title: Influence of the Side Chain in the Structure and Fragmentation of Amino Acids Radical Cations.
abstract: The conformational properties of ionized amino acids (Gly, Ala, Ser, Cys, Asp, Gln, Phe, Tyr, and His) have been theoretically analyzed using the hybrid B3LYP and the hybrid-meta MPWB1K functionals as well as with the post-Hartree Fock CCSD(T) level of theory. As a general trend, ionization is mainly localized at the -NH2 group, which becomes more planar and acidic, the intramolecular hydrogen bond in which -NH2 acts as proton donor being strengthened upon ionization. For this reason, the so-called conformer IV(+) becomes the most stable for nonaromatic amino acid radical cations. Aromatic amino acids do not follow this trend because ionization takes place mainly at the side chain. For these amino acids for which ionization of the side chain prevails over the -NH2 group, structures III(+) and II(+) become competitive. The Cα-X fragmentations of the ionized systems have also been studied. Among the different decompositions considered, the one that leads to the loss of COOH(•) is the most favorable one. Nevertheless, for aromatic amino acids fragmentations leading to R(•) or R(+) start being competitive. In fact, for His and Tyr, results indicate that the fragmentation leading to R(+) is the most favorable process.
authors.affiliation: Departament de Química, Universitat Autonoma de Barcelona, Bellaterra 08193, Spain, and Institut de Química Computacional, Departament de Química, Universitat de Girona, Girona 17071, Spain.
authors.forename: Adrià
authors.initials: A
authors.lastname: Gil
authors.affiliation: Departament de Química, Universitat Autonoma de Barcelona, Bellaterra 08193, Spain, and Institut de Química Computacional, Departament de Química, Universitat de Girona, Girona 17071, Spain.
authors.forename: Sílvia
authors.initials: S
authors.lastname: Simon
authors.affiliation: Departament de Química, Universitat Autonoma de Barcelona, Bellaterra 08193, Spain, and Institut de Química Computacional, Departament de Química, Universitat de Girona, Girona 17071, Spain.
authors.forename: Luis
authors.initials: L
authors.lastname: Rodríguez-Santiago
authors.affiliation: Departament de Química, Universitat Autonoma de Barcelona, Bellaterra 08193, Spain, and Institut de Química Computacional, Departament de Química, Universitat de Girona, Girona 17071, Spain.
authors.forename: Juan
authors.initials: J
authors.lastname: Bertrán
authors.affiliation: Departament de Química, Universitat Autonoma de Barcelona, Bellaterra 08193, Spain, and Institut de Química Computacional, Departament de Química, Universitat de Girona, Girona 17071, Spain.
authors.forename: Mariona
authors.initials: M
authors.lastname: Sodupe