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.     

Collision-Induced dissociations of deprotonated peptides. Dipeptides containing aspartic or glutamic acids

Deprotonated dipeptides, on collisional activation, fragment by the characteristic process NH₂CH(R¹) CONHCH(R²)CO₂^? → NH₂^?C(R¹)CONHCH(R²)CO₂H → ^?NHCH(R²)CO₂H + NH₂C(R¹)?C?O, when R¹ and R² = H or alkyl. However, when one of the constituent amino acids is either aspartic acid or glutamic acid, the standard cleavage becomes minor in comparison with fragmentation through the α-side-chain of Asp or Glu. For example, [Asp-Leu - H]^? and [Leu-Asp - H]^? both fragment principally by loss of water, a fragmentation not normally noted for peptides. In addition, [Leu-Asp - H]^? loses CO₂ and also forms HO₂CCH?CHCO₂^?˙. These fragmentations establish that Asp is the C-terminal amino acid. In contrast, isomeric Glu dipeptides, e.g. [Glu-Ala - H]^? and [Ala-Glu - H]^? undergo similar fragmentation, both competitively losing H₂O and CO₂. Both spectra also contain a product ion at m/z 128, identified as the pyroglutamate anion. Product ion and deuterium-labelling studies have been used in an attempt to elucidate the complex fragmentation mechanisms in these systems.     

Hydrogen bond catalyzed enantioselective vinylogous Mukaiyama aldol reaction

[chemical reaction: see text]. The concept of hydrogen bonding catalysis was extended to the vinylogous Mukaiyama aldol reaction, which gives rapid access to polyketide derivatives. The reaction of the silyldienol ether shown and a range of aldehydes catalyzed by TADDOL proceeds regiospecifically to produce the addition products in good yields and enantiomeric excesses.     

Studies on pyrrolidinones. Michael reaction of glutamic acid and diethylglutamate

Starting from glutamic acid or diethyl glutamate, some derivatives of N-(2-carboxyethyl), N-(3-oxobutyl) and N-(2-cyanoethyl)pyroglutamic acid were synthesized.     

Hybrid peptide design. Hydrogen bonded conformations in peptides containing the stereochemically constrained gamma-amino acid residue, gabapentin

The crystal structure of 12 peptides containing the conformationally constrained 1-(aminomethyl)cyclohexaneacetic acid, gabapentin (Gpn), are reported. In all the 39 Gpn residues conformationally characterized so far, the torsion angles about the Calpha-Cbeta and Cbeta-Cgamma bonds are restricted to the gauche conformation (+/-60 degrees ). The Gpn residue is constrained to adopt folded conformations resulting in the formation of intramolecularly hydrogen-bonded structures even in short peptides. The peptides Boc-Ac6c-Gpn-OMe 1 and Boc-Gpn-Aib-Gpn-Aib-OMe 2 provide examples of C7 conformation; peptides Boc-Gpn-Aib-OH 3, Boc-Ac6c-Gpn-OH 4, Boc-Val-Pro-Gpn-OH 5, Piv-Pro-Gpn-Val-OMe 6, and Boc-Gpn-Gpn-Leu-OMe 7 provide examples of C9 conformation; peptide Boc-Ala-Aib-Gpn-Aib-Ala-OMe 8 provides an example of C12 conformation and peptides Boc-betaLeu-Gpn-Val-OMe 9 and Boc-betaPhe-Gpn-Phe-OMe 10 provide examples of C13 conformation. Gpn peptides provide examples of backbone expanded mimetics for canonical alpha-peptide turns like the gamma (C7) and the beta (C10) turns. The hybrid betagamma sequences provide an example of a mimetic of the C13 alpha-turn formed by three contiguous alpha-amino acid residues. Two examples of folded tripeptide structures, Boc-Gpn-betaPhe-Leu-OMe 11 and Boc-Aib-Gpn-betaPhg-NHMe 12, lacking internal hydrogen bonds are also presented. An analysis of available Gpn residue conformations provides the basis for future design of folded hybrid peptides.     

