Gas-phase proton-transport self-catalysed isomerisation of glutamine radical cation: The important role of the side-chain

The gas-phase isomerisation reaction of glutamine radical cation from [NH2CH (CH2CH2CONH2) COOH ] +• to [ NH2C (CH2CH2CONH 2) C (OH)2]+• has been studied theoretically using the MPWB1K functional approach. The [ NH2 C (CH2CH2CONH2) C (OH)2] +• diol species has been found to be the most stable isomer for glutamine radical cation. Moreover, it has been observed that glutamine has a long enough side-chain with basic groups that acts as a solvent molecule favouring the proton-transfer from C α to COOH group. This fact reduces dramatically the isomerisation energy barriers compared to the same process for glycine radical cation in gas phase. Thus, this reaction can be considered as an example of gas-phase proton-transport catalysed reaction in which the proton-transport is carried out by the reactant molecule itself instead of any solvent. © 2007 Springer-Verlag.