Asymmetric Photolysis of Amino Acids and Its Relation to the Origin of Homochirality
Takamasa Momose
Professor, Department of Chemistry
The University of British Columbia
Wednesday, September 18, 2024
11 a.m.
In-person: C2-361
Abstract: Objects are chiral when they cannot be superimposed on their mirror images, such as our left and right hands. All proteinogenic amino acids, except for glycine, are chiral molecules. It is known that the human body is composed of only one type of amino acid chirality, a phenomenon known as homochirality. However, the origin of homochirality in biological systems remains an open question. Some suggest that its origin might be extraterrestrial, specifically due to the exposure of chiral molecules to circularly polarized photons in interstellar space, which could cause an initial population imbalance leading to the homochirality observed today. However, this extraterrestrial hypothesis has not been widely accepted, largely due to the belief that molecular optical rotatory dispersion is too insignificant to create the substantial imbalance required for homochirality. Our group has investigated the photolysis of amino acids in their non-zwitterionic form using matrix isolation spectroscopy. Recently, we found that specific conformers of neutral amino acids exhibit significant asymmetry in the chiral-destroying dissociation rate induced by circularly polarized photons. The observed anisotropy factor for the lowest energy conformer of leucine was remarkably large, reaching 0.1 – a factor 15 times larger than observed for zwitterionic leucine in solid films, and nearly 40 times greater than the anisotropy reported in the electronic absorption spectrum of gas-phase leucine ensembles at room temperature. This significant finding indicates that even if reported anisotropy values in the electronic absorption spectrum are low, the dissociation asymmetry of certain conformers can still be substantial. An anisotropy factor of 0.1 could result in an initial enantiomeric excess exceeding 10%, even with a 90% extent of reaction. This result suggests that asymmetric photodissociation of amino acids may have been a crucial factor in the emergence of biological homochirality. The importance of the results of this experiment will be discussed.