Infrared spectroscopy of arginine cation complexes: direct observation of gas-phase zwitterions

The structures of cationized arginine complexes [Arg + M]+, (M = H, Li, Na, K, Rb, Cs, and Ag) and protonated arginine methyl ester [ArgOMe + H]+ have been investigated in the gas phase using calculations and infrared multiple-photon dissociation spectroscopy between 800 and 1900 cm-1 in a Fourier transform ion cyclotron resonance mass spectrometer. The structure of arginine in these complexes depends on the identity of the cation, adopting either a zwitterionic form (in salt-bridge complexes) or a non-zwitterionic form (in charge-solvated complexes). A diagnostic band above 1700 cm-1, assigned to the carbonyl stretch, is observed for [ArgOMe + H]+ and [Arg + M]+, (M = H, Li, and Ag), clearly indicating that Arg in these complexes is non-zwitterionic. In contrast, for the larger alkali-metal cations (K+, Rb+, and Cs+) the measured IR-action spectra indicate that arginine is a zwitterion in these complexes. The measured spectrum for [Arg + Na]+ indicates that it exists predominantly as a salt bridge with zwitterionic Arg; however, a small contribution from a second conformer (most likely a charge-solvated conformer) is also observed. While the silver cation lies between Li+ and Na+ in metal-ligand bond distance, it binds as strongly or even more strongly to oxygen-containing and nitrogen-containing ligands than the smaller Li+. The measured IR-action spectrum of [Arg + Ag]+ clearly indicates only the existence of non-zwitterionic Arg, demonstrating the importance of binding energy in conformational selection. The conformational landscapes of the Arg-cation species have been extensively investigated using a combination of conformational searching and electronic structure theory calculations [MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p)]. Computed conformations indicate that Ag+ is di-coordinated to Arg, with the Ag+ chelated by both the N-terminal nitrogen and Neta of the side chain but lacks the strong M+-carbonyl oxygen interaction that is present in the tri-coordinate Li+ and Na+ charge-solvation complexes. Experiment and theory show good agreement; for each ion species investigated, the global-minimum conformer provides a very good match to the measured IR-action spectrum.     

Effects of alkaline earth metal ion complexation on amino acid zwitterion stability: results from infrared action spectroscopy

The structures of isolated alkaline earth metal cationized amino acids are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and theory. These results indicate that arginine, glutamine, proline, serine, and valine all adopt zwitterionic structures when complexed with divalent barium. The IRMPD spectra for these ions exhibit bands assigned to carboxylate stretching modes, spectral signatures for zwitterionic amino acids, and lack bands attributable to the carbonyl stretch of a carboxylic acid functional group. Structural and spectral assignments are strengthened through comparisons with absorbance spectra calculated for low-energy structures and the IRMPD spectra of analogous ions containing monovalent alkali metals. Many bands are significantly red-shifted from the corresponding bands for amino acids complexed with monovalent metal ions, owing to increased charge transfer to divalent metal ions. The IRMPD spectra of arginine complexed with divalent strontium and barium are very similar and indicate that arginine adopts a zwitterionic form in both ions. Calculations indicate that nonzwitterionic forms of arginine are lowest in free energy in complexes with smaller alkaline earth metal cations and that zwitterionic forms are preferentially stabilized with increasing metal ion size. B3LYP and MP2 calculations indicate that zwitterionic forms of arginine are lowest in free energy for M = Ca, Sr, and Ba.     

