Radical-Cationic Gaseous Amino Acids:  A Theoretical Study Kailee N. Sutherland Philippe C. Mineau Galina Orlova 10.1021/jp071168a.s002 https://acs.figshare.com/articles/journal_contribution/Radical_Cationic_Gaseous_Amino_Acids_A_Theoretical_Study/2990671 Three major forms of gaseous radical-cationic amino acids (RCAAs), keto (COOH), enolic (C(OH)OH), and zwitterionic (COO<sup>-</sup>), as well as their tautomers, are examined for aliphatic Ala<sup>•+</sup>, Pro<sup>•+</sup>, and Ser<sup>•+</sup>, sulfur-containing Cys<sup>•+</sup>, aromatic Trp<sup>•+</sup>, Tyr<sup>•+</sup>, and Phe<sup>•+</sup>, and basic His<sup>•+</sup>. 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<sup>•+</sup>, 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<sub>α</sub>−C(O−)O moiety occurs upon H-transfer from C<sub>α</sub> to the carboxylic group to yield the captodative enolic form, which is the lowest-energy isomer for Ala<sup>•+</sup>, Pro<sup>•+</sup>, Ser<sup>•+</sup>, Cys<sup>•+</sup>, Tyr<sup>•+</sup>, and Phe<sup>•+</sup>. This H-transfer occurs in a single step as a 1,3-shift through the σ-system. For His<sup>•+</sup>, the lowest-energy isomer is formed upon H-transfer from C<sub>α</sub> to the basic side chain, which results in a keto form, with spin delocalized along the NC<sub>α</sub>CO fragment. Trp<sup>•+</sup> 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<sup>•+</sup> is a keto form, with no H-transfer. 2007-08-16 00:00:00 C α side chain RCAA factor delocalization Trp Pro keto form COOH isomer COO 3LYP CCSD Ser Cy Phe Tyr captodative enolic form Ala