Mass spectral study of alkali-cationized Boc-carbo-beta3-peptides by electrospray tandem mass spectrometry

Electrospray ionization tandem mass spectrometric study of protonated and alkali- cationized α/ε-hybrid peptides: differentiation of a pair of dipeptide positional isomers

A new class of Boc-N-protected hybrid peptides derived from L- Ala and ε⁶-Caa (L-Ala = L-Alanine, Caa = C-linked carboamino acid derived from D-xylose) have been studied by positive ion electrospray ionization (ESI) ion-trap tandem mass spectrometry (MS/MS). MSⁿ spectra of protonated and alkali-cationized hybrid peptides produce characteristic fragmentation involving the peptide backbone, the tert-butyloxycarbonyl (Boc) group, and the side chain. The dipeptide positional isomers are differentiated by the collision-induced dissociation (CID) of the protonated and alkali-cationized peptides. The CID of [M + H]⁺ ion of Boc-NH-L-Ala-ε-Caa- OCH₃ (1) shows a prominent [M + H - C₄H₈]⁺ ion, which is totally absent for its positional isomer Boc-NH-ε-Caa-L-Ala-OCH₃ (6), which instead shows significant loss of t-butanol. The formation of the [M + Cat - C₄H₈]⁺ ion is totally absent and [M + Cat - Boc + H]⁺ is prominent in the CID of the [M + Cat]⁺ ion of Boc-NH-L-Ala-ε-Caa- OCH₃ (1), whereas the former is highly abundant and the latter is of low abundance for its positional isomer Boc-NH-ε-Caa-L-Ala-OCH₃ (6). It is observed that 'b' ions are abundant when oxazolone structures are formed through a five-membered cyclic transition state in tetra-, penta-, and hexapeptides and the cyclization process for larger 'b' ions led to an insignificant abundance. However, the significant 'b' ion is formed in ε,α-dipeptide, which may have a seven-membered substituted 2-oxoazepanium ion structure. The MSⁿ spectra of [M + Cat - Boc + H]⁺ ions of these peptides are found to be significantly different to those of [M + H - Boc + H]⁺ ions. The CID spectra of [M + Cat - Boc + H]⁺ ions of peptide acids containing L-Ala at the C-terminus show an abundant N-terminal rearrangement ion, [bⁿ + 17 + Cat]⁺, which is absent for the peptide acids containing ε-Caa at the C-terminus. Thus, the results of these hybrid peptides provide sequencing information, the structure of the cyclic intermediate involved in the formation of the rearrangement ion, and distinguish a pair of dipeptide positional isomers.

Mass spectrometry in India

This review emphasizes the mass spectrometry research being performed at academic and established research institutions in India. It consists of three main parts covering the work done in organic, atomic and biological mass spectrometry. The review reveals that the use of mass spectrometry techniques started in the middle of the 20th century and was applied to research in the fields of organic, nuclear, geographical and atomic chemistry. Later, with the advent of soft and atmospheric ionization techniques it has been applied to pharmaceutical and biological research. In due course, several research centers with advanced mass spectrometry facilities have been established for specific areas of research such as gas-phase ion chemistry, ion-molecule reactions, proscribed chemicals, pesticide residues, pharmacokinetics, protein/peptide chemistry, nuclear chemistry, geochronological studies, archeology, petroleum industry, proteomics, lipidomics and metabolomics. Day-by-day the mass spectrometry centers/facilities in India have attracted young students for their doctoral research and other advanced research applications.