One water molecule stabilizes the cationized arginine zwitterion

Singly hydrated clusters of lithiated arginine, sodiated arginine, and lithiated arginine methyl ester are investigated using infrared action spectroscopy and computational chemistry. Whereas unsolvated lithiated arginine is nonzwitterionic, these results provide compelling evidence that attachment of a single water molecule to this ion makes the zwitterionic form of arginine, in which the side chain is protonated, more stable. The experimental spectra of lithiated and sodiated arginine with one water molecule are very similar and contain spectral signatures for protonated side chains, whereas those of lithiated arginine and singly hydrated lithiated arginine methyl ester are different and contain spectral signatures for neutral side chains. Calculations at the B3LYP/6-31++G** level of theory indicate that solvating lithiated arginine with a single water molecule preferentially stabilizes the zwitterionic forms of this ion by 25-32 kJ/mol and two essentially isoenergetic zwitterionic structure are most stable. In these structures, the metal ion either coordinates with the N-terminal amino group and an oxygen atom of the carboxylate group (NO coordinated) or with both oxygen atoms of the carboxylate group (OO coordinated). In contrast, the OO-coordinated zwitterionic structure of sodiated arginine, both with and without a water molecule, is clearly lowest in energy for both ions. Hydration of the metal ion in these clusters weakens the interactions between the metal ion and the amino acid, whereas hydrogen-bond strengths are largely unaffected. Thus, hydration preferentially stabilizes the zwitterionic structures, all of which contain strong hydrogen bonds. Metal ion size strongly affects the relative propensity for these ions to form NO or OO coordinated structures and results in different zwitterionic structures for lithiated and sodiated arginine clusters containing one water molecule.     

Structure of cationized arginine (arg.m, m = h, li, na, k, rb, and cs) in the gas phase: further evidence for zwitterionic arginine

The gas-phase structures of cationized arginine, Arg.M(+), M = Li, Na, K, Rb, and Cs, were studied both by hybrid method density functional theory calculations and experimentally using low-energy collisionally activated and thermal radiative dissociation. Calculations at the B3LYP/LACVP++** level of theory show that the salt-bridge structures in which the arginine is a zwitterion (protonated side chain, deprotonated C-terminus) become more stable than the charge-solvated structures with increasing metal ion size. The difference in energy between the most stable charge-solvated structure and salt-bridge structure of Arg.M(+) increases from -0.7 kcal/mol for Arg.Li(+) to +3.3 kcal/mol for Arg.Cs(+). The stabilities of the salt-bridge and charge-solvated structures reverse between M = Li and Na. These calculations are in good agreement with the results of dissociation experiments. The low-energy dissociation pathways depend on the cation size. Arginine complexed with small cations (Li and Na) loses H(2)O, while arginine complexed with larger cations (K, Rb, and Cs) loses NH(3). Loss of H(2)O must come from a charge-solvated ion, whereas the loss of NH(3) can come from the protonated side chain of a salt-bridge structure. The results of dissociation experiments using several cationized arginine derivatives are consistent with the existence of these two distinct structures. In particular, arginine methyl esters, which cannot form salt bridges, dissociate by loss of methanol, analogous to loss of H(2)O from Arg.M(+); no loss of NH(3) is observed. Although dissociation experiments probe gas-phase structure indirectly, the observed fragmentation pathways are in good agreement with the calculated lowest energy isomers. The combination of the results from experiment and theory provides strong evidence that the structure of arginine-alkali metal ion complexes in the gas phase changes from a charge-solvated structure to a salt-bridge structure as the size of the metal ion increases.     

Gas phase hydrogen/deuterium exchange of arginine and arginine dipeptides complexed with alkali metals

The hydrogen/deuterium (H/D) exchange of protonated and alkali-metal cationized Arg-Gly and Gly-Arg peptides with D(2)O in the gas phase was studied using electrospray ionization quadropole ion trap mass spectrometry. The Arg-Gly and Gly-Arg alkali metal complexes exchange significantly more hydrogens than protonated Arg-Gly and Gly-Arg. We propose a mechanism where the peptide shifts between a zwitterionic salt bridge and nonzwitterionic charge solvated conformations. The increased rate of H/D exchange of the alkali metal complexes is attributed to the peptide metal complexes' small energy difference between the salt-bridge conformation and the nonzwitterionic charge-solvated conformation. Implications for the applicability of this mechanism to other zwitterionic systems are discussed.     

