Research Summary: Study of the structural features of AGG and its ion using IRMPD (Infrared Multiple Photon Dissociation) spectroscopy.
Author Affiliations:
Sung Hwan Yoon, Julia Chamot-Rooke, Brittany R. Perkins, Amy E. Hilderbrand, John C. Poutsma, Vicki H. Wysocki from the University of Arizona, Ecole Polytechnique, and The College of William and Mary.
Date Received: August 31, 2008.
Techniques like NMR and X-ray crystallography are benchmarks for biological structure analysis.
Increasing use of mass spectrometry (MS) for structural investigations:
Soft ionization techniques (MALDI, ESI) enable the examination of biological systems from small molecules to complex proteins.
Mass spectrometry allows for analysis in a solvent-free environment, studying intrinsic features and interactions.
Main structures proposed for b2+ ions:
Diketopiperazine structure (formed by N-terminal amino group attack).
Oxazolone structure (formed by carbonyl oxygen attack).
Research has investigated how ions and peptide structures can be evaluated:
Techniques such as MS/MS, isotope labeling, gas-phase hydrogen/deuterium exchange, ion mobility, and computational modeling.
Past studies and indirect methods suggest that many simple peptide b2+ ions are oxazolones
IRMPD spectroscopy: has shown potential for probing structures effectively.
Comparison of Protonated AGG and cAG:
IRMPD spectra of protonated cAG (known diketopiperazine structure) revealed fragments at m/z 101 (loss of CO) and other significant ions.
The study used quantum chemical calculations and performed at the B3LYP/6-311++G(d,p) level to determine structures.
Key findings in IRMPD spectrum:
Carbonyl stretching bands and their shifts provide structural insights.
Diketopiperazine structures protonated at amide oxygens (Gly and Ala) were found to have lower free energies than nitrogen protonated counterparts:
Suggests major isomers involve amide oxygen protonation.
The protonation effect on carbonyl stretching shifts can be utilized to derive structural conclusions.
Key Bands Observed:
Band at 1775 cm-1 indicates an unprotonated carbonyl amide, with shifts indicating protonation effects.
Presence of specific bands (1202 cm-1) indicates the stability and nature of the diketopiperazine structure.
The b2+ ion IR spectrum revealed distinct patterns differing from cAG:
Blue-shifted carbonyl stretching bands at about 1970 cm-1, indicating an oxazolone structure.
Additional bands (1630 cm-1 and 1339 cm-1) suggest NH2 scissoring and twisting modes characteristic of oxazolones.
As a conclusion:
Protonated cAG retains a diketopiperazine structure, while b2+ ion from AGG conclusively adopts an oxazolone structure.
This confirms previous structural hypotheses based on theoretical models and prior experiments.
Thanks to CLIO team for assistance.
Research supported by NIH and NSF grants, and European Commission EPITOPES project.
Availability of additional figures and spectra details to support findings.
References provided for further exploration.