Can cluster structure affect kinetic method measurements? The curious case of glutamic acid's gas-phase acidity.
The gas-phase acidities of aspartic, glutamic, and 2-aminoadipic acid have been determined by the kinetic method in a triple-quadrupole instrument. Although aspartic acid behaves in the conventional way and gives a DeltaH(acid) value of 1340 kJ mol(-1), glutamic and 2-aminoadipic acids give kinetic method plots with two distinct slopes. This leads to DeltaH(acid) values of 1350 and 1366 kJ mol(-1) for glutamic acid, and 1355 and 1369 kJ mol(-1) for 2-aminoadipic acid. The value for aspartic acid and the low collision energy value for glutamic acid are consistent with recent measurements by Poutsma and co-workers in a quadrupole ion trap. The experiments are supported by calculations at the G3(MP2) and OLYP/aug-cc-pVTZ levels. Computational studies of model clusters of the amino acids with trifluoroacetate suggest there are distinct preferences. Glutamic and 2-aminoadipic acid prefer clusters where the amino acid adopts a zwitterion-like structure whereas aspartic acid prefers to adopt a conventional (canonic) structure in its clusters. This result along with the computed stabilities of zwitterion-like conformations of the deprotonated amino acids leads to the following explanation for the presence of two slopes in the kinetic method plots. At low collision energies, the deprotonated amino acid dissociates from the cluster, with rearrangement if necessary, to give its preferred conformation, but at high collision energies, the deprotonated amino acid directly dissociates in the conformation preferred in the cluster. For glutamic and 2-aminoadipic acids, this is a zwitterion-like structure that is about 20 kJ mol(-1) less stable than the global minimum.
Fournier
Francoise
F
Université Pierre et Marie Curie-Paris6, UMR 7613 Synthèse, Structure et Fonction de Molécules Bioactives, Paris, France.
Afonso
Carlos
C
Fagin
Adelaide E
AE
Gronert
Scott
S
Tabet
Jean-Claude
JC
D015074:2-Aminoadipic Acid / Q000737:chemistry; D000596:Amino Acids / Q000737:chemistry; D001224:Aspartic Acid / Q000737:chemistry; D004951:Esterification; D005740:Gases; D018698:Glutamic Acid / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D010455:Peptides / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization; D013816:Thermodynamics
D000596:Amino Acids; D005740:Gases; D010455:Peptides; D015074:2-Aminoadipic Acid; D001224:Aspartic Acid; D018698:Glutamic Acid
J Mass Spectrom. 2004 Sep;39(9):1004-15
15386748
J Am Soc Mass Spectrom. 2006 Sep;17(9):1275-81
16829127
J Mass Spectrom. 2004 Sep;39(9):998-1003
15386749
J Mass Spectrom. 2007 Jan;42(1):25-35
17149792
J Mass Spectrom. 2004 Sep;39(9):1016-24
15386747
J Am Chem Soc. 2001 Dec 12;123(49):12255-65
11734026
J Am Soc Mass Spectrom. 2000 May;11(5):371-9
10790840
J Am Soc Mass Spectrom. 1999 Jan;10(1):1-8
9888180