Infrared spectroscopy of cationized lysine and epsilon-N-methyllysine in the gas phase: effects of alkali-metal ion size and proton affinity on zwitterion stability

The gas-phase structures of protonated and alkali-metal-cationized lysine (Lys) and epsilon-N-methyllysine (Lys(Me)) are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. IRMPD spectra of Lys.Li(+) and Lys.Na(+) are similar, but the spectrum for Lys.K(+) is different, indicating that the structure of lysine in these complexes depends on the metal ion size. The carbonyl stretch of a carboxylic acid group is clearly observed in each of these spectra, indicating that lysine is nonzwitterionic in these complexes. A detailed comparison of these spectra to those calculated for candidate low-energy structures indicates that the bonding motif for the metal ion changes from tricoordinated for Li and Na to dicoordinated for K, clearly revealing the increased importance of hydrogen-bonding relative to metal ion solvation with increasing metal ion size. Spectra for Lys(Me).M(+) show that Lys(Me), an analogue of lysine whose side chain contains a secondary amine, is nonzwitterionic with Li and zwitterionic with K and both forms are present for Na. The proton affinity of Lys(Me) is 16 kJ/mol higher than that of Lys; the higher proton affinity of a secondary amine can result in its preferential protonation and stabilization of the zwitterionic form.     

Alkali metal ion binding to glutamine and glutamine derivatives investigated by infrared action spectroscopy and theory

The gas-phase structures of alkali-metal cationized glutamine are investigated by using both infrared multiple photon dissociation (IRMPD) action spectroscopy, utilizing light generated by a free electron laser, and theory. The IRMPD spectra contain many similarities that are most consistent with glutamine adopting nonzwitterionic forms in all ions, but differences in the spectra indicate that the specific nonzwitterionic forms adopted depend on metal-ion identity. For ions containing small alkali metals, the metal ion is solvated predominantly by the amino group, the carbonyl oxygen of the carboxylic acid group, and the carbonyl oxygen of the amide group. With increasing alkali-metal-ion size, additional structures are present in which the carboxylic acid group donates a hydrogen bond to the amino group and the metal ion is solvated only by the amide and carboxylic acid groups. The effects of alkylation of the amino and amide groups on the proton affinity of isolated glutamine and the relative zwitterion stability of sodiated glutamine were examined computationally. Methylation of the amino group increases the proton affinity of isolated glutamine and preferentially stabilizes the zwitterionic form of sodiated glutamine by roughly 20 kJ/mol. Ethylation and isopropylation of the amide group each increase the proton affinity of isolated glutamine by roughly 13 kJ/mol but preferentially stabilize the zwitterionic form of sodiated glutamine by less than 3 kJ/mol. These results indicate that effects of proton affinity on relative zwitterion stability compete with effects of metal-ion solvation.     

Gas-phase conformations of cationized poly(styrene) oligomers

The gas-phase conformations of poly(styrene) oligomers cationized by Li+, Na+, Cu+, and Ag+ (M+ PSn) were examined using ion mobility experiments and molecular mechanics/ dynamics calculations. M+PSn ions were formed by MALDI and their ion-He collision cross-sections were measured by ion mobility methods. The experimental collision cross-sections of each M+PS n-mer were similar for all four metal cations and increased linearly with n. Molecular modeling of selected M+ PS oligomers cationized by Li+ and Na+ yielded quasi-linear structures with the metal cation sandwiched between two phenyl groups. The relative energies of the structures were approximately 2-3 kcal/mol more stable when the metal cation was sandwiched near the middle of the oligomer chain than when it was near the ends of the oligomer. The cross-sections of these theoretical structures agree well with the experimental values with deviations typically around 1-2%. The calculations also show that the metal cation tends to align the phenyl groups on the same side of the -CH2-CH- backbone. Calculations on neutral poly(styrene), on the other hand, showed structures in which the phenyl groups were more randomly positioned about the oligomer backbone. The conformations and metal-oligomer binding energies of M+PS are also used to help explain CID product distributions and fragmentation mechanisms of cationized PS oligomers. etry     

Fragmentation study of rutin, a naturally occurring flavone glycoside cationized with different alkali metal ions, using post-source decay matrix-assisted laser desorption/ionization mass spectrometry.

