Fragmentation chemistry of [M + Cu]+ peptide ions containing an N-terminal arginine

[M + Cu]+ peptide ions formed by matrix-assisted laser desorption/ionization from direct desorption off a copper sample stage have sufficient internal energy to undergo metastable ion dissociation in a time-of-flight mass spectrometer. On the basis of fragmentation chemistry of peptides containing an N-terminal arginine, we propose the primary Cu+ ion binding site is the N-terminal arginine with Cu+ binding to the guanidine group of arginine and the N-terminal amine. The principal decay products of [M + Cu]+ peptide ions containing an N-terminal arginine are [a(n) + Cu - H]+ and [b(n) + Cu - H]+ fragments. We show evidence to suggest that [a(n) + Cu - H]+ fragment ions are formed by elimination of CO from [b(n) + Cu - H]+ ions and by direct backbone cleavage. We conclude that Cu+ ionizes the peptide by attaching to the N-terminal arginine residue; however, fragmentation occurs remote from the Cu+ ion attachment site involving metal ion promoted deprotonation to generate a new site of protonation. That is, the fragmentation reactions of [M + Cu]+ ions can be described in terms of a "mobile proton" model. Furthermore, proline residues that are adjacent to the N-terminal arginine do not inhibit formation of [b(n) + Cu - H]+ ion, whereas proline residues that are distant to the charge carrying arginine inhibit formation of [b(n) + Cu - H]+ ions. An unusual fragment ion, [c(n) + Cu + H]+, is also observed for peptides containing lysine, glutamine, or asparagine in close proximity to the Cu+ carrying N-terminal arginine. Mechanisms for formation of this fragment ion are also proposed.     

Influence of the labeling group on ionization and fragmentation of carbohydrates in mass spectrometry

The ionization and fragmentation behaviors of carbohydrate derivatives prepared by reaction with 2-aminobenzamide (AB), 1-phenyl-3-methyl-5-pyrazolone (PMP), and phenylhydrazine (PHN) were compared under identical mass spectrometric conditions. It has been shown that the intensities of signals in MS spectra depend on the kind of saccharides investigated and reducing end labels used. PMP sialyllactose, when ionized by ESI/MALDI, produced a mixture of [M + H]+, [M + Na]+, [M - H + 2Na]+ ions in the positive mode and [M - H]-, [M + Na - 2H]- ions in the negative mode. The AB and PHN derivatives formed abundant [M + H]+ and [M - H]- ions in ESI, and by matrix-assisted laser desorption/ionization (MALDI) produced abundant [M + Na]+ ions. PMP- and reduced AB-sialyllactose produced only Y-type fragment ions under both MS/MS sources. In the electrospray ionization (ESI)-MS/MS spectrum of PHN-sialyllactose, abundant ions corresponded to B, Z cleavages and in its MALDI-MS/MS spectrum, the abundant ions were consistent with Y glycosidic cleavages with the concurrence of B, C, and cross-ring fragment ions. In the MALDI-MS spectra of oligosaccharides acquired immediately after derivatization, it was possible to detect only PHN derivatives. After purification, spectra of all three types of derivatives showed high signal-to-noise ratios with the most abundant ions observed for AB reduced saccharides. [M + Na]+ ions were the dominant products and their fragmentation patterns were influenced by the type of the labeling and the kind of oligosaccharide considered. In the MALDI-PSD and -MS/MS spectra of AB-derivatized glycans, higher m/z fragment ions corresponded to B and Y cleavages and the loss of bisecting GlcNAc appeared as a weak signal or was not detected at all. Fragmentation patterns observed in the spectra of hybrid/complex PHN and PMP glycans were more comparable-higher m/z fragments corresponded to B and C glycosidic cleavages. For PHN glycans, the abundance of ions resulting from the loss of bisecting GlcNAc depended on the number of residues linked to the 6-positioned mannose. Also, PHN and PMP derivatives produced cross-ring cleavages with abundances higher than observed in the spectra of AB derivatized oligosaccharides. For high-mannose glycans, the most informative cleavages were provided by AB and PHN type of labeling. Here, PMP produced dominant Y-cleavages from the chitobiose while other ions produced weak signals.     

