Thermodynamics and Mechanisms of Protonated Diglycine Decomposition: A Guided Ion Beam Study

We report the use of unimolecular dissociation by infrared radiation for gaseous multiphoton energy transfer to determine relative activation energy (E a,laser ) for dissociation of peptide sequence ions. The sequence ions of interest are mass-isolated; the entire ion cloud is then irradiated with a continuous wave CO 2 laser, and the first order rate constant, k d , is determined for each of a series of laser powers. Provided these conditions are met, a plot of the natural logarithm of k d versus the natural logarithm of laser power yields a straight line, whose slope provides a measure of E a,laser . This method reproduces the E a values from blackbody radiative dissociation (BIRD) for the comparatively large, singly and doubly protonated bradykinin ions (nominally y 9 and y 9 2+ ). The comparatively small sequence ion systems produce E a,laser values that are systematic underestimates of theoretical barriers calculated with density functional theory (DFT). However, the relative E a,laser values are in qualitative agreement with the mobile proton model and available theory. Additionally, novel protonated cyclic-dipeptide (diketopiperazine) fragmentation reactions are analyzed with DFT. FT-ICR MS provides access to sequence ions generated by electron capture dissociation, infrared multiphoton dissociation, and collisional activation methods (i.e., b n , y m , c n , z m• ions).

Section: Ecd Fragmentssupporting

confidence: 59%

Relative Stability of Peptide Sequence Ions Generated by Tandem Mass Spectrometry

Bythell

Hendrickson

Marshall

2012

“…In the following paper [11], guided ion beam tandem mass spectrometry is used to quantitatively characterize the energetics for H + GG decomposition for the first time. Six ionic products are formed by loss of CO, loss of H 2 O forming the b 2 ion, combined loss of H 2 O + CO forming the a 2 ion, combined loss of CO + NH 3 , and formation of the y 1 (H + G) and a 1 (CH 2 NH 2 + ) ions.…”

mentioning

confidence: 99%

Thermodynamics and Mechanisms of Protonated Diglycine Decomposition: A Computational Study

Armentrout

Heaton

2011

Self Cite

We present a full computational description of the fragmentation reactions of protonated diglycine (H + GG). Relaxed potential energy surface scans performed at B3LYP/6-31 G(d) or B3LYP/6-311+G(d,p) levels are used to map the reaction coordinate surfaces and identify the transition states (TSs) and intermediate reaction species for seven reactions observed experimentally in the succeeding companion paper. All structures are optimized at the B3LYP/ 6-311+G(d,p) level, with single point energies of the key optimized structures calculated at B3LYP and MP2(full) levels using a 6-311+G(2 d,2p) basis set. These theoretical structures and energies are compared with extensive calculations in the literature. Although the pathways elucidated here are generally in agreement with those previously outlined, new details and, for some reactions, lower energy transition states are located. Further, the mechanism for the combined loss of carbon monoxide and ammonia is explored for the first time.

“…Unfortunately, data are seldom available from either of these experimental techniques. The best experiment for comparison with our model is threshold CID, which yields the zero-temperature energy threshold for each primary product ion (and sometimes also for secondary ions) [64][65][66]. However, data analysis is complex for such experiments; tripeptides are the largest peptides studied so far.…”

Section: Comparison With Experimentsmentioning

confidence: 99%

“…The authors thank Professor Peter Armentrout for a preprint of reference [66] and are especially grateful to the anonymous reviewers for helpful comments and suggestions.…”

Section: Acknowledgmentsmentioning

confidence: 99%

N-Protonated Isomers as Gateways to Peptide Ion Fragmentation

Hæffner

Merle

Irikura

2011

According to the popular "mobile proton model" for peptide ion fragmentation in tandem mass spectrometry, peptide bond cleavage is typically preceded by intramolecular proton transfer from basic sites to an amide nitrogen in the backbone. If the intrinsic barrier to dissociation is the same for all backbone sites, the fragmentation propensity at each amide bond should reflect the stability of the corresponding N-protonated isomer. This hypothesis was tested by using ab initio and force-field computations on several polyalanines and Leu-enkephalin. The results agree acceptably with experimental reports, supporting the hypothesis. It was found that backbone Nprotonation is most favorable near the C-terminus. The preference for C-terminal N-protonation, which is stronger for longer polyalanines, may be understood in terms of the well known "helix macrodipole" in the corresponding helical conformations. The opposite stability trend is found for peptides constrained to be linear, which is initially surprising but turns out to be consistent with the reversed direction of the macrodipole in the linear conformation.