Differentiation of α- or β-aspartic isomers in the heptapeptides by the fragments of [M + Na]+ using ion trap tandem mass spectrometry

Electrospray ionization coupled with low energy collision induced dissociation (CID) in an ion trap mass spectrometer was used to examine the fragmentation patterns of the [M + Na](+) of eight pairs of heptapeptides containing α- or β-Asp residues in second and sixth amino acid positions, respectively. Selective cleavages at the peptide backbone C-terminal to two Asp residues were observed, which generated a series of C-terminal y(5) ions and N-terminal b(6) ions. Two typical ions: [y5 + Na - H]+ and [b6 + Na + OH]+, produced by α-Asp containing peptides were noted to be much more abundant than those of the peptides with β-Asp, which could be used for distinction of the isomers in Asp2 and Asp6, respectively. In addition, a series of internal ions generated by simultaneous cleavages at Asp residues were detected. Competitive reactions of carboxylic groups occurred between Asp6 side chain and C-terminus. Formation mechanisms of most product ions are proposed. The results obtained in this work are significant since low energy CID has been demonstrated to be effective for the distinction of Asp isomers.

Differentiation of positional isomers of hybrid peptides containing repeats of β-nucleoside derived amino acid (β-Nda-) and L-amino acids by positive and negative ion electrospray ionization tandem mass spectrometry (ESI-MSn)

A new class of positional isomeric pairs of -Boc protected oligopeptides comprised of alternating nucleoside derived β-amino acid (β-Nda-) and L-amino acid residues (alanine, valine, and phenylalanine) have been differentiated by both positive and negative ion electrospray ionization ion-trap tandem mass spectrometry (ESI-MS(n)). The protonated dipeptide positional isomers with β-Nda- at the N-terminus lose CH(3)OH, NH(3), and C(2)H(4)O(2), whereas these processes are absent for the peptides with L-amino acids at the N-terminus. Instead, the presence of L-amino acids at the N-terminus results in characteristic retro-Mannich reaction involving elimination of imine. A good correlation has been observed between the conformational structure of the peptides and the abundance of y(n)(+) and b(n)(+) ions in MS(n) spectra. In the case of tetrapeptide isomers that are reported to form helical structures in solution phase, no y(n)(+) and b(n)(+) ions are observed when the corresponding amide -NH- participates in the helical structures. In contrast, significant y(n)(+) and b(n)(+) ions are formed when the amide -NH- is not involved in the H-bonding. In the case of tetra- and hexapeptides, it is observed that abundant b(n)(+) ions are formed, presumably with stable oxazolone structures when the C-terminus of the b(n)(+) ions possessed L-amino acid and the β-Nda- at the C-terminus appears to prevent the cyclization process leading to the absence of corresponding b(n)(+) ions.

Characterization of Nα-Fmoc-protected ureidopeptides by electrospray ionization tandem mass spectrometry (ESI-MS/MS): differentiation of positional isomers

Four pairs of positional isomers of ureidopeptides, FmocNH-CH(R(1))-ϕ(NH-CO-NH)-CH(R(2))-OY and FmocNH-CH(R(2))-ϕ(NH-CO-NH)-CH(R(1))-OY (Fmoc = [(9-fluorenyl methyl)oxy]carbonyl; R(1) = H, alkyl; R(2) = alkyl, H and Y = CH(3)/H), have been characterized and differentiated by both positive and negative ion electrospray ionization (ESI) ion-trap tandem mass spectrometry (MS/MS). The major fragmentation noticed in MS/MS of all these compounds is due to --N--CH(R)--N--bond cleavage to form the characteristic N- and C-terminus fragment ions. The protonated ureidopeptide acids derived from glycine at the N-terminus form protonated (9H-fluoren-9-yl)methyl carbamate ion at m/z 240 which is absent for the corresponding esters. Another interesting fragmentation noticed in ureidopeptides derived from glycine at the N-terminus is an unusual loss of 61 units from an intermediate fragment ion FmocNH = CH(2) (+) (m/z 252). A mechanism involving an ion-neutral complex and a direct loss of NH(3) and CO(2) is proposed for this process. Whereas ureidopeptides derived from alanine, leucine and phenylalanine at the N-terminus eliminate CO(2) followed by corresponding imine to form (9H-fluoren-9-yl)methyl cation (C(14)H(11) (+)) from FmocNH = CHR(+). In addition, characteristic immonium ions are also observed. The deprotonated ureidopeptide acids dissociate differently from the protonated ureidopeptides. The [M - H](-) ions of ureidopeptide acids undergo a McLafferty-type rearrangement followed by the loss of CO(2) to form an abundant [M - H - Fmoc + H](-) which is absent for protonated ureidopeptides. Thus, the present study provides information on mass spectral characterization of ureidopeptides and distinguishes the positional isomers.