A post-source decay matrix-assisted laser desorption/ionization mass spectrometric (PSD-MALDI-MS) study of rutin, a naturally occurring flavone glycoside cationized with different alkali metal ions, is reported. The fragmentations of rutin were performed by selecting the [R + Cat]+ peaks for PSD, where R represents a rutin molecule and Cat an alkali metal ion (Li+, Na+, K+). The PSD-MALDI mass spectra showed, depending on Cat, different fragmentation patterns with respect to both the quality and quantity of the fragment ions formed. The intensity of fragmentation decreased in the order Li+ > Na+ > K+. The fragmentation mechanism and an explanation for the observed differences are suggested.     

Infrared multiple photon dissociation spectroscopy of cationized histidine: effects of metal cation size on gas-phase conformation

The gas phase structures of cationized histidine (His), including complexes with Li(+), Na(+), K(+), Rb(+), and Cs(+), are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser, in conjunction with quantum chemical calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G(d,p) (Li(+), Na(+), and K(+) complexes) and B3LYP/HW*/6-311+G(d,p) (Rb(+) and Cs(+) complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. On the basis of these experiments and calculations, the only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li(+)(His) and Na(+)(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO,N(α),N(1)], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K(+)(His), Rb(+)(His), and Cs(+)(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li(+)(His) and Na(+)(His). For these complexes, the bidentate [CO,N(1)] conformer in which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,N(α),N(1)] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb(+)(His) and Cs(+)(His) indicate that both [CO,N(1)] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers.     

Experimental and theoretical investigation of alkali metal cation interactions with hydroxyl side-chain amino acids

Absolute bond dissociation energies of serine (Ser) and threonine (Thr) to alkali metal cations are determined experimentally by threshold collision-induced dissociation of M+AA complexes, where M+=Li+, Na+, and K+ and AA=Ser and Thr, with xenon in a guided ion beam tandem mass spectrometer. Experimental results show that the binding energies of both amino acids to the alkali metal cations are very similar to one another and follow the order of Li+>Na+>K+. Quantum chemical calculations at three different levels, B3LYP, B3P86, and MP2(full), using the 6-311+G(2d,2p) basis set with geometries and zero-point energies calculated at the B3LYP/6-311+G(d,p) level show good agreement with the experimental bond energies. Theoretical calculations show that all M+AA complexes have charge-solvated structures (nonzwitterionic) with [CO, N, O] tridentate coordination.     

Effect of metal ion and water coordination on the structure of a gas-phase amino acid.

The mode of metal ion and water binding to the amino acid valine is investigated using both theory and experiment. Computations indicate that without water, the structure of valine is nonzwitterionic. Both Li(+) and Na(+) are coordinated to the nitrogen and carbonyl oxygen (NO coordination), whereas K(+) coordinates to both oxygens (OO coordination) of nonzwitterionic valine. The addition of a single water molecule does not significantly affect the relative energies calculated for the cationized valine clusters. Experimentally, the rates of water evaporation from clusters of Val.M(+)(H(2)O)(1), M = Li, Na, and K, are measured using blackbody infrared radiative dissociation. The dissociation rate from the valine complex is compared to water evaporation rates from model complexes of known structure. These results indicate that the metal ion in the lithiated and the sodiated clusters is NO-coordinated to nonzwitterionic valine, while that in the potassiated cluster has OO coordination, in full agreement with theory. The zwitterionic vs nonzwitterionic character of valine in the potassiated cluster cannot be distinguished experimentally. Extensive modeling provides strong support for the validity of inferring structural information from the kinetic data.     