Fragmentation of the deprotonated ions of peptides containing cysteine, cysteine sulfinic acid, cysteine sulfonic acid, aspartic acid, and glutamic acid

We examined the fragmentation of the electrospray-produced [M-H]- and [M-2H]2- ions of a number of peptides containing two acidic amino acid residues, one being aspartic acid (Asp) or glutamic acid (Glu), and the other being cysteine sulfinic acid [C(SO2H)] or cysteine sulfonic acid [C(SO3H)], on an ion-trap mass spectrometer. We observed facile neutral losses of H2S and H2SO2 from the side chains of cysteine and C(SO2H), respectively, whereas the corresponding elimination of H2SO3 from the side chain of C(SO3H) was undetectable for most peptides that we investigated. In addition, the collisional activation of the [M-H]- ions of the C(SO2H)-containing peptides resulted in the cleavage of the amide bond on the C-terminal side of the C(SO2H) residue. Moreover, collisional activation of the [M-2H]2- ions of the above Asp-containing peptides led to the cleavage of the backbone N-Calpha bond of the Asp residue to give cn and/or its complementary [zn-H2O] ions. Similar cleavage also occurred for the singly deprotonated ions of the otherwise identical peptides with a C-terminal amide functionality, but not for the [M-H]- ions of same peptides with a free C-terminal carboxylic acid. Furthermore, ab initio calculation results for model cleavage reactions are consistent with the selective cleavage of the backbone N-Calpha bond in the Asp residue.     

Further studies on the fragmentation of protonated ions of peptides containing aspartic acid, glutamic acid, cysteine sulfinic acid, and cysteine sulfonic acid

Here we examined the fragmentation, on a quadrupole ion-trap mass spectrometer, of the protonated ions of a group of peptides containing one arginine and two different acidic amino acids, one being aspartic acid (Asp) or glutamic acid (Glu) and the other being cysteine sulfinic acid [C(SO2H)] or cysteine sulfonic acid [C(SO3H)]. Our results showed that, upon collisional activation, the cleavage of the peptide bond C-terminal to C(SO2H) is much more facile than that of the peptide bond C-terminal to Asp, Glu, or C(SO3H). There is no significant difference, however, in susceptibility to cleavage of peptide bonds that are C-terminal to Asp, Glu, and C(SO3H). To understand these experimental observations, we carried out B3LYP/6-31G* density functional theory calculations for a model cleavage reaction of GXG --> b2 + Gly, in which X is Asp, Glu, C(SO2H), or C(SO3H). Our calculation results showed that the cleavage reaction is thermodynamically more favorable when X = C(SO2H) than when X = Asp or C(SO3H). We attributed the less facile cleavage of the amide bond after Glu to that the formation of a six-membered ring b ion for Glu-bearing peptides is kinetically not as favorable as the formation of a five-membered ring b ion for peptides containing the other three acidic amino acids. The results from this study may provide useful tools for peptide sequencing.     

Peptides containing aminophosphonic acids. IV—mass spectra of the peptides containing aminomethylphosphonic acid

The fragmentation behavious under electron of di- and tri-peptide analogues containing amino acid (aa) and aminomethylphosphonic acid (ap) groups are described. Successive fragmentations of the molecular ions provided a convenient method for the sequence determination of peptides containing aa and ap units.     