Low-energy collisionally activated decomposition and structural characterization of cyclic heptapeptide microcystins by electrospray ionization mass spectrometry

Characteristics of electrospray ionization mass spectrometry/collision-induced dissociation (ESIMS/CID) mass spectra of microcystins, cyanobacterial cyclic heptapeptide hepatoxins, were examined. The collision conditions showed remarkable effects on the quality of the CID mass spectra, which were divided into three patterns according to the number of Arg residues. A characteristic cleavage reaction and neutral losses of MeOH, NH3 and guanidine group(s) from the (2S,3S,8S,9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4 E,6E-dienoic acid (Adda) and Arg residues were observed in the ESI and ESIMS/CID mass spectra, suggesting the most probable protonation sites in [M + H]+ and [M + 2H]2+ ions of microcystins. Microcystins with no Arg residue showed only [M + H]+ ions with a proton reacting at the methoxyl group in the Adda residue, and the ESIMS/CID/MS data revealed their structures unambiguously. The protonation site in [M + H]+ ions of microcystins with Arg residue(s) was the guanidine group. The [M + 2H]2+ ions of microcystins possessing one Arg residue had one proton on the Arg residue and probably another proton on the Adda residue, while the [M + 2H]2+ ions of microcystins having two Arg residues showed protonation at both Arg residues and the ESIMS/CID/MS data assigned their sequences. Structures of microcystins possessing one Arg residue can be assigned by ESIMS/CID/MS of [M + H]+ ions combined with those of [M + 2H]2+ ions.     

Comparison of intensities between doubly charged ions [M + 2H]2+ and singly charged ions [M + H]+ of gramicidin S by electrospray mass spectrometry.

The reason why the intensity of doubly charged ions [M + 2H]2+ of gramicidin S is higher than that of singly charged ions [M + H]+ in electrospray is investigated by ion evaporation theory. As a result of comparison between the total free energies of extracting [M + 2H]2+ and [M + H]+ from a charged droplet to infinity, it is found that the total free energy of [M + 2H]2+ is estimated to be lower than that of [M + H]+. This clearly supports the experimental result. In addition, the importance of the electrostatic contribution in electrospray is demonstrated by showing the result that the total free energy of [M + 2H]2+ without electrostatic contribution is higher than that of [M + H]+.     

Determination of the fatty acyl profiles of phosphatidylethanolamines by tandem mass spectrometry of sodium adducts

Phosphatidylethanolamines (PEs) are one of the major constituents of cellular membranes, and, along with other phospholipid classes, have an essential role in the physiology of cells. Profiling of phospholipids in biological samples is currently done using mass spectrometry (MS). In this work we describe the MS fragmentation of sodium adducts of 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphatidylethanolamine (POPE) and 2-linoleoyl-1-palmitoyl-sn-glycero-3-phosphatidylethanolamine (PLPE). This study was performed by electrospray ionization tandem mass spectrometry (ESI-MS/MS) using three different instruments and also by matrix-assisted laser desorption/ionization tandem mass spectrometry (MALDI-MS/MS). All MS/MS spectra show product ions related to the polar head fragmentation and product ions related to the loss of acyl chains. In ESI-MS/MS spectra, the product ions [M+Na-R1COOH-43]+ and [M+Na-R2COOH-43]+ show different relative abundance, as well as [M+Na-R1COOH]+ and [M+Na-R2COOH]+ product ions, allowing identification of both fatty acyl residues of PEs, and their specific location. MALDI-MS/MS shows the same product ions reported before and other ions generated by charge-remote fragmentation of the C3-C4 bond (gamma-cleavage) of fatty acyl residues combined with loss of 163 Da. These fragment ions, [M+Na-(R2-C2H3)-163]+ and [M+Na-(R1-C2H3)-163]+, show different relative abundances, and the product ion formed by the gamma-cleavage of sn-2 is the most abundant. Overall, differences noted that are important for identification and location of fatty acyl residues in the glycerol backbone are: relative abundance between the product ions [M+Na-R1COOH-43]+ > [M+Na-R2COOH-43]+ in ESI-MS/MS spectra; and relative abundance between the product ions [M+Na-(R2-C2H3)-163]+ > [M+Na-(R1-C2H3)-163]+ in MALDI-MS/MS spectra.     

Elucidation of O-Phosphoryl and N-Phosphoryl Amino Acids by Electrospray Ionization Tandem Mass Spectrometry

Mass spectroscopic characteristics of phosphoryl amino acids were studied in detail by positive and negative electrospray ionization mass spectrometry (ESI-MS) in conjunction with tandem mass spectrometry (MS/MS). Besides N-diisopropyloxyphosphoryl amino acids (N-DIPP-AA), O-phospho- and O-diisopropyloxyphosphoryl amino acids (O-DIPP-AA) were studied and compared to N-DIPP-AA. The fragmentation pathways of [M H]^ ,[M Na]^ and [M-H]^- ions of phosphoryl amino acids were summarized. In addition to several similar patterns, each of them showed its characteristic fragmention.     

