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Alex G Harrison

Publications and source records attributed to Alex G Harrison.

12 recordsLinked to original sources
PubMed2006

Scrambling of sequence information in collision-induced dissociation of peptides.

Collision-induced dissociation (CID) of protonated YAGFL-NH2 leads to nondirect sequence fragment ions that cannot directly be derived from the primary peptide structure. Experimental and theoretical evidence indicate that primary fragmentation of the intact peptide leads to the linear YAGFLoxa b5 ion with a C-terminal oxazolone ring that is attacked by the N-terminal amino group to induce formation of a cyclic peptide b5 isomer. The latter can undergo various proton transfer reactions and opens up to form something other than the YAGFLoxa linear b5 isomer, leading to scrambling of sequence information in the CID of protonated YAGFL-NH2.

Amino Acid Sequence↗
PubMed2005

Fragmentation reactions of deprotonated peptides containing proline. The proline effect.

The collision-induced dissociation (CID) fragmentation reactions of a variety of deprotonated peptides containing proline have been studied in detail using MS(2) and MS(3) experiments, deuterium labelling and accurate mass measurements when necessary. The [M--H--CO(2)](-) (a(2)) ion derived from H-Pro-Xxx-OH dipeptides shows an unusual fragmentation involving loss of C(2)H(4); this fragmentation reaction is not observed for larger peptides. The primary fragmentation reactions of deprotonated tripeptides with an N-terminal proline are formation of a(3) and y(1) ions. When proline is in the central position of tripeptides, a(3), y(2) and y(1) ions are the primary fragmentation products of [M--H](-), while when the proline is in the C-terminal position, a(3)and y(1) ions are the major primary products. In the latter case, the a(3) ion fragments primarily to the ''b(2) ion; further evidence is presented that the ''b(2) ions have a deprotonated oxazolone structure. Larger deprotonated peptides having at least two amino acid residues N-terminal to proline show a distinct preference for cleavage of the amide bond N-terminal to proline to form, mainly, the appropriate y ion. This proline effect is compared and contrasted with the similar proline effect observed in the fragmentation of protonated peptides containing proline.

Peptides↗
PubMed2004

Characterization of alpha- and gamma-glutamyl dipeptides by negative ion collision-induced dissociation.

The low-energy CID mass spectra of the [M-H](-) ions of a variety of dipeptides containing glutamic acid have been obtained using cone-voltage collisional activation. Dipeptides with the gamma-linkage, H-Glu(Xxx-OH)-OH, are readily distinguished from those with the alpha-linkage, H-Glu-Xxx-OH, by the much more prominent elimination of H-Xxx-OH from the [M-H](-) ions of the former isomers, resulting in formation of m/z 128, presumably deprotonated pyroglutamic acid. Dipeptides with the reverse linkage, H-Xxx-Glu-OH, show distinctive fragmentation reactions of the [M-H](-) ions including enhanced elimination of CO(2) and formation of deprotonated glutamic acid. Exchange of the labile hydrogens for deuterium has shown that there is considerable interchange of C-bonded hydrogens with labile (N- and O-bonded) hydrogens prior to most fragmentation reactions. All dipeptides show loss of H(2)O from [M-H](-). MS(3) studies show that the [M-H-H(2)O](-) ion derived from H-Glu-Gly-OH has the structure of deprotonated pyroglutamylglycine while the [M-H-H(2)O](-) ions derived from H-Glu(Gly-OH)-OH and H-Gly-Glu-OH show a different fragmentation behaviour indicating distinct structures for the fragment ions.

Dipeptides↗
PubMed2004

Site of alkylation of N-methyl- and N-ethylaniline in the gas phase: a tandem mass spectrometric study.

