Prediction of electronically nonadiabatic decomposition mechanisms of isolated gas phase nitrogen-rich energetic salt: Guanidium-triazolate

Mass spectral and theoretical investigations of N-Cα bond cleavages in the disulfide-containing peptide TTCPYCKK and its analogues

RATIONALE

For disulfide-containing peptides, mass spectrometric analyses are rarely comparably studied between their dithiol and disulfide forms. Persulfide ions afforded from peptides with a disulfide ring are from either an unusual N-C^α bond cleavage or a canonical peptide bond cleavage; their isomeric structures are, however, not identified just from peaks of mass spectra.

METHODS

Isomeric structures of [C³ P⁴ X⁵ |C⁶ _M ], [C³ _MA P⁴ X⁵ |C⁶ _MB ] and [P⁴ X⁵ C⁶ |C³ _M ] were identified from a series of the X⁵ substituted dicysteine octapeptides using electrospray ionization tandem mass spectrometry for both their dithiol and disulfide forms. Formation mechanisms of different persulfide ions were investigated systematically by theoretical methods. Moreover, electrostatic potential-mapped molecular van der Waals surfaces were used to determine the stabilities of the intermediates, which gave a further evaluation of favored bond cleavage.

RESULTS

Mass spectral analyses indicated that the fragmented ions changed largely when an intramolecular disulfide bond was formed. New types of disulfide-containing fragmented ions [C³ P⁴ X⁵ |C⁶ _M ] or [C³ _MA P⁴ X⁵ |C⁶ _MB ] were thus proposed. Energy analysis showed that the N-C^α cleavage was not competitive energetically with that of the amide bond for Y⁵ and its phosphorylated analogue. However, the N-C^α cleavage products dominated for the S⁵ - and T⁵ -containing peptides. Stabilities of the intermediates were found to be related with the electrostatic potential-mapped molecular van der Waals surfaces.

CONCLUSIONS

Persulfide ions containing more residues than previously found were proposed not only from b₇ ions but also from y₆ ions. In addition, a new kind of phosphorylated analogue, [C³ P⁴ _p Y⁵ |C⁶ _M ], is reported in this work. Our study provides convincing results for separating isomeric structures in the cases of N-C^α cleavages, which greatly assists in the structural identification of disulfide-containing peptides.

Direct Pyrolysis of a Manganese-Triazolate Metal-Organic Framework into Air-Stable Manganese Nitride Nanoparticles

Although metal-organic frameworks (MOFs) are being widely used to derive functional nanomaterials through pyrolysis, the actual mechanisms involved remain unclear. In the limited studies to date, elemental metallic species are found to be the initial products, which limits the variety of MOF-derived nanomaterials. Here, the pyrolysis of a manganese triazolate MOF is examined carefully in terms of phase transformation, reaction pathways, and morphology evolution in different conditions. Surprisingly, the formation of metal is not detected when manganese triazolate is pyrolyzed in an oxygen-free environment. Instead, a direct transformation into nanoparticles of manganese nitride, Mn2N x embedded in N-doped graphitic carbon took place. The electrically conductive Mn2N x nanoparticles show much better air stability than bulk samples and exhibit promising electrocatalytic performance for the oxygen reduction reaction. The findings on pyrolysis mechanisms expand the potential of MOF as a precursor to derive more functional nanomaterials.

Mass spectral and theoretical investigations of the transient proton-bound dimers on the cleavage processes of the peptide GHK and its analogues

Fragmentation mechanisms of the singly protonated peptides GHK, GHKH and HGHK have been investigated by mass spectrometry and theoretical calculations. Fragmentation behavior of the protonated H-K amide bond in GHK was changed completely when a histidinyl residue was introduced into the C-terminus of GHK. The H-K amide bond breaking was a predominant pathway in the case of GHK and GHKH. For HGHK, the histidinyl residue at the N-terminus hampered significantly breaking of the H-K amide bond resulting in a high potential energy barrier; calculations indicated that this histidinyl effect played a vital role for the H-K amide bond fragmentation. Subsequent analysis of the fragmentation mechanism revealed that recombination processes of the hydrogen bonding for the intermediate products were all exergonic. Formation of a proton-bound dimer (PBD) lowering the energy barriers from a thermodynamic perspective for all the designed fragmentation pathways was demonstrated to be feasible by our systematic calculations. Moreover, the involvement of different PBDs was further confirmed by analyses of the reduced density gradient (RDG) isosurfaces and scatter maps. A dynamically favored pathway was likely via six-membered ring or nine-membered ring structures generated by the diketopiperazine as revealed by atom-in-molecules (AIM) analyses, since the steric interaction energies in the newly formed ring were estimated to be relatively small when compared to the products generated from a lactam and/or an oxazolone pathway. This is the first feasibility investigation from a dynamic viewpoint for formation of different rings involved in the lactam, oxazolone or diketopiperazine pathways in the fragmentation mechanisms proposed.

Insight into nucleophilic fragmentation mechanisms by glutamic acid side chain in singly protonated glutathione and related peptidyl ions

Fragmentation mechanisms of the singly protonated glutathione (γ-ECG) and its synthetic analogue peptides (ECG and PPECG) have been investigated by liquid chromatography tandem-mass spectrometry and theoretical calculations. In the mass spectra, similar fragmentation patterns were observed for γ-ECG and ECG, but a completely different one was found in the case of PPECG. The E-C amide bond cleavage is the predominant pathway for the fragmentation of γ-ECG and ECG, whereas the additional N-terminal prolyl residues in PPECG significantly suppress the E-C amide bond cleavage. Theoretical calculations reveal that the fragmentation efficiencies of the E-C bonds in the protonated γ-ECG and ECG are much higher than that in the protonated PPECG, being attributed to their lower barriers of the potential energy; clearly the introduction of two prolyl residues can increase substantially the potential energy barrier. In the proposed mechanism, the protonated E-C amide bonds in the three peptides are first weakened followed by a nucleophilic addition by the glutamyl carboxyl oxygen atom in side chain, leading to the breaking of the E-C amide bonds. However, the processes of E-C bond fragmentation for three protonated analogs were not collaborative. Protonated amide bonds first fragment, then the nucleophilic addition by the side chain of glutamyl carboxyl oxygen atom takes places. On the other hand, the prolyl residues in PPECG can largely diminish the nucleophilic addition, resulting in a much lower efficiency of its E-C amide bond breaking. Distance analysis indicates that breaking the E-C amide bonds in the protonated γ-ECG, ECG, and PPECG ions could not occur without the assistance from the nucleophilic attack, highlighting an asynchronous collaborative process in the bond breakings.