On the question of salt bridges of cationized amino acids in the gas phase: glycine and arginine

The geometrical shapes of the sodiated and cesiated amino acids glycine and arginine were probed in the gas phase by using the ion mobility based ion chromatography method. The data were compared to those obtained for alkali cationized methyl esters and for all the protonated species. Molecular mechanics, semiempirical, and ab initio/density functional theory (DFT) calculations were carried out to generate model structures for comparison with experiment and to determine the relative energies of different structures. For alkali cationized glycine the experimental cross sections agreed with charge solvation structures which were found by calculation to be the most stable forms as well. Both experiment and theory indicated that sodium is solvated by both the amino and the carbonyl groups, while cesium is solvated by one or both oxygen(s) of the carboxyl group. Alkali cationized arginine was found to form a salt bridge structure. The carboxylate group is stabilized by both the charged guanidinium group and the alkali ion. High level (6-311++G7 and DZVP) ab initio/DFT calculations carried out on sodiated and rubidiated N amidinoglycine, which contains a guanidino group and which was used as a model for the larger arginine molecule, indicated that the salt bridge structures are ∼10 kcal/mol more stable than the charge solvation forms for both alkali ions. The structure of protonated arginine, i.e. salt bridge or charge solvation, could not be unambiguously determined.     

Comparative analysis of the gas-phase reactions of cylindrospermopsin and the difference in the alkali metal cation mobility

Cylindrospermopsin (CYN) belongs to a group of toxins produced by several strains of freshwater cyanobacteria. It is a compact zwitterionic molecule composed of a uracil section and a tricyclic guanidinium portion with a primarily hepatotoxic effect. Using low multi-stage and high-resolution mass spectrometry, the gas-phase reactions of this toxin have been investigated. Our data show that collision-induced dissociation (CID) spectra of CYN are dominated by neutral losses, and three major initial fragmentation pathways are clearly distinguishable. Interestingly, comparative analysis of protonated and cationizated molecules showed a significant difference in the balance of the SO3 and terminal ring elimination. These data indicate that the differential ion mobility of H+, Li+, Na+ and K+ leads to different fragmentation pathways, giving rise to mass spectra with different profiles.     

Structures and hydration enthalpies of cationized glutamine and structural analogues in the gas phase

The structures of lithiated and sodiated glutamine, both with and without a water molecule, are investigated using experiment and theory. Loss of water from these complexes and from lithiated and sodiated complexes of asparagine methyl ester, asparagine ethyl ester, and glutamine methyl ester is probed with blackbody infrared radiative dissociation experiments performed over a wide temperature range. Threshold dissociation energies, E(o), for loss of a water molecule from these complexes are obtained from master equation modeling of these data. The values of E(o) are 63 +/- 1 and 53 +/- 1 kJ/mol for the lithiated and sodiated glutamine complexes, respectively. These values are similar to those for the nonzwitterionic model complexes and are in excellent agreement with calculated values. In contrast, water binding to the zwitterionic form is calculated to be significantly higher. These results indicate that glutamine in these lithiated and sodiated complexes with a water molecule are nonzwitterionic. Complexes with the asparagine side chain have slightly higher E(o) values than those with the glutamine side chain, a result consistent with more effective solvation of the metal ion due to the slightly longer side chain of glutamine. Calculations indicate that lithiated and sodiated glutamine are nonzwitterionic, with the metal ion interacting with the amine nitrogen and carbonyl oxygen from the amino acid backbone and the amide oxygen of the side chain. Addition of a water molecule does not affect the lowest-energy structure of lithiated glutamine, whereas, for sodiated glutamine, the lowest-energy zwitterionic and nonzwitterionic structures are essentially isoenergetic.     

