Radical-Cationic Gaseous Amino Acids:  A Theoretical Study

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_α−C(O−)O moiety occurs upon H-transfer from C_α 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 σ-system. For His^•+, the lowest-energy isomer is formed upon H-transfer from C_α to the basic side chain, which results in a keto form, with spin delocalized along the NC_α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.