Gas-phase anionic complexes of alkali metal ions and peptides: Structure and collision activated decompositions

Formation and fragmentation of unsaturated fatty acid [M - 2H + Na]- ions: stabilized carbanions for charge-directed fragmentation

Fatty acids are long-chain carboxylic acids that readily produce [M - H](-) ions upon negative ion electrospray ionization (ESI) and cationic complexes with alkali, alkaline earth, and transition metals in positive ion ESI. In contrast, only one anionic monomeric fatty acid-metal ion complex has been reported in the literature, namely [M - 2H  +  Fe(II)Cl](-). In this manuscript, we present two methods to form anionic unsaturated fatty acid-sodium ion complexes (i.e., [M - 2H  +  Na](-)). We find that these ions may be generated efficiently by two distinct methods: (1) negative ion ESI of a methanolic solution containing the fatty acid and sodium fluoride forming an [M - H  +  NaF](-) ion. Subsequent collision-induced dissociation (CID) results in the desired [M - 2H  +  Na](-) ion via the neutral loss of HF. (2) Direct formation of the [M - 2H  +  Na](-) ion by negative ion ESI of a methanolic solution containing the fatty acid and sodium hydroxide or bicarbonate. In addition to deprotonation of the carboxylic acid moiety, formation of [M - 2H  +  Na](-) ions requires the removal of a proton from the fatty acid acyl chain. We propose that this deprotonation occurs at the bis-allylic position(s) of polyunsaturated fatty acids resulting in the formation of a resonance-stabilized carbanion. This proposal is supported by ab initio calculations, which reveal that removal of a proton from the bis-allylic position, followed by neutral loss of HX (where X = F(-) and (-)OH), is the lowest energy dissociation pathway.

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.

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.

Characterization of [peptide+(Ag)n]+ complexes using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Silver ion complexes of peptides [M + (Ag)(n) ](+) , M = angiotensin I or substance P where n = 1-8 and 17-23 for angiotensin I and n = 1-5 for substance P, are identified and characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The Ag(+) coordination number exceeds the number of available amino acid residues in angiotensin I whereas the number of observed complexes in substance P is less than the number of amino acid residues in it. The larger coordination number of angiotensin I with Ag(+) indicates the simultaneous binding of several Ag(+) ions to the amino acid residue present in it. The lower number of observed complexes in substance P suggests the binding of two or more residues to one Ag(+) ion. The presence of trifluoroacetic acid in the peptide samples reduces the Ag(+) coordination ability in both the peptides which indicates that the basic residues in it are already protonated and do not participate in the Ag(+) -binding process. The Ag(+) ion also forms a complex with the α-cyano-4-hydroxycinnamic acid (CHCA) matrix and is observed in the MALDI mass spectra and the formation of [CHCA + Ag](+) , [CHCA + AgNO(3) ](+) and [(CHCA)(2)  + Ag](+) ions is due to the high binding affinity of Ag(+) to the CN group of CHCA.

Generation of [La(peptide)]3+ complexes in the gas phase: determination of the number of binding sites provided by dipeptide, tripeptide, and tetrapeptide ligands

Gas-phase complexes [La(peptide)](3+) containing 2-4 amino-acid residues have been investigated by electrospraying solutions containing La(3+) and the peptide; only complexes in which the peptide contained an arginine residue were observed. Using the coordination number of eight for La(3+) [Shi, T.; Hopkinson, A. C.; Siu, K. W. M. Chem. Eur. J. 2007, 13, 1142-1151] and the relative abundances of the hydrates [La(peptide)(H(2)O)(n)](3+), the number of binding sites provided by the peptides was deduced: Leu-Trp-Met-Arg, 7; Met-Arg-Phe-Ala, 6; Gly-Arg-Gly, 4; Gly-Gly-Arg 4; and Met-Arg, 4. Density Functional Theory calculations show that the zwitterionic form of Gly-Gly-Arg preferentially binds La(3+) through four coordination sites-the two amide oxygens and the two carboxy oxygens.

Study of peptide on-line complexation with transition-metal ions generated from sacrificial electrodes in thin-chip polymer microsprays

A miniaturized polymer electrospray-type interface is used to study metal-ion chelation with model peptides. Taking advantage of the intrinsic electrochemical behavior of electrospray, a sacrificial electrode is used to generate at the same time electrospray and transition-metal ions coming from the anodic dissolution of the electrode. The microspray interface provides enhanced mass transport due to its small dimensions, increasing the yield of possible reactions, in particular complex formation. Transition-metal electrodes, e.g. copper, zinc, nickel, iron and silver, are used to obtain on-line complexation with model peptides. It is demonstrated that the use of in-reservoir sacrificial electrodes is an efficient way to generate metal ions in order to form and study complexes with peptides, avoiding the addition of metallic salts.

Metal ion adducts in the structural analysis of ginsenosides by electrospray ionization with multi-stage mass spectrometry

The effect of metal (Li+, Na+, K+, Ag+) cationization on collision-induced dissociation of ginsenosides was investigated by electrospray ionization mass spectrometry combined with multi-stage mass spectrometry (ESI-MS(n)). The fragments of sodiated and lithiated molecules give valuable structural information regarding the nature of the aglycone and the sequence and linkage information of sugar moieties. However, the number and relative abundances of fragment ions from lithiated ginsenosides are significantly greater than for the sodiated species. The K+ adducts undergo glycosidic cleavages and very limited cross-ring reactions. The silver ion adducts fragment mainly through glycosidic cleavages.

