Structure and Intramolecular Proton Transfer of Alanine Radical Cations

The structures of the four lowest alanine conformers, along with their radical cations and the effect of ionization on the intramolecular proton transfer process, are studied using the density functional theory and MP2 method. The energy order of the radical cations of alanine differs from that of the corresponding neutral conformers due to changes in the basicity of the NH2 group upon ionization. Ionization favors the intramolecular proton transfer process, leading to a proton-transferred radical-cation structure, [NH3 + -CHCH3-COO], which contrasts with the fact that a proton-transferred zwitterionic conformer is not stable for a neutral alanine in the gas phase. The energy barrier during the proton transfer process is calculated to be about 6 kcal/mol.