Analysis of neutral oligosaccharides for structural characterization by matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight mass spectrometry

We have acquired multi-stage mass spectra (MSn) of four branched N-glycans derived from human serum IgG by matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight mass spectrometry (MALDI-QIT-TOF-MS) in order to demonstrate high sensitivity structural analysis. [M+H]+ and [M+Na]+ ions were detected in the positive mode. The detection limit of [M+Na]+ in MS/MS and MS3 measurements for structural analysis was found to be 100 fmol, better than that for [M+H]+. The [M+H]+ ions subsequently fragmented to produce predominantly a Y series of fragments, whereas [M+Na]+ ions fragmented to give a complex mixture of B and Y ions together with some cross-ring fragments. Three features of MALDI-QIT-CID fragmentation of [M+Na]+ were cleared by the analysis of MS/MS, MS3 and MS4 spectra: (1) the fragment ions resulting from the breaking of a bond are more easily generated than that from multi-bond dissociation; (2) the trimannosyl-chitobiose core is either hardly dissociated, easily ionized or it is easy to break a bond between N-acetylglucosamine and mannose; (3) the fragmentation by loss of only galactose from the non-reducing terminus is not observed. We could determine the existence ratios of candidates for each fragment ion in the MS/MS spectrum of [M+Na]+ by considering these features. These results indicate that MSn analysis of [M+Na]+ ions is more useful for the analysis of complicated oligosaccharide structures than MS/MS analysis of [M+H]+, owing to the higher sensitivity and enhanced structural information. Furthermore, two kinds of glycans, with differing branch structures, could be distinguished by comparing the relative fragment ion abundances in the MS3 spectrum of [M+Na]+. These analyses demonstrate that the MSn technology incorporated in MALDI-QIT-TOF-MS can facilitate the elucidation of structure of complex branched oligosaccharides.     

两种氨基酸中[MH-CO2H2]+的特征质谱碎裂

运用低能碰撞诱导解离（ＣＩＤ）研究了电子轰击（ＥＩ）、快原子轰击（ＦＡＢ）电离条件下质子化亮氨酸与异亮氨酸解离产生亚稳离子「ＭＨ－ＣＯ２Ｈ２」的单分子质谱破裂，二种异构体呈现出了各自不同的解离特征，根据ＣＩＤ的特征碎片离子和氘代同位素标记实验，提出了其破裂过程的存在离子／中性（碎片）复合物中间体机理，并对有关的特征离子的形成进行了讨论。     

Comparative studies of 193-nm photodissociation and TOF-TOFMS analysis of bradykinin analogues: The effects of charge site(s) and fragmentation timescales

The dissociation reactions of [M + H]+, [M + Na]+, and [M + Cu]+ ions of bradykinin (amino acid sequence RPPGFSPFR) and three bradykinin analogues (RPPGF, RPPGFSPF, PPGFSPFR) are examined by using 193-nm photodissociation and post-source decay (PSD) TOF-TOF-MS techniques. The photodissociation apparatus is equipped with a biased activation cell, which allows us to detect fragment ions that are formed by dissociation of short-lived (<1 mus) photo-excited ions. In our previously reported photodissociation studies, the fragment ions were formed from ions dissociating with lifetimes that exceeded 10 mus; thus these earlier photofragment ion spectra and post-source decay (PSD) spectra [composite of both metastable ion (MI) and collision-induced dissociation (CID)] were quite similar. On the other hand, short-lived photo-excited ions dissociate by simple bond cleavage reactions and other high-energy dissociation channels. We also show that product ion types and abundances vary with the location of the charge on the peptide ion. For example, H+ and Na+ cations can bind to multiple polar functional groups (basic amino acid side chains) of the peptide, whereas Cu+ ions preferentially bind to the guanidino group of the arginine side-chain and the N-terminal amine group. Furthermore, when Cu+ is the charge carrier, the abundances of non-sequence informative ions, especially loss of small neutral molecules (H2O and NH3) is decreased for both photofragment ion and PSD spectra relative to that observed for [M + H]+ and [M + Na]+ peptide ions.     