N-Methylaniline (NMA) was ethylated and N-ethylaniline (NEA) was methylated under chemical ionization conditions using C(2)H(5)I and CH(3)I, respectively, as reagent gases. The structures of the resulting m/z 136 adduct ions have been probed using metastable ion and collision-induced dissociation (CID) methods. From the similarity of the spectra obtained and from the presence of structure-diagnostic ions at m/z 59 (CH(3)NHC(2)H(5) (+*)) and m/z 44 (CH(3)NHCH(2) (+)), it is concluded that predominantly N-alkylation occurs in both systems. This interpretation was aided by the use of C(2)D(5)I and CD(3)I as reagents. Adduct ions of m/z 136 were also formed by ethylation of the isomeric toluidines and by methylation of the ring-ethylanilines. The resulting CID mass spectra were distinctly different from those obtained for the m/z 136 ions obtained by alkylation of NMA and NEA. Protonation of N-ethyl-N-methylaniline using CH(3)C(==O)CH(3) as Brønsted acid reagent produced an m/z 136 species whose CID mass spectrum also featured intense ion signals at m/z 59 and 44. This observation led to the conclusion that protonation with acetone as reagent results, in this case, in dominant N-protonation. However, the CID mass spectrum of the m/z 136 ion formed when CH(3)OH was the protonating agent featured a weak signal at m/z 44 and no signal at m/z 59. Hence it was concluded that the latter m/z 136 ion contains a larger contribution from the ring-protonated adduct.

Journal Article↗
PubMed2004

Formation of iminium ions by fragmentation of a2 ions.

Tandem mass spectrometric experiments have been carried out on the protonated amides H-Gly-Ala-NH2, H-Ala-Gly-NH2, H-Ala-Val-NH2, H-Val-Ala-pNA, H-Leu-Phe-NH2, H-Phe-Leu-NH2, H-Phe-Tyr-NH2 and H-Tyr-Phe-NH2 with particular emphasis on the fragmentation of the isomeric a2 ions derived therefrom. Primary fragmentation reactions of the protonated amides involve formation of the y1" and b2 ions with further fragmentation of the b2 ion to form the a2 ion which fragments to form iminium ions. Collision-induced dissociation studies of the mass-selected a2 ions were carried out. For the Gly-Ala, Ala-Gly and Val-Ala a2 ions, weak signals were observed corresponding to loss of CO from the a2 ion. With the exception of the Gly-Ala, Ala-Gly and Val-Ala a2 ions, both possible iminium ions (a1 and the internal iminium ion) are observed with the most abundant being that formed by proton attachment to the imine of higher proton affinity. The results provide strong support for the recently proposed (El Aribi et al. J. Am. Chem. Soc. 2003; 125: 9229) mechanism of fragmentation of a2 ions which involves elimination of CO from the a2 ion to form a proton-bound complex of two imines. Based on this mechanism ab initio calculations of the total energies of the a2 ions and the transition states for fragmentation have been carried out giving the energy barrier for fragmentation of each a2 ion. The experimental results are interpreted in terms of these energetics data, unimolecular rate constants calculated by using the RRKM theory, and the imine proton affinities.

Gas Chromatography-Mass Spectrometry↗
PubMed2004

Fragmentation of deprotonated N-benzoylpeptides: formation of deprotonated oxazolones.

The fragmentation reactions of deprotonated N-benzoyl peptides, specifically hippurylglycine, hippurylglyclyclycine, and hippurylphenylalanine (hippuryl = N-benzoylGly) have been studied using MS2 and MS3 experiments as well as deuterium labeling. A major fragment ion is observed at m/z 160 ([C9H6NO2]-) which, upon collisional activation, mainly eliminates CO2 indicating that the two oxygen atoms have become bonded to the same carbon. This observation is rationalized in terms of formation of deprotonated 2-phenyl-5-oxazolone. Various pathways to the deprotonated oxazolone have been elucidated through MS3 experiments. Fragmentation of deprotonated N-acetylalanylalanine gives a relatively weak signal at m/z 112 which, upon collisional activation, fragments, in part, by loss of CO2 leading to the conclusion that the m/z 112 ion is deprotonated 2,4-dimethyl-5-oxazolone.

Glycine↗
PubMed2004

Fragmentation of protonated oligoalanines: amide bond cleavage and beyond.

