Ab initio study of the two-photon circular dichroism in chiral natural amino acids.

Two-photon circular dichroism spectra calculated within an origin-invariant density functional theory approximation in the absorption region where the lowest electronic excited states appear are presented for all 19 essential amino acids in the gas phase. A comparison of intensities and characteristic features is made with the corresponding two-photon absorption and one-photon circular dichroism spectra for each species. Also, the contributions of the electric dipole, magnetic dipole, and electric quadrupole transitions to the rotational strengths are analyzed in some detail. The remarkable fingerprinting capabilities of the two-photon circular dichroism spectroscopy are highlighted.