Alkali-metal ion/molecule association reactions and their applications to mass spectrometry

This review will be concerned primarily with thermal alkali-metal ion association reactions of the general type: A+ + M + N reversible (A + M)+ + N. Where A denotes a positively charged alkali metal ion, M is neutral species and N works as a third body. As indicated, most association reactions are reversible, and the A-M bond derives primarily from electrostatic forces whose energy (affinity) is typically 50 kcal/mol or less, often much less. The review includes reaction mechanism, instrumentation, and application to mass spectrometry, together with reaction rates and alkali ion affinities of the classified compounds.

A study of metal chelation of dinucleotide analogs in the gas phase by fast-atom bombardment mass spectrometry

Fast-atom bombardment (FAB) mass spectrometry was used to investigate the interaction of proton and alkali metal ions with dinucleotide analogs such as T-n-T (T = thymine moiety, n = polyether chain, e.g., triethylene, tetraethylene, pentaethylene, and hexaethylene ether 1-4), A-n-T (A = adenine unit 5-8), and T-n-OMe (9-12) in 3-nitrobenzyl alcohol matrix. The [M + H](+) ion is the most abundant ion for the A-n-T series, whereas in 1-4 and 9-12 the (TC2H4)(+) ion is the most abundant. Formation of [M + H -C2H4O](+) ions, a characteristic fragmentation of crown ethers under electron ionization, is observed for compounds 1-12 and is more pronounced in 6 and 7. An abundant [M - H](-) ion is observed for all the compounds studied under negative ion FAB due to the presence of the (-CO-NH-CO-) group of thymine, an indication of existence of intramolecular H bonding. The FAB mass spectra of 1-12 with alkali metal ions (Li(+), Na(+), K(+), Rb(+), and Cs(+)) showed formation of abundant metal-coordinated ions ([M + Met](+) and [TC2H4 + Met](+)). Compounds 3, 4, 6, 7, and 10-12 showed ions due to the substitution of the thymine moiety by a hydroxyl group ([M + Met - 108](+), Met = metal ion). For compound 3 alone, substitution of two thymine groups ([M + Met - 216](+)) was observed. Metastable ion studies were used to elucidate the structures of these potentially significant ions, and the ion formule were confirmed with high resolution measurements. Selectivity toward metal complexation with ligand size was seen in the T-n-T and A-n-T series and was even more pronounced in A-n-T series. These dinucleotide analogs fall in the following order of chelation of alkali metal ions, acyclic glymes < dinucleotide analogs (acyclic glymes substituted with nitrogen bases) < crown ethers, which places them in perspective as receptor models.

Observation of alkali-metal-carboxylate radical anions in fast-atom bombardment

The existence of a series of 32 molecular radical anions from carboxylic acids salts RCOO-Cat(-), where R = CH3, CH3CH2, CH3CH2CH2, (CH3)2CH, C6H5, o-CH3-C6H4, m-CH3-C6H4, and p-CH3-C6H4 and Cat = Li, Na, K, Rb, has been proven by the observation of their fragmentation in negative-ion fast-atom bombardment tandem mass spectrometry. These species occur at very low abundance and are not detected in the fast-atom bombardment spectra because they are hidden in the background. However, the collision-induced dissociation fragmentation of ions selected at the mass-to-charge ratio values that correspond to these species display characteristic signals that are completely different from the signals observed from pure matrix or after addition of corresponding metal hydroxide to the matrix. The main fragmentation observed is the loss of the neutral metal atom from RCOOCat(-·), followed by a loss of CO2 for the aromatic compounds. Neutral loss experiments also confirm the existence of these radical anions. Scans for the loss of a selected metal after addition of one of the carboxylic acid salts to the matrix display abundant peaks at mass-to-charge ratio values that correspond to the salt radical anions. Some weaker peaks appear at other mass-to-charge ratio values independent of the salt that is used and also are observed from the matrix when the corresponding metal hydroxide is added. When alkali metal salts from some deuterated acids are analyzed, the predicted shifts are observed. The loss of the neutral metal is more pronounced from RCOONa(-·), RCOOK(-·), and RCOORb(-·) than from RCOOLi(-·).

Interaction of angiotensin peptides and zinc metal ions probed by electrospray ionization mass spectrometry

Electrospray ionization-tandem mass spectrometry experiments were used to provide evidence regarding the sites of interactions between zinc metal ions and angiotensin peptides. The electrospray ionization mass spectra of histidine-containing human angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) and angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu) in the presence of zinc show abundant multiply charged ions for the zinc-attached peptide [M + aZn(2+) +(c - 2a)H(+)](c+), where a = 1, 2 and c is charge. From collisionally activated dissociation experiments, with both low energy (triple quadrupole mass spectrometry) and high energy collisions (linked scan at constant B/E with a double focusing instrument) of the [M + Zn](2+) and [M + Zn + H](3+) ions for angiotensin II, a [b 6 + Zn](2+) species is produced as the most abundant product ion, suggesting that the zinc interaction site is in the vicinity of the His(6) residue. Additionally, tandem mass spectra from the zinc-attached ions for angiotensin I show abundant [b 6 + Zn](2+) and [b 9 + Zn](2+) products, providing evidence that both His(6) and His(9) are involved in zinc coordination.