The fragmentation reactions of the singly-protonated oligoalanines trialanine to hexaalanine have been studied using energy-resolved mass spectrometry in MS(2) and MS(3) experiments. The primary fragmentation reactions are rationalized in terms of the b(x)-y(z) pathway of amide bond cleavage which results in formation of a proton-bound complex of an oxazolone and a peptide/amino acid; on decomposition of this complex the species of higher proton affinity preferentially retains the proton. For protonated pentaalanine and protonated hexaalanine the major primary fragmentation reaction involves cleavage of the C-terminal amide bond to form the appropriate b ion. The lower mass b ions originate largely, if not completely, by further fragmentation of the initially formed b ion. MS(3) energy-resolved experiments clearly show the fragmentation sequence b(n) --> b(n-1) --> b(n-2). A more minor pathway for the alanines involves the sequence b(n) --> a(n) --> b(n-1) --> b(n-2). The a(5) ion formed from hexaalanine loses, in part, NH(3) to begin the sequence of fragmentation reactions a(5) --> a(5)* --> a(4)* --> a(3)* where a(n)* = a(n) - NH(3). The a(3)* ion also is formed from the b(3) ion by the sequence b(3) --> a(3) --> a(3)* with the final step being sufficiently facile that the a(3) ion is not observed with significant intensity in CID mass spectra. A cyclic structure is proposed for the a(3)* ion.

Alanine↗
PubMed2003

Fragmentation reactions of protonated peptides containing glutamine or glutamic acid.

A variety of protonated dipeptides and tripeptides containing glutamic acid or glutamine were prepared by electrospray ionization or by fast atom bombardment ionization and their fragmentation pathways elucidated using metastable ion studies, energy-resolved mass spectrometry and triple-stage mass spectrometry (MS(3)) experiments. Additional mechanistic information was obtained by exchanging the labile hydrogens for deuterium. Protonated H-Gln-Gly-OH fragments by loss of NH(3) and loss of H(2)O in metastable ion fragmentation; under collision-induced dissociation (CID) conditions loss of H-Gly-OH + CO from the [MH - NH(3)](+) ion forms the base peak C(4)H(6)NO(+) (m/z 84). Protonated dipeptides with an alpha-linkage, H-Glu-Xxx-OH, are characterized by elimination of H(2)O and by elimination of H-Xxx-OH plus CO to form the glutamic acid immonium ion of m/z 102. By contrast, protonated dipeptides with a gamma-linkage, H-Glu(Xxx-OH)-OH, do not show elimination of H(2)O or formation of m/z 102 but rather show elimination of NH(3), particularly in metastable ion fragmentation, and elimination of H-Xxx-OH to form m/z 130. Both the alpha- and gamma-dipeptides show formation of [H-Xxx-OH]H(+), with this reaction channel increasing in importance as the proton affinity (PA) of H-Xxx-OH increases. The characteristic loss of H(2)O and formation of m/z 102 are observed for the protonated alpha-tripeptide H-Glu-Gly-Phe-OH whereas the protonated gamma-tripeptide H-Glu(Gly-Gly-OH)-OH shows loss of NH(3) and formation of m/z 130 as observed for dipeptides with the gamma-linkage. Both tripeptides show abundant formation of the y(2)'' ion under CID conditions, presumably because a stable anhydride neutral structure can be formed. Under metastable ion conditions protonated dipeptides of structure H-Xxx-Glu-OH show abundant elimination of H(2)O whereas those of structure H-Xxx-Gln-OH show abundant elimination of NH(3). The importance of these reaction channels is much reduced under CID conditions, the major fragmentation mode being cleavage of the amide bond to form either the a(1) ion or the y(1)'' ion. Particularly when Xxx = Gly, under CID conditions the initial loss of NH(3) from the glutamine containing dipeptide is followed by elimination of a second NH(3) while the initial loss of H(2)O from the glutamic acid dipeptide is followed by elimination of NH(3). Isotopic labelling shows that predominantly labile hydrogens are lost in both steps. Although both [H-Gly-Glu-Gly-OH]H(+) and [H-Gly-Gln-Gly-OH]H(+) fragment mainly to form b(2) and a(2) ions, the latter also shows elimination of NH(3) plus a glycine residue and formation of protonated glycinamide. Isotopic labelling shows extensive mixing of labile and carbon-bonded hydrogens in the formation of protonated glycinamide.