Collisional activation of ions formed by [Li]+ ion attachment

Since no unimolecular fragmentation is observed with [M+Li]⁺ ions under normal operating conditions the collisional activation method was used to study the fragmentation behaviour of these ions. It was found that the liberation of the [Li]⁺ ion is a dominant process only with smaller molecules. In addition, direct bond cleavages and new types of rearrangement reactions lead to fragment ions in which the lithium is normally retained. The decomposition behaviour of [M+Li]⁺ ions represents an intermediate case between that of [M]+ ions and excited neutral molecules and is quite different from that of [M+H]⁺ ions.     

Structural characterization of isomeric triazolocoumarins by high- and low-energy collision spectrometry of M+˙, [M + H]+ and [M − H]− species

Two monometayl- and four dimethyl-triazolocoumarin isomers were characterized by their electron impact mass spectra and by low-energy collision experiments performed on molecular ions M⁺˙ and other fragment ions with an ion-trap mass spectrometer. High-energy collision-activated dissociation measurements were performed on the protonated [M + H]⁺ and deprotonated [M ? H]^? molecular ion obtained by fast atom bombardment and M⁺˙ species produced by electron impact ionization on a double-focusing, reverse-geometry instrument. The data obtained allowed unequivocal structural identification of all the compounds investigated.     

Differentiation of Boc- alpha,beta- and beta,alpha-peptides and a pair of diastereomeric beta,alpha-dipeptides by positive and negative ion electrospray tandem mass spectrometry (ESI-MS/MS)

Positive and negative ion electrospray ionization (ESI) tandem mass spectral study of a new series of hybrid peptides, viz, BocN-alpha,beta-peptides and BocN-beta,alpha-peptides, synthesized from C-linked carbo-beta3-amino acids [Caa (S)] and L-Ala has been carried out. The alpha,beta-peptides have been differentiated from beta,alpha-peptides by the collision-induced dissociation (CID) of [M + H]+ and [M - H]- ions in positive and negative ion ESI-MS respectively. The fragment ion [M + H - C(CH3)3 + H]+ formed from [M + H]+ ions by the loss of 2-methyl-prop-2-ene in alpha,beta-peptides with L-Ala at the N-terminus is insignificant or totally absent for beta,alpha-peptides which have the Caa (S) at N-terminus. The fragment ion [M - H-C(CH3)3OH - HNCO]- formed from [M - H]- of beta,alpha-peptide acids is totally absent for alpha,beta-peptide acids. This has been attributed to the absence of the beta-methylene group in alpha,beta-peptides, and the participation of the beta-methylene group in the loss of HNCO in beta,alpha-peptide acids is confirmed by the deuteration experiments. The CID of [M + H-Boc + H]+ ions of these peptides also produce characteristic fragmentation. In the CID spectra of alpha,beta-peptides, the b(n)+ ions and the resulting y(n)+ ions occur at a mass difference of 243 and 71 Da corresponding to the successive losses of Caa and L-Ala, whereas a mass difference of 71 and 243 Da is observed for beta,alpha-peptides. In contrast to the CID of protonated peptides, the CID of [M - H]- ions of the alpha,beta- and beta,alpha-peptide acids do not give b(n)- ions and show abundant z(n) (-) ions. Further, a pair of diastereomeric dipeptide esters and acids have been distinguished by the CID of [M + H]+ ions. The loss of 2-methyl-prop-2-ene is more pronounced for Boc-NH-Caa(R)-D-Ala-OCH3 (21) and Boc-NH-Caa(R)-D-Ala-OH (23) with Caa (R) at the N-terminus, whereas it is totally absent for Boc-NH-Caa (S)-D-Ala-OCH3 (22) and Boc-NH-Caa(S)-D-Ala-OH (24) peptides, which have Caa (S) at the N-terminus. Thus, on the basis of our previous and present studies, we propose that the CID of [M + H]+ ions provides a simple and useful method for distinguishing the configuration of Caa (S or R) at the N-terminus of BocN-carbo beta,alpha- and beta,beta-dipeptides.     

