title: Radical-cationic gaseous amino acids: a theoretical study.
abstract: Three major forms of gaseous radical-cationic amino acids (RCAAs), keto (COOH), enolic (C(OH)OH), and zwitterionic (COO(-)), as well as their tautomers, are examined for aliphatic Ala(.+), Pro(.+), and Ser(.+), sulfur-containing Cys(.+), aromatic Trp(.+), Tyr(.+), and Phe(.+), and basic His(.+). The hybrid B3LYP exchange-correlation functional with various basis sets along with the highly correlated CCSD(T) method is used. For all RCAAs considered, the main stabilizing factor is spin delocalization; for His(.+), protonation of the basic side chain is equally important. Minor stabilizing factors are hydrogen bonding and 3e-2c interactions. An efficient spin delocalization along the N-C(alpha)-C(O-)O moiety occurs upon H-transfer from C(alpha) to the carboxylic group to yield the captodative enolic form, which is the lowest-energy isomer for Ala(.+), Pro(.+), Ser(.+), Cys(.+), Tyr(.+), and Phe(.+). This H-transfer occurs in a single step as a 1,3-shift through the sigma-system. For His(.+), the lowest-energy isomer is formed upon H-transfer from C(alpha) to the basic side chain, which results in a keto form, with spin delocalized along the N-C(alpha)-C=O fragment. Trp(.+) is the only RCAA that favors spin delocalization over an aromatic system given the low ionization energy of indole. The lowest-energy isomer of Trp(.+) is a keto form, with no H-transfer.
authors.affiliation: Department of Chemistry, St. Francis Xavier University, Nova Scotia, Canada, B2G 2W5.
authors.forename: Kailee N
authors.initials: KN
authors.lastname: Sutherland
authors.forename: Philippe C
authors.initials: PC
authors.lastname: Mineau
authors.forename: Galina
authors.initials: G
authors.lastname: Orlova
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D002412:Cations; D005609:Free Radicals; D005740:Gases / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008962:Models, Theoretical
chemical_list: D000596:Amino Acids; D002412:Cations; D005609:Free Radicals; D005740:Gases