Deuterium↗
PubMed2003

Amide bond cleavage in deprotonated tripeptides: a newly discovered pathway to "b2 ions.

The fragmentation reactions of the [M-H](-) ions of the tripeptides H-Gly-Leu-Sar-OH, H-Leu-Gly-Pro-OH and H-Gly-Leu-Gly-OH have been investigated in detail using energy-resolved mass spectrometry, isotopic labelling and MS(3) experiments. It is shown that the major route to the "b(2) ions involves loss of a neutral amine from the a(3) ([M-H-CO(2)](-)) ion rather than being formed directly by fragmentation of the [M-H](-) ion. When there is no C-terminal amidic hydrogen (Sar, Pro), loss of a neutral amine is the dominant primary fragmentation reaction of the a(3) ion. However, when there is a C-terminal amidic hydrogen (Gly), elimination of the N-terminal amino acid residue is the major fragmentation reaction of the a(3) ion and formation of the "b(2) ion is greatly reduced in importance. It is proposed that the "b(2) ions are deprotonated oxazolones.

Amides↗
PubMed2003

Experimental and theoretical investigation of the main fragmentation pathways of protonated H-Gly-Gly-Sar-OH and H-Gly-Sar-Sar-OH.

The fragmentation pathways of protonated H-Gly-Gly-Sar-OH and H-Gly-Sar-Sar-OH are investigated by using both computational and experimental techniques. The main goal of these studies is to further investigate which factors determine the branching ratio of the b2-y1 (Paizs, B.; Suhai, S. Rapid Commun. Mass Spectrom. 2002, 16, 375.) and "diketopiperazine" (Cordero, M. M.; Houser, J. J.; Wesdemiotis, C. Anal. Chem. 1993, 65, 1594.) pathways of protonated tripeptides. Protonated H-Gly-Sar-Sar-OH represents a sensitive test for the branching ratio of the b2-y1 and "diketopiperazine" pathways since this ion cannot produce y1 ions on the b(-y1 channel but only b2 ions. Protonated H-Gly-Gly-Sar-OH and H-Gly-Sar-Sar-OH exhibit very different fragmentation behavior under the investigated experimental conditions. The former fragments forming mainly y1 ions (maximum abundance of the b2 and y2 ions is approximately 15%), while the latter produces mainly b2 ions while at larger internal energies the a2, y2, and y1 ions become also moderately abundant. Theoretical modeling and analysis of the main fragmentation pathways indicate that the majority of the b2 and y1 ions of protonated H-Gly-Gly-Sar-OH and the b2 ions of H-Gly-Sar-Sar-OH are formed on the b2-y1 pathway.

Chemical Phenomena↗
PubMed2002

Effect of phenylalanine on the fragmentation of deprotonated peptides.

The fragmentation reactions of a variety of deprotonated dipeptides and tripeptides containing phenylalanine have been studied using energy-resolved collision-induced dissociation, isotopic labeling and MS/MS/MS experiments. The benzyl a-group has a substantial effect on the fragmentation reactions observed. When the phenylalanine is in the C-terminal position of dipeptides or tripeptides a major fragmentation reaction is elimination of neutral cinnamic acid to from a deprotonated amino acid amide (c1 ion) for dipeptides and a deprotonated dipeptide amide (c2 ion) for tripeptides. Fragmentation of the [M - H]- ions of tripeptides with phenylalanine in the central position also results in substantial formation of the deprotonated amide of the N-terminal amino acid residue. When the phenylalanine residue is in the N-terminal position elimination of C7H8 from the [M - H - CO2]- ion and formation of the benzyl anion become important fragmentation pathways. Sequence ions frequently observed are the y1 ions, "b2 ions and a3-Nt ions.

Peptides↗