Matrix-assisted laser desorption/ionization time-of-flight and nano-electrospray ionization ion trap mass spectrometric characterization of 1-cyano-2-substituted-benz[f]isoindole derivatives of peptides for fluorescence detection

A series of hexa- to decapeptides (molecular mass range 800-1200) were labeled with naphthalene-2,3-dicarboxaldehyde, which preferentially reacts with the primary amino groups of a peptide. A highly stable peptide conjugate is formed, which allows selective analysis by fluorescence at excitation and emission wavelengths of 420 and 490 nm, respectively. After removal of unreacted compounds, the peptide conjugates were characterized by matrix-assisted laser desorption/ionization (MALDI) time-of-flight and nano-electrospray ionization (ESI) ion trap mass spectrometry. They readily form both [M + H]+ ions by MALDI and both [M + H]+ and [M + 2H]2+ ions by ESI. Furthermore, the fragmentation behavior of the N-terminally tagged peptides, exhibiting an uncharged N-terminus, was investigated applying post-source decay fragmentation with a curved field reflector and collision-induced dissociation with a quadrupole ion trap. Fragmentation is dominated in both cases by series of a-, b- and y-type ions and [M + H - HCN]+ ions. Peptide bonds adjacent to the fluorescence label were less susceptible to cleavage than the bonds of the non-derivatized peptide ions. In general, the resulting fragment ion patterns were less complex than those of the underivatized peptides.     

FAB and chemical ionization in the production of [M+ H]+ and [M− H]− species from 2-aryl-2-methyl-1,3-dithianes

The unimolecular fragmentations of [M + H]⁺ and [M – H]^? ions from four 2-aryl-2-methyl-1,3-dithianes are described and clarified with the aid of deuterated derivatives. Comparison of the MIKE spectra of [M + H]⁺ species obtained under chemical ionization and fast atom bombardment (FAB) conditions reveals differences which are attributed to the different energetics involved in the two ionization processes. It is suggested that FAB is a ‘softer’ ionization technique but, at the same time, it provides, for the possibility of solvation, reaction sites not available in gas-phase protonation. [M – H]^? species and anionic fragments thereof were generally not obtained under FAB(?) conditions. [M – H]^? ions are readily produced in gas-phase reactions with OH^? via proton abstraction from C(4) or C(5), and from the 2-methyl substituent; and they fragment according to several reaction pathways.     

[M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

The N₂ negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]^?˙. This addition is also observed in the N₂ positive chemical ionization (PCI) mass spectra. An ion at [M – 15]^? is found in the NICI spectra of aminoaromatics such as aniline, 1- and 2-aminonaphthalene and 1- and 2-aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]^?˙, [M – 15]^?˙ and [M + 11]^+˙ ions result from the ionization of analytes altered by surface-assisted reactions. Experiments with ¹⁵N₂, [¹⁵N] aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆] aniline show that the [M + 11]^?˙ ion corresponds to [M + N – 3H]^?˙. The added nitrogen originates from the N₂ buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N₂ PCI mass spectrum of aniline suggest that the neutral product of the surface-assisted reaction is 1,4-dicyanobuta-1,3-diene. Experiments with diamino-substituted aromatics show analogous reactions resulting in the formation of [M – 4H]^?˙ ions for aromatics with ortho-amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.     

A new copper containing MALDI matrix that yields high abundances of [Peptide + Cu]+ ions

The dinuclear copper complex (α-cyano-4-hydroxycinnamic acid (CHCA) copper salt (CHCA)₄Cu₂), synthesized by reacting CHCA with copper oxide (CuO), yields increased abundances of [M + xCu − (x−1)H]⁺ (x = 1–6) ions when used as a matrix for matrix-assisted laser desorption ionization (355 nm Nd:YAG laser). The yield of [M + xCu − (x−1)H]⁺ (x = 1∼6) ion is much greater than that obtained by mixing peptides with copper salts or directly depositing peptides onto oxidized copper surfaces. The increased ion yields for [M + xCu − (x−1)H]⁺ facilitate studies of biologically important copper binding peptides. For example, using this matrix we have investigated site-specific copper binding of several peptides using fragmentation chemistry of [M + Cu]⁺ and [M + 2Cu − H]⁺ ions. The fragmentation studies reveal interesting insight on Cu binding preferences for basic amino acids. Most notable is the fact that the binding of a single Cu⁺ ion and two Cu⁺ ions are quite different, and these differences are explained in terms of intramolecular interactions of the peptide-Cu ionic complex.     

Origin of the [M − H]+ ion in flavanone

Studies of the genesis of the [M ? H]⁺ ion in flavanone and 2′-hydroxychalcone, performed with the aid of metastable decompositions and deuterium labelling, allow new structural notations to be postulated for the [M ? H]+ ions, which in turn provide evidence for the pathways in the [M ? H – ketene]+ fragmentation routes for these compounds.     

