Significant Chiral Asymmetry Observed in Neutral Amino Acid Ultraviolet Photolysis

Area of Science:

• Astrobiology
• Chemical Physics
• Origins of Life

Background:

• The origin of homochirality (the predominance of one enantiomer) in biological systems is a fundamental unsolved question.
• The extraterrestrial hypothesis suggests circularly polarized light in space could induce homochirality, but this is debated due to perceived low optical rotatory dispersion.
• Previous studies indicated limited asymmetry in molecular photodissociation, hindering acceptance of extraterrestrial origins for homochirality.

Purpose of the Study:

• To investigate the potential for circularly polarized photons to induce significant chiral asymmetry in amino acid dissociation.
• To experimentally determine if specific amino acid conformers exhibit substantial asymmetry in photodissociation rates.
• To assess the viability of asymmetric photodissociation as a mechanism for establishing biological homochirality.

Main Methods:

• Experimental measurement of chiral-destroying dissociation rates of neutral amino acid conformers using circularly polarized photons.
• Quantification of the anisotropy factor, a measure of photodissociation asymmetry.
• Comparison of observed anisotropy factors with previously reported values for different amino acid states and conditions.

Main Results:

• Specific conformers of neutral amino acids demonstrated significant asymmetry in dissociation rates when exposed to circularly polarized photons.
• The lowest energy conformer of leucine exhibited a remarkably large anisotropy factor of 0.1.
• This observed anisotropy is substantially greater than values reported for zwitterionic leucine or gas-phase leucine ensembles.

Conclusions:

• Asymmetric photodissociation of specific amino acid conformers can generate substantial enantiomeric excess.
• A significant anisotropy factor, even if seemingly small, can lead to considerable enantiomeric imbalance.
• This provides strong experimental support for asymmetric photodissociation as a plausible mechanism for the origin of biological homochirality.