Diastereomeric differentiation of norbornene amino acid peptides by electrospray ionization tandem mass spectrometry

A new class of diastereomeric pairs of non-natural amino acid peptides derived from butyloxycarbonyl (Boc-)protected cis-(2S,3R)- and trans-(2S,3S)-beta-norbornene amino acids including a monomeric pair have been investigated by electrospray ionization (ESI) tandem mass spectrometry using quadrupole time-of-flight (Q-TOF) and ion-trap mass spectrometers. The protonated cis-BocN-beta-nbaa (2S,3R) (1) (betanbaa = beta-norbornene amino acid) eliminates the Boc group to form [M+H-Boc+H](+), whereas an additional ion [M+H-C(4)H(8)](+) is formed from trans-BocN-beta-nbaa (2S,3S) (2). Similarly, it is observed that the peptide diastereomers (di-, tri- and tetra-), with cis-BocN-beta-nbaa (2S,3R)- at the N-terminus, initially eliminate the Boc group to form [M+H-Boc+H](+) which undergo further fragmentation to give a set of product ions that are different for the peptides with trans-BocN-beta-nbaa (2S,3S)- at the N-terminus. Thus the Boc group fragments differently depending on the configuration of the amino acid present at the N-terminus. It is also observed that the peptide bond cleavage in these peptides is less favoured and most of the product ions are formed due to retro-Diels-Alder fragmentation. Interestingly, sodium-cationized peptide diastereomers mainly yield a series of retro-Diels-Alder fragment ions which are different for each diastereomer as they are formed starting from [M+Na-Boc+H](+) in peptides with cis-BocN-beta-nbaa (2S,3R)- at the N-terminus, and [M+Na-C(4)H(8)](+) in peptides with trans-BocN-beta-nbaa (2S,3S)- at the N-terminus. All these results clearly indicate that these diastereomeric pairs of peptides yield characteristic product ions which help distinguish the isomers.

Electrospray tandem mass spectrometry of alkali-cationized BocN-carbo-alpha,beta- and -beta,alpha-peptides: Differentiation of positional isomers

Dissociation pathways of a series of alkali-cationized hybrid peptides, viz., Boc-alpha,beta- and -beta,alpha-carbopeptides, synthesized from C-linked carbo-beta3-amino acids [Caa (S)] and alpha-alanine (L-Ala), have been investigated by electrospray ionization tandem mass spectrometry. The positional isomers (six pairs) of the cationized alpha,beta- and beta,alpha-peptides can be differentiated by the collision-induced dissociation (CID) spectra of their [M + Cat-Boc + H]+ ions which give characteristic series of alkali-cationized C- (x(n)+, y(n)+, z(n)+) and N-terminal (a(n)+, b(n)+, c(n)+) ions. Another noteworthy difference is cationized beta,alpha-peptides eliminate a molecule of ammonia whereas this pathway is absent for alpha,beta-peptides. This is useful for identifying the presence of a beta-amino acid at the N-terminus. The CID spectra of [M + Cat-Boc + H]+ ions of these peptide acids show abundant rearrangement [b(n) + 17 + Cat]+ (n = 1 to n-1) ions which is diagnostic for distinguishing between alpha- and beta-amino acid at the C-terminus. MS(n) experiments of [b(n) + Li-H]+ ions from these hybrid peptides showed the loss of CO and 72 u giving rise to [a(n) + Li-H]+ and cationized nitrile product ions which render support to earlier proposals that b(n)+ or [b(n) + Cat-H]+ ions have protonated or cationized oxazolinone structures, respectively.