Photoelectron spectroscopy and circular dichroism in chiral biomolecules: L-alanine

A theoretical treatment of the photoionization of the chiral amino acid L-alanine is presented. Particular attention is paid to a previously unobserved circular dichroism which should be detectable in the photoelectron angular distribution (CDAD) from randomly oriented molecules. Numerical estimates of this difference in the differential cross-sections for left- and right-circularly polarized light range as large as 40% of the mean cross-section. Three different low-energy conformational structures are considered. Further comparisons with the experimental photoelectron spectrum suggest, however, that only one dominates in the gas phase. This concurs with other experimental data but disagrees with conclusions drawn from previous molecular orbital calculations. The magnitude of the predicted CDAD effect, especially when ionizing skeletal bonding orbitals, is sufficient to suggest that it may provide an experimental means for successfully distinguishing optical and conformational isomers.