Influence of the Side Chain in the Structure and Fragmentation of Amino Acids Radical Cations

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• 作者：Adrià ; Gil ; Sí ; lvia Simon ; Luis Rodrí ; guez-Santiago ; Juan Bertr&aacute ; n ; Mariona Sodupe
• 刊名：Journal of Chemical Theory and Computation
• 出版年：2007
• 出版时间：November 2007
• 年：2007
• 卷：3
• 期：6
• 页码：2210 - 2220
• 全文大小：473K
• 年卷期：v.3,no.6(November 2007)
• ISSN：1549-9626

文摘

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. Asa general trend, ionization is mainly localized at the -NH₂ group, which becomes more planarand acidic, the intramolecular hydrogen bond in which -NH₂ acts as proton donor beingstrengthened upon ionization. For this reason, the so-called conformer IV(+) becomes the moststable for nonaromatic amino acid radical cations. Aromatic amino acids do not follow this trendbecause ionization takes place mainly at the side chain. For these amino acids for whichionization of the side chain prevails over the -NH₂ group, structures III(+) and II(+) becomecompetitive. The C-X fragmentations of the ionized systems have also been studied. Amongthe different decompositions considered, the one that leads to the loss of COOH is the mostfavorable one. Nevertheless, for aromatic amino acids fragmentations leading to R or R⁺ startbeing competitive. In fact, for His and Tyr, results indicate that the fragmentation leading to R⁺is the most favorable process.