Establishment of mass spectrometric fingerprints of novel synthetic cholesteryl neoglycolipids: The presence of a unique <Emphasis Type="Italic">C</Emphasis>-glycoside species during electrospray ionization and during collision-induced dissociation tandem mass spectrometry

In this study we evaluated the fragmentation pattern of 16 novel amphiphilic neoglycolipid cholesteryl derivatives that can be efficiently used to increase cationic liposomal stability and to enhance gene transfer ability. These neoglycolipids bear different sugar moieties, such as D-glucosamine, N-acetyl-D-glucosamine, N-trideuterioacetyl-D-glucosamine, N-acetyllactosamine, L-fucose, N-allyloxycarbonyl-D-glucosamine, and some of their per-O-acetylated derivatives. Regardless of the structure of the tested neoglycolipid, QqToF-MS analysis using electrospray ionization (ESI) source showed abundant protonated [M+H]+ species. We also identified by both QqToF-MS and low-energy collision tandem mass spectrometry (CID-MS/MS) of the [M+H]+ ion, the presence of specific common fingerprint fragment ions: [Cholestene]+, sugar [oxonium]+, [(Sugar-spacer-OH)+H]+, [oxonium-H2O]+, and [(Cholesterol-spacer-OH)+H]+. In addition, we observed a unique ion that could not be rationally explained by the expected fragmentation of these amphiphilic molecules. The structure of this ion was tentatively proposed with that of a C-glycoside species formed by a chemical reaction between the sugar portion and the cholesterol. MS/MS analysis of this unique [C-glycoside]+ confirmed the validity of the proposed structure of this ion. The presence of an amino group at position C-2 and free hydroxyl groups of the sugar motif is crucial for the formation of a "reactive" sugar oxonium ion that can form the [C-glycoside]+ species. In summary, we precisely established the fragmentation patterns of the tested series of neoglycolipid cholesteryl derivatives and authenticated their structure as well; moreover, we speculated on the formation of a C-glycoside with the ESI source under atmospheric pressure and in the collision cell during MS/MS analysis.     

Tandem mass spectra of transition-metal ion adducts of glycosyl dithioacetals; distinction among stereoisomers

Electrospray ionization mass spectra of some glycosyl dithioacetals recorded in the presence of transition-metal chlorides, XCl2 (where X = Co, Mn and Zn), give abundant adduct ions such as [M+XCl]+ and [2M-H+X]+ and minor ions such as [M-H+X]+ and [2M+XCl]+. The tandem mass spectra of these adducts show characteristic elimination of neutral molecules such as H2O, HCl, EtSH, CH2O, C2H4O2/C2H4O. [M+XCl]+ ions fragment readily and the fragmentation appears to be stereochemically controlled as the relative abundances of the fragments are different for three stereoisomers. The added metal is lost as neutral molecules in the form of XCl(OH) and XCl(SEt). This is a predominant pathway in the ZnCl+ adducts. [2M+XCl]+ ions fragment preferentially by elimination of HCl, indicating strong metal interactions in the resulting dimeric [2M-H+X]+ ion. As there are several electron-rich centers in the molecule, the dimeric complex [2M-H+X]+ can have several structures and the observed fragmentations may reflect the sum of those of all these structures. The dimeric complexes fragment by elimination of neutral molecules leaving the dimeric interactions intact. The extent of fragmentation varies for the stereoisomers, leading to stereochemical differentiation.     

Rapid identification of Mg-chelated chlorins by on-line high performance liquid chromatography/atmospheric pressure chemical ionization mass spectrometry

On-line high performance liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (HPLC/APCI-MS) has been applied to the identification of a number of tetrapyrrolic pigments as well as several magnesium-free analogs. Mass spectra, acquired both in positive (+) and negative (-) ionization mode, allow not only the determination of the structural features of the pigments, but also a very easy differentiation of the Mg-chelated pigments from the free bases. In the positive ionization mode, all pigments show mainly a [M + H]+ ion and a [M + H - C20H38]+ fragment ion corresponding to the loss of the phytyl chain via a hydrogen rearrangement. In the negative ionization mode, on the other hand, although all pigments give an abundant molecular anion [M]-*, only the Mg-chelated chlorin spectra show a prominent fragment [M - C20H39]- produced by a formal loss of the phytyl radical.