title: An Experimental and Computational Study of the Gas-Phase Acidities of the Common Amino Acid Amides.
abstract: Using proton-transfer reactions in a Fourier transform ion cyclotron resonance mass spectrometer and correlated molecular orbital theory at the G3(MP2) level, gas-phase acidities (GAs) and the associated structures for amides corresponding to the common amino acids have been determined for the first time. These values are important because amino acid amides are models for residues in peptides and proteins. For compounds whose most acidic site is the C-terminal amide nitrogen, two ions populations were observed experimentally with GAs that differ by 4-7 kcal/mol. The lower energy, more acidic structure accounts for the majority of the ions formed by electrospray ionization. G3(MP2) calculations predict that the lowest energy anionic conformer has a cis-like orientation of the [-C(═O)NH](-) group whereas the higher energy, less acidic conformer has a trans-like orientation of this group. These two distinct conformers were predicted for compounds with aliphatic, amide, basic, hydroxyl, and thioether side chains. For the most acidic amino acid amides (tyrosine, cysteine, tryptophan, histidine, aspartic acid, and glutamic acid amides) only one conformer was observed experimentally, and its experimental GA correlates with the theoretical GA related to side chain deprotonation.
authors.affiliation: Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States.
authors.forename: Chelsea E
authors.initials: CE
authors.lastname: Plummer
authors.affiliation: Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States.
authors.forename: Michele L
authors.initials: ML
authors.lastname: Stover
authors.affiliation: Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States.
authors.forename: Samantha S
authors.initials: SS
authors.lastname: Bokatzian
authors.affiliation: Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States.
authors.forename: John T M
authors.initials: JT
authors.lastname: Davis
authors.affiliation: Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States.
authors.forename: David A
authors.initials: DA
authors.lastname: Dixon
authors.affiliation: Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States.
authors.forename: Carolyn J
authors.initials: CJ
authors.lastname: Cassady
mesh_terms: D000577:Amides / Q000737:chemistry*; D000596:Amino Acids / Q000737:chemistry*; D005740:Gases / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D008968:Molecular Conformation; D011522:Protons; D011789:Quantum Theory
chemical_list: D000577:Amides; D000596:Amino Acids; D005740:Gases; D011522:Protons