Search for regular polypeptides possessing esterase properties. I. Polypeptides containing tyrosine and glutamic acid residues

An investigation has been made of the catalytic properties in relation to the hydrolysis of p-NPA of six polypeptides of regular structure: H-[Glu-Tyr]_n-OH (1), H-[Glu₂-Tyr]_n-OH (2), H-[Glu-Tyr₃]_n-OH (3), H-[Glu₃-Tyr]_n-OH (4), H-[Glu₃-Tyr]_n-OH (5), and H-[Glu-Tyr₂]_n]-OH (6). It has been shown that polypeptides (I), (III), and (IV) catalyze the hydrolysis of p-NPA (p-nitrophenyl acetate). An enzyme-like type of catalysis has been found. Some catalytic characteristics have been calculated and the dependence of the rate of hydrolysis on the pH of the medium, the temperature, and the concentration of p-NPA have been discussed. The structures of the catalytically active and catalytically inactive polypeptides have been studied by the circular dichroism method. It has been shown that under conditions in which the catalytic properties of polypeptides are shown to the maximum degree there is a structure of the random coil type. The catalytic activity falls or disappears completely when ordered fragments of the -helix and -structure types appear in the structure. It has been found that polypeptide (I) possesses the maximum catalytic activity. It exceeds the activity of a copolymer of the same amino acids by an order of magnitude.Institute of Molecular Biology, Academy of Sciences of the USSR, Moscow. Tadzhik State University, Dushanbe. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 687–699, September, October, 1979.     

Search for regular polypeptides possessing esterase properties. I. Polypeptides containing tyrosine and glutamic acid residues

An investigation has been made of the catalytic properties in relation to the hydrolysis of p-NPA of six polypeptides of regular structure: H-[Glu-Tyr]_n-OH (1), H-[Glu₂-Tyr]_n-OH (2), H-[Glu-Tyr₃]_n-OH (3), H-[Glu₃-Tyr]_n-OH (4), H-[Glu₃-Tyr]_n-OH (5), and H-[Glu-Tyr₂]_n]-OH (6). It has been shown that polypeptides (I), (III), and (IV) catalyze the hydrolysis of p-NPA (p-nitrophenyl acetate). An enzyme-like type of catalysis has been found. Some catalytic characteristics have been calculated and the dependence of the rate of hydrolysis on the pH of the medium, the temperature, and the concentration of p-NPA have been discussed. The structures of the catalytically active and catalytically inactive polypeptides have been studied by the circular dichroism method. It has been shown that under conditions in which the catalytic properties of polypeptides are shown to the maximum degree there is a structure of the random coil type. The catalytic activity falls or disappears completely when ordered fragments of the α-helix and β-structure types appear in the structure. It has been found that polypeptide (I) possesses the maximum catalytic activity. It exceeds the activity of a copolymer of the same amino acids by an order of magnitude.     

Synthesis of a hexapeptide containing valine, leucine, and glutamic acid

Summary The previously unreported methyl ester of N-benzyloxycarbonyl-L-phenylalanyl-L-valyl-O-methyl-L-glutamyl-L-leucyl-L-valyl-L-alanine has been synthesized.T. G. Shevchenko Kiev State University. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 785–788, November–December, 1975.     

Quinoxaline-specific enantioselective sulfa-michael reaction catalyzed by chiral phosphoric acid

Highly enantioselective sulfa-Michael additions (SMA) between 2-alkenyl quinoxalines and aromatic thiols are accomplished using a low loading of chiral phosphoric acid catalyst (1 mol%). It was confirmed by an investigation of a lot of azaarenes that the two C=N units of quinoxalines are indispensable for controlling the reaction enantioselectivities. A series of non-terminal 2-alkenes substituted with aryls or alkyls, even other electro-withdrawing groups such as ketones, esters, or amides, selectively reacted and afforded the desired SMA products (48 examples) in good regioselectivities with high yields (up to 99%) and good ee values (up to 97%).     

Synthesis of tripeptides containing serine,glutamic acid,and imino acids

Conclusions By the mixed-anhydride and carbodiimide methods the following tripeptides were synthesized: glycylDL-seryl-L-proline, glycyl-L-prolyl-DL-serine, glycyl-DL-seryl-L-hydroxyproline, glycyl-L-hydroxy-prolyl-DL-serine, glycyl-L-hydroxyprolyl-L-glutamic acid, glycyl-L-prolyl-L-glutamic acid, and glycyl-DL-seryl-glycine.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1823–1825, August, 1970.