Interactions of mono- and divalent metal ions with aspartic and glutamic acid investigated with IR photodissociation spectroscopy and theory

The interaction of metal ions with aspartic (Asp) and glutamic (Glu) acid and the role of gas-phase acidity on zwitterionic stability were investigated using infrared photodissociation spectroscopy in the spectral range 950-1900 cm (-1) and by hybrid density functional theory. Lithium ions interact with both carbonyl oxygen atoms and the amine nitrogen for both amino acids, whereas cesium interacts with both of the oxygen atoms of the C-terminus and the carbonyl oxygen of the side chain for Asp. For Glu, this structure is competitive, but a structure in which the cesium ion interacts with just the carbonyl oxygen atoms is favored and the calculated spectrum for this structure is more consistent with the experimentally measured spectrum. In complexes with either of these metal ions, both amino acids are non-zwitterionic. In contrast, Glu*Ca (2+) and Glu*Ba (2+) both adopt structures in which Glu is zwitterionic and the metal ion interacts with both oxygens of the C-terminal carboxylate and the carbonyl oxygen in the side chain. Assignment of the zwitterionic form of Glu is strengthened by comparisons to the spectrum of the protonated form, which indicate spectral features associated with a protonated amino nitrogen. Comparisons with results for glutamine, which adopts nearly the same structures with these metal ions, indicate that the lower Delta H acid of Asp and Glu relative to other amino acids does not result in greater relative stability of the zwitterionic form, a result that is directly attributed to effects of the metal ions which disrupt the strong interaction between the carboxylic acid groups in the isolated, deprotonated forms of these amino acids.     

Structures of lithiated lysine and structural analogues in the gas phase: effects of water and proton affinity on zwitterionic stability

The structures of lithiated lysine, ornithine, and related molecules, both with and without a water molecule, are investigated using both density functional theory and blackbody infrared radiative dissociation experiments. The lowest-energy structure of lithiated lysine without a water molecule is nonzwitterionic; the metal ion interacts with both nitrogen atoms and the carbonyl oxygen. Structures in which lysine is zwitterionic are higher in energy by more than 29 kJ/mol. In contrast, the singly hydrated clusters with the zwitterionic and nonzwitterionic forms of lysine are more similar in energy, with the nonzwitterionic form more stable by only approximately 7 kJ/mol. Thus, a single water molecule can substantially stabilize the zwitterionic form of an amino acid. Analogous molecules that have methyl groups attached to either the N-terminus (NMeLys) or the side-chain amine (Lys(Me)) have proton affinities greater than that of lysine. In the lithiated clusters with a water molecule attached, the zwitterionic forms of NMeLys and Lys(Me) are calculated to be approximately 4 and approximately 11 kJ/mol more stable than the nonzwitterionic forms, respectively. Calculations of the potential-energy pathway for interconversion between the different forms of lysine in the lithiated complex indicate multiple stable intermediates with an overall barrier height of approximately 83 kJ/mol between the lowest-energy nonzwitterionic form and the most accessible zwitterionic form. Experimentally determined binding energies of water are similar for all these complexes and range from 57 to 64 kJ/mol. These results suggest that loss of a water molecule from the lysine complexes is both energetically and entropically favored compared to interconversion between the nonzwitterionic and zwitterionic structures. Comparisons to calculated binding energies of water to the various structures show that the experimental results are most consistent with the nonzwitterionic forms.     

Influence of halogenation on the properties of uracil and its noncovalent interactions with alkali metal ions. Threshold collision-induced dissociation and theoretical studies

