Chiral Asymmetry in the Photoionization of Gas-Phase Amino-Acid Alanine at Lyman-α Radiation Wavelength

Gas-phase pure enantiomers of alanine, the simplest proteinaceous chiral amino acid, are investigated by photoelectron circular dichroism, a direct chiroptical, orbital-sensitive effect giving rise to large asymmetries in the photoelectron angular distribution upon photoionization by circularly polarized light. Here we report electron imaging measurements made at the Lyman-alpha radiation photon energy (10.2 eV) that reveal a strong overall asymmetry for the outermost orbital. Despite the anticipated presence of different conformers, this asymmetry is effectively independent of sample temperature (and hence of conformer population). Furthermore, because of the associated recoiling of the corresponding ion, photoionization by circularly polarized light can generate an asymmetric flux of gas-phase alanine cations, allowing us to deduce an enantiomeric excess, in a given line of sight, of up to 4%. In addition to the implications for the origin of biomolecular asymmetry, these studies pave the way for future chiroptical analytical studies of more complex biomolecules such as peptides.