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Abstract No.: FrOr-06
Speaker: Peter B. Armentrout
Session: Activation and Dissociation
Presentation date: Fri, Sep 1, 2006
Presentation time: 10:50 – 11:10

Activation and Decomposition of Sodiated Acidic Amino Acids and Their Amide Derivatives

Peter B. Armentrout1, Amy Heaton1, Robert M. Moision1

1 University of Utah, Salt Lake City, United States

Correspondence address: Peter B. Armentrout, University of Utah, Chemistry, 315 S. 1400 E. Rm 2020, Salt Lake City, Utah, 84112-0850 United States.

Web site: http://www.chem.utah.edu/faculty/armentrout/index.html

Keywords: Bond Energies; Calculations, Ab Initio; Collision-Induced Dissociation (CID); Interactions, Protein/Peptide with Metal ions.

Novel aspect: Energetics for dissocation of Na+ bound to acidic amino acids and their amide derivatives. Experimental and theoretical studies. Differences in the dissociation observed for complexes formed by electrospray ionization vs. condensation.

 

Objectives
The dissocation of sodium cations bound to acidic amino acids, aspartic acid (Asp) and glutamic acid (Glu), and their amide derivatives, asparagine (Asn) and glutamine (Gln), are studied both experimentally and theoretically. Significantly, differences in the dissociation behavior are found for these complexes when formed by electrospray ionization (ESI) vs. condensation in a flow tube source. These differences provide insight into the decomposition mechanisms of these amino acids, which are further elucidated by comparisons to theoretical calculations.

Methods
Our guided ion beam tandem mass spectrometer is used to study the energy-resolved dissociation of Na+(AA) where AA is an amino acid as induced by collisions with Xe. Complexes are formed by ESI and in a flow tube ion source by condensation. For comparison, we use ab initio theory to calculate bond energies for the species observed. Geometry optimizations are performed at the B3LYP/6-311+G(d,p) level followed by single point calculations at the MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. Final bond dissociation energies (BDEs) are corrected for zero point energies and basis set superposition errors.

Results and Conclusions
Reactions of Na+(AA), where AA is Asp, Glu, Asn, or Gln, with Xe were studied at kinetic energies ranging from 0 to 5 eV. When formed by ESI, these complexes dissociate to form only Na+. The kinetic energy dependences of the cross sections are analyzed using methods developed previously and provide threshold energies that equal the 0 K Na+-AA BDEs (kJ/mol): 199, Asp; 204, Glu; 213, Asn; and 221, Gln, all ± 10 kJ/mol. Clearly, BDEs are larger for the amide derivatives and increase for the larger Glx ligands. Ab initio calculations at the B3P86 level agree very well with these BDEs, whereas B3LYP and MP2 values are systematically high and low, respectively, by 8 kJ/mol. The theoretical results reveal that the lowest energy conformation in all cases is tridentate with the metal ion binding to the backbone carbonyl and amide group along with the sidechain carbonyl. Clearly, the CONH2 sidechain is a better donor than the COOH sidechain, and the longer sidechain in Glx allows less steric problems than for Asx.

When the complexes are formed by condensation of Na+ with AA (heated to 170 - 210 ° C) in a flow tube source, very different fragmentation patterns are observed. At low energies, complexes lose XH where XH = H2O for Asp and Glu and XH = NH3 for Asn and Gln. At higher energies, Na+ and Na+(XH) are observed. Analyses of these data yield thermodynamic information compared to theoretical results for possible decomposition pathways suggests that XH loss form cyclic five-membered rings: an anhydride for Asx and 5-oxo proline for Glx. As the deamination of asparagine is believed to be important in the aging process, investigations of these decomposition reactions are particularly interesting.