Differentiation of Boc‐ α,β‐ and β,α‐peptides and a pair of diastereomeric β,α‐dipeptides by positive and negative ion electrospray tandem mass spectrometry (ESI‐MS/MS)

Abstract

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.

Differentiation of three pairs of Boc‐β,γ‐ and γ,β‐hybrid peptides by electrospray ionization tandem mass spectrometry

“…34 -38 We have reported earlier ESI MS/MS of a series of Boc-protected carbopeptides, 34 and differentiated both positional and diastereomers. 35,36 We have also shown that MS/MS of some of the˛, -/ ,˛-37 peptides can be used to identify the -NH hydrogens that participate in the 'H' bonding leading to helical structures in solution phase. Recently, Talaty et al reported an interesting mass spectral study on the influence of the presence and position of an alternative amino acid including two nonnatural amino acids viz.ˇ-alanine (X), -aminobutyric acid (X) on the formation of b n C and y n C ions from a series of protonated tetrapeptides with general sequence XAAG, AXAG, and AAXG.…”

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)

“…The negative ion ESI of di-(11-16) tetra-(17-18) and hexa -ions [39,41,[43][44][45][46]54]. The formation of these ions may presumably be due to a 1,3-H migration from -NH-to the ‛ O of the butyloxy group leading to the elimination of tert-butanol by charge-remote fragmentation [46].…”

“…The tandem mass spectrometry (MS/MS) of protonated [21][22][23][24][25][26][27], deprotonated peptides [28][29][30][31][32][33] in electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) [34][35][36][37] have been used for structure elucidation of peptides. There are very few reports in the literature on the mass spectral study of peptides derived from non-natural amino acids [38][39][40][41][42][43][44][45][46][47][48]. We have reported ESI-MS/MS of a series of Boc-protected carbopeptides [39][40][41][42][43][44][45][46][47][48], and differentiated both positional and diastereomeric isomers [39][40][41][42][43][44][45][46]48].…”

“…There are very few reports in the literature on the mass spectral study of peptides derived from non-natural amino acids [38][39][40][41][42][43][44][45][46][47][48]. We have reported ESI-MS/MS of a series of Boc-protected carbopeptides [39][40][41][42][43][44][45][46][47][48], and differentiated both positional and diastereomeric isomers [39][40][41][42][43][44][45][46]48]. Recently, we have reported a study on the effect of Nterminal β-and γ-carbo amino acids on the fragmentation of γ-amino butyric acid (γ-Abu) containing hybrid peptides [46].…”

Canα- andβ-alanine containing peptides Be distinguished based on the CID spectra of their protonated ions?

“…In contrast, despite the growing interest in the use of ␤-amino acids in constructing peptides that are protease resistant [8] or that have unique architectures [9], the gas-phase chemistry of protonated ␤-amino acids and their peptides have received much less attention. Key exceptions include studies that have shown that ␤-alanine has a higher proton affinity than ␣-alanine [10]; protonated ␣-and ␤-alanine (1 and 2; see Scheme 1) fragment via different pathways [11]; the relative abundances of sequence ions in ␣-and ␤-alanine containing peptides can be different [12]; and ␤-alanine peptides can be identified via CID of the Boc-protected derivatives [13]. Furthermore, Seebach's group has shown that tandem mass spectrometry of protonated ␤-peptides can be used to confirm their structures via the formation of sequence ions [14].…”