Radical formation of amino acid precursors in interstellar regions? Ser, Cys and Asp

Abstract

It is proposed that the glycine precursor NH₂CH₂CN may be synthesised in interstellar dust clouds by the radical combination reactions NH₂˙ + ˙CH₂CN → NH₂CH₂CN (ΔG = −302 kJ mol⁻¹) and/or NH₂CH₂˙ + ˙CN → NH₂CH₂CN (ΔG = −414 kJ mol⁻¹). All calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G(d) level of theory. This paper extends that concept to radical/radical coupling reactions to form Ser, Cys and Asp precursor nitriles. The hydrogen abstraction process NH₂CH₂CN + HO˙ → NH₂˙CHCN + H₂O (ΔG = −130 kJ mol⁻¹) is suggested to precede the radical coupling reactions NH₂˙CHCN + R˙ → NH₂CHRCN (R˙ = ˙CH₂OH, ˙CH₂SH and ˙CH₂CN) to form nitrile precursors of the amino acids Ser, Cys and Asp. These three reactions are all favourable (ΔG = −240, −227 and −223 kJ mol⁻¹). The radical species ˙CH₂NH₂, ˙CH₂OH, ˙CH₂SH and ˙CH₂CN are shown to be stable for the microsecond timeframe by a combination of theoretical calculations and the experimental mass spectrometric neutralization/reionization procedure.