The influence of halogenation on the properties of uracil and its noncovalent interactions with alkali metal ions is investigated both experimentally and theoretically. Bond dissociation energies of alkali metal ion-halouracil complexes, M+(XU), are determined using threshold collision-induced dissociation techniques in a guided ion beam mass spectrometer, where M+ = Li+, Na+, and K+ and XU = 5-fluorouracil, 5-chlorouracil, 6-chlorouracil, 5-bromouracil, and 5-iodouracil. The structures and theoretical bond dissociation energies of these complexes are determined from ab initio calculations. Theoretical calculations are also performed to examine the influence of halogenation on the acidities, proton affinities, and Watson-Crick base pairing energies. Halogenation of uracil is found to produce a decrease in the proton affinity, an increase in the alkali metal ion binding affinities, an increase in the acidity, and stabilization of the A::U base pair. In addition, alkali metal ion binding is expected to lead to an increase in the stability of nucleic acids by reducing the charge on the nucleic acid in a zwitterion effect as well as through additional noncovalent interactions between the alkali metal ion and the nucleobases.     

Novel fragmentation pathway for CID of (b<Subscript>n</Subscript>−1+Cat)<Superscript>+</Superscript> ions from model,metal cationized peptides

We report a new fragmentation pathway for the CID of (b3 - 1 + Cat)+ product ions derived from the model peptide AXAG, where X = beta-alanine, gamma-aminobutyric acid, epsilon-amino-n-caproic acid, or 4-aminomethylbenzoic acid. By changing the amino acid to the C-terminal side of the amino acid X, and incorporating 15N and 13C labeled residues at the same position, we conclude that the dissociation pathway most likely leads to a metal cationized nitrile. With respect to the various amino acids at position X, the putative nitrile product becomes more prominent, relative to the conventional (a3 - 1 + Cat)+ species, in the order beta-alanine < gamma-aminobutyric acid < epsilon-aminocaproic acid < 4-aminomethylbenzoic acid. The pathway is not observed for peptides with alpha-amino acids at position X. The product ion is observed most prominently during the CID of Li+ and Na+ cationized peptides, only to a small extent for Ag+ cationized peptides, and not at all from protonated analogues.     

Multi-stage tandem mass spectrometry of metal cationized leucine enkephalin and leucine enkephalin amide

We have examined the multi-stage collision induced dissociation (CID) of metal cationized leucine enkephalin, leucine enkephalin amide, and the N-acetylated versions of the peptides using ion trap mass spectrometry. In accord with earlier studies, the most prominent species observed during the multi-stage CID of alkali metal cationized leucine enkephalin are the [b(n) + 17 + Cat]+ ions. At higher CID stages (i.e. >MS(4)), however, dissociation of the [b2 + 17 + Cat]+ ion, a cationized dipeptide, results in the production of [a(n) -1 + Cat]+ species. The multi-stage CID of Ag+ cationized leucine enkephalin can be initiated with either the [b(n) -1 + Ag]+ or [b(n) + 17 + Ag]+ ions produced at the MS/MS stage. For the former, sequential CID stages cause, in general, the loss of CO, and then the loss of the imine of the C-terminal amino acid, to reveal the amino acid sequence. Similar to the alkali cationized species, CID of [b2 -1 + Ag]+ produces prominent [a(n) -1 + Ag]+ ions. The multi-stage CID of argentinated peptides is reminiscent of fragmentation observed for protonated peptides, in that a series of (b(n)) and (a(n)) type ions are generated in sequential CID stages. The Ag+ cation is similar to the alkali metals, however, in that the [b(n) + 17 + Ag]+ product is produced at the MS/MS and MS3 stages, and that sequential CID stages cause the elimination of amino acid residues primarily from the C-terminus. We found that N-acetylation of the peptide significantly influenced the fragmentation pathways observed, in particular by promoting the formation of more easily interpreted (in the context of unambiguous sequence determination) dissociation spectra from the [b2 + 17 + Li]+, [b2 + 17 + Na]+ and [b2 -1 + Ag]+ precursor ions. Our results suggest, therefore, that N-acetylation may improve the efficacy of multi-stage CID experiments for C-terminal peptide sequencing in the gas phase. For leucine enkephalin amide, only the multi-stage CID of the argentinated peptide allowed the complete amino acid sequence to be determined from the C-terminal side.