine complexes. Although the N-Cα bond in the Zn(2+)-polyhistidine complex was cleaved by ETD, ECD of Zn(2+)-polyhistidine induced peptide bond cleavage accompanied with hydrogen atom release. The different fragmentation modes by ECD and ETD originated from the different electronic states of the charge-reduced complexes resulting from these processes. The details of the fragmentation processes were investigated by density functional theory. 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.

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.

Formation of Ternary Complexes of Iron(III) Cations in Solution and Gas Phase

Formation of labile 1 : 1 : 1 ternary mononuclear complexes of iron(iii) cation with η3-terdentate meridional binders was studied using electrospray ionisation mass spectrometry (ESI-MS) titration and UV-Vis titration in solution phase. Low selectivities towards formation of ternary heteroleptic complexes in the solution phase vs. symmetric 2 : 1 complexes were obtained with combinations of dianionic 2,6-bis[hydroxy(methyl)amino]-1,3,5-triazine (BHT) ligands with monoanionic terdentate ligands such as 2-[(2-pyridinylmethylene)amino]phenol. Moderate selectivities were observed in formation of ternary iron(iii) complexes of iron(iii) between BHT ligands and neutral terdentate ligands such as pyridin-2-ylmethylpyridin-2-ylmethyleneamine. Results obtained by MS titrations were in a reasonable agreement with UV titration data indicating that quantitative ESI MS spectrometry can be applied to these labile iron(iii) complexes.

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.

Tunable charge tags for electron-based methods of peptide sequencing: design and applications

Charge tags using basic auxiliary functional groups 6-aminoquinolinylcarboxamido, 4-aminopyrimidyl-1-methylcarboxamido, 2-aminobenzoimidazolyl-1-methylcarboxamido, and the fixed-charge 4-(dimethylamino)pyridyl-1-carboxamido moiety are evaluated as to their properties in electron transfer dissociation mass spectra of arginine C-terminated peptides. The neutral tags have proton affinities that are competitive with those of amino acid residues in peptides. Charge reduction by electron transfer from fluoranthene anion-radicals results in peptide backbone dissociations that improve sequence coverage by providing extensive series of N-terminal c-type fragments without impeding the formation of C-terminal z fragments. Comparison of ETD mass spectra of free and tagged peptides allows one to resolve ambiguities in fragment ion assignment through mass shifts of c ions. Simple chemical procedures are reported for N-terminal tagging of Arg-containing tryptic peptides.

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.

Gas-phase doubly charged complexes of cyclic peptides with copper in +1, +2 and +3 formal oxidation states: formation, structures and electron capture dissociation

Copper complexes with a cyclic D-His-β-Ala-L-His-L-Lys and all-L-His-β-Ala-His-Lys peptides were generated by electrospray which were doubly charged ions that had different formal oxidation states of Cu(I), Cu(II) and Cu(III) and different protonation states of the peptide ligands. Electron capture dissociation showed no substantial differences between the D-His and L-His complexes. All complexes underwent peptide cross-ring cleavages upon electron capture. The modes of ring cleavage depended on the formal oxidation state of the Cu ion and peptide protonation. Density functional theory (DFT) calculations, using the B3LYP with an effective core potential at Cu and M06-2X functionals, identified several precursor ion structures in which the Cu ion was threecoordinated to pentacoordinated by the His and Lys side-chain groups and the peptide amide or enolimine groups. The electronic structure of the formally Cu(III) complexes pointed to an effective Cu(I) oxidation state with the other charge residing in the peptide ligand. The relative energies of isomeric complexes of the [Cu(c-HAHK + H)](2+) and [Cu(c-HAHK - H)](2+) type with closed electronic shells followed similar orders when treated by the B3LYP and M06-2X functionals. Large differences between relative energies calculated by these methods were obtained for open-shell complexes of the [Cu(c-HAHK)](2+) type. Charge reduction resulted in lowering the coordination numbers for some Cu complexes that depended on the singlet or triplet spin state being formed. For [Cu(c-HAHK - H)](2+) complexes, solution H/D exchange involved only the N-H protons, resulting in the exchange of up to seven protons, as established by ultra-high mass resolution measurements. Contrasting the experiments, DFT calculations found the lowest energy structures for the gas-phase ions that were deprotonated at the peptide C(α) positions.

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).

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

Peptides adducted with different divalent Group IIB metal ions (Zn(2+), Cd(2+), and Hg(2+)) were found to give very different ECD mass spectra. ECD of Zn(2+) adducted peptides gave series of c-/z-type fragment ions with and without metal ions. ECD of Cd(2+) and Hg(2+) adducted model peptides gave mostly a-type fragment ions with M(+•) and fragment ions corresponding to losses of neutral side chain from M(+•). No detectable a-ions could be observed in ECD spectra of Zn(2+) adducted peptides. We rationalized the present findings by invoking both proton-electron recombination and metal-ion reduction processes. As previously postulated, divalent metal-ions adducted peptides could adopt several forms, including (a) [M + Cat](2+), (b) [(M + Cat - H) + H](2+), and (c) [(M + Cat - 2H) + 2H](2+). The relative population of these precursor ions depends largely on the acidity of the metal-ion peptide complexes. Peptides adducted with divalent metal-ions of small ionic radii (i.e., Zn(2+)) would form predominantly species (b) and (c); whereas peptides adducted with metal ions of larger ionic radii (i.e., Hg(2+)) would adopt predominantly species (a). Species (b) and (c) are believed to be essential for proton-electron recombination process to give c-/z-type fragments via the labile ketylamino radical intermediates. Species (c) is particularly important for the formation of non-metalated c-/z-type fragments. Without any mobile protons, species (a) are believed to undergo metal ion reduction and subsequently induce spontaneous electron transfer from the peptide moiety to the charge-reduced metal ions. Depending on the exothermicity of the electron transfer reaction, the peptide radical cations might be formed with substantial internal energy and might undergo further dissociation to give structural related fragment ions.

Electron predators are hydrogen atom traps. Effects of aryl groups on N-C(α) bond dissociations of peptide radicals

Effects of substituted aryl groups on dissociations of peptide aminoketyl radicals were studied computationally for model tetrapeptide intermediates GXD(•) G where X was a cysteine residue that was derivatized by S-(3-nitrobenzyl), S-(3-cyanobenzyl), S-(3,5-dicyanobenzyl), S-(2,3,4,5,6-pentafluorobenzyl), and S-benzyl groups. The aminoketyl radical was placed within the Asp amide group. Aminoketyl radicals having the S-(3-nitrobenzyl) group were found to undergo spontaneous and highly exothermic migration of the hydroxyl hydrogen atom onto the nitro group in conformers allowing interaction between these groups. Competing reaction channels were investigated for aminoketyl radicals having the S-(3-cyanobenzyl) and S-(3,5-dicyanobenzyl) groups, e.g. H-atom migration to the C and N atoms of the C≡N group, migration to the C-4 position of the phenyl ring, and dissociation of the radical-activated NC(α) bond between the Asp and Gly residues. RRKM kinetic analysis on the combined B3LYP and ROMP2/6-311++G(2d,p) potential energy surface indicated > 99% H-atom transfer to the C≡N group forming a stable iminyl intermediate. The NC(α) bond dissociation was negligible. In contrast, peptides with the S-(2,3,4,5,6-pentafluorobenzyl) and S-benzyl groups showed preferential NC(α) bond dissociation that outcompeted H-atom migration to the C-4 position and fluorine substituents in the phenyl ring. These computational results are used to suggest an alternative mechanism for the quenching effect on electron-based peptide backbone dissociations of benzyl groups with electron-withdrawing substitutents, as reported recently.