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Title: Gas-phase Acidities of Aspartic Acid, Glutamic Acid, and their Amino Acid Amides.

Abstract

Gas-phase acidities (GA or ΔG acid) for the two most acidic common amino acids, aspartic acid and glutamic acid, have been determined for the first time. Because of the amide linkage’s importance in peptides and as an aid in studying side chain versus main chain deprotonation, aspartic acid amide and glutamic acid amide were also studied. Experimental GA values were measured by proton transfer reactions in an electrospray ionization/Fourier transform ion cyclotron resonance mass spectrometer. Calculated GAs were obtained by density functional and molecular orbital theory approaches. The best agreement with experiment was found at the G3MP2 level; the MP2/CBS and B3LYP/aug-cc-pVDZ results are 3–4 kcal/mol more acidic than the G3MP2 results. Experiment shows that aspartic acid is more acidic than glutamic acid by ca. 3 kcal/mol whereas the G3MP2 results show a smaller acidity difference of 0.2 kcal/mol. Similarly, aspartic acid amide is experimentally observed to be ca. 2 kcal/mol more acidic than glutamic acid amide whereas the G3MP2 results show a correspondingly smaller energy difference of 0.7 kcal/mol. The computational results clearly show that the anions are all ring-like structures with strong hydrogen bonds between the OH or NH 2 groups and the CO 2 - group frommore » which the proton is removed. The two amino acids are main-chain deprotonated. In addition, use of the COSMO model for the prediction of the free energy differences in aqueous solution gave values in excellent agreement with the most recent experimental values for pK a. Glutamic acid is predicted to be more acidic than aspartic acid in aqueous solution due to differential solvation effects.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
921823
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: International Journal of Mass Spectrometry; Journal Volume: 265; Journal Issue: 2-3
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; AMIDES; AMINO ACIDS; ANIONS; AQUEOUS SOLUTIONS; ASPARTIC ACID; FREE ENERGY; FUNCTIONALS; GLUTAMIC ACID; HYDROGEN; ION CYCLOTRON-RESONANCE; MASS SPECTROMETERS; PEPTIDES; PH VALUE; PROTONS; SOLVATION; TRANSFER REACTIONS; Environmental Molecular Sciences Laboratory

Citation Formats

Li, Zhong, Matus, Myrna H, Velazquez, Hector A, Dixon, David A, and Cassady, Carolyn J. Gas-phase Acidities of Aspartic Acid, Glutamic Acid, and their Amino Acid Amides.. United States: N. p., 2007. Web. doi:10.1016/j.ijms.2007.02.009.
Li, Zhong, Matus, Myrna H, Velazquez, Hector A, Dixon, David A, & Cassady, Carolyn J. Gas-phase Acidities of Aspartic Acid, Glutamic Acid, and their Amino Acid Amides.. United States. doi:10.1016/j.ijms.2007.02.009.
Li, Zhong, Matus, Myrna H, Velazquez, Hector A, Dixon, David A, and Cassady, Carolyn J. Wed . "Gas-phase Acidities of Aspartic Acid, Glutamic Acid, and their Amino Acid Amides.". United States. doi:10.1016/j.ijms.2007.02.009.
@article{osti_921823,
title = {Gas-phase Acidities of Aspartic Acid, Glutamic Acid, and their Amino Acid Amides.},
author = {Li, Zhong and Matus, Myrna H and Velazquez, Hector A and Dixon, David A and Cassady, Carolyn J},
abstractNote = {Gas-phase acidities (GA or ΔGacid) for the two most acidic common amino acids, aspartic acid and glutamic acid, have been determined for the first time. Because of the amide linkage’s importance in peptides and as an aid in studying side chain versus main chain deprotonation, aspartic acid amide and glutamic acid amide were also studied. Experimental GA values were measured by proton transfer reactions in an electrospray ionization/Fourier transform ion cyclotron resonance mass spectrometer. Calculated GAs were obtained by density functional and molecular orbital theory approaches. The best agreement with experiment was found at the G3MP2 level; the MP2/CBS and B3LYP/aug-cc-pVDZ results are 3–4 kcal/mol more acidic than the G3MP2 results. Experiment shows that aspartic acid is more acidic than glutamic acid by ca. 3 kcal/mol whereas the G3MP2 results show a smaller acidity difference of 0.2 kcal/mol. Similarly, aspartic acid amide is experimentally observed to be ca. 2 kcal/mol more acidic than glutamic acid amide whereas the G3MP2 results show a correspondingly smaller energy difference of 0.7 kcal/mol. The computational results clearly show that the anions are all ring-like structures with strong hydrogen bonds between the OH or NH2 groups and the CO2- group from which the proton is removed. The two amino acids are main-chain deprotonated. In addition, use of the COSMO model for the prediction of the free energy differences in aqueous solution gave values in excellent agreement with the most recent experimental values for pKa. Glutamic acid is predicted to be more acidic than aspartic acid in aqueous solution due to differential solvation effects.},
doi = {10.1016/j.ijms.2007.02.009},
journal = {International Journal of Mass Spectrometry},
number = 2-3,
volume = 265,
place = {United States},
year = {Wed Feb 14 00:00:00 EST 2007},
month = {Wed Feb 14 00:00:00 EST 2007}
}