A new study provides new insights into the cosmic evolution of amino acids

All biological amino acids on Earth appear exclusively in their left form, but the reason behind this observation is elusive. Recently, scientists from Japan have discovered new clues about the cosmic origin of this asymmetry. Based on the optical properties of the amino acids found in the Murchison meteorite, they ran physics-based simulations, revealing that precursors to biological amino acids may have determined amino acid homology during the early phase of galactic evolution.

If you look at your hands, you’ll notice that they are mirror images of each other. However, no matter how hard you try to flip and rotate one hand, you will never be able to fit it perfectly on the other hand. Many molecules have a similar property called “chirality”, which means that the “left” (L) copy of a molecule cannot be superimposed on the “right” mirror image (D). Although both versions of a chiral molecule, called “homologous,” have the same chemical formula, the way they interact with other molecules, especially with other chiral molecules, can differ greatly.

Interestingly, one of the many mysteries surrounding the origin of life as we know it has to do with karlism. It turns out that biological amino acids (AAs) – the building blocks of proteins – appear on Earth only in one of their two possible isoforms, L-form. However, if you synthesize AAs artificially, both the L and D forms will be produced in equal amounts. This indicates that, at an early point in the past, L-AAs must have dominated the heterotrophic world. This phenomenon is known as “spiral symmetry breaking”.

Against this background, a research team led by Assistant Professor Mitsuo Shoji of the University of Tsukuba, Japan, conducted a study aimed at solving this mystery. As described in their paper published in Journal of Physical Chemistry Lettersthe team sought to find evidence supporting a cosmological origin for homozygous AAs on Earth, as well as reconciling some of the inconsistencies and inconsistencies in our previous understanding.

“The idea that homeopathy may have originated in space was proposed after AAs were found in the Murchison meteorite that fell in Australia in 1969,” explains Dr. Shoji. Curiously, in the samples obtained from this meteorite, both L-enantiomers were more prevalent than their D-enantiomer counterpart. One common explanation for this is that the asymmetry is caused by circularly polarized ultraviolet (CPL) light in our galaxy’s star-forming regions. Scientists have verified that this type of radiation can actually lead to asymmetric photochemical reactions which, given enough time, will favor the production of L-AAs over D-AAs. However, the adsorption properties of isovaline AA are inconsistent with those of other AAs, implying that an interpretation based on UV alone is either insufficient or incorrect.

Against this background, Dr. Shoji’s team pursued an alternative hypothesis. Rather than the far ultraviolet, they hypothesized that the chiral asymmetry was, in fact, caused specifically by the CP Lyman-α (Lyα) emission line, a spectral line of hydrogen atoms that permeated the early Milky Way. Furthermore, rather than focusing solely on the photoactive interactions in AAs, the researchers investigated the possibility of chiral asymmetry starting with the precursors of AAs, namely the aminopropanals (APs) and amino nitriles (ANs).

Through quantum mechanical calculations, the team analyzed Lyα-induced reactions to produce AAs along the chemical pathway involved in Strecker synthesis. Then they observed the ratios of the L- to D-enantiomers of the AAs, APs, and ANs at each step of the process.

The results show that the L-enantiomers of ANs are preferentially formed under right CP (R-CP) Lyα irradiation, with their enantiomers matching those of the corresponding AAs. “Together, our findings suggest that homozygous genes underlie the origin of homosexual personality,” notes Dr. Shoji. “More specifically, irradiation of the AN precursor with R-CP Lyα radiation leads to a higher proportion of L-enantiomers. The subsequent predominance of L-AAs is possible through interactions induced by water and heat molecules.”

Thus, the study brings us one step closer to understanding the complex history of our own biochemistry. The team stresses that further studies focusing on ANs should be conducted on future samples of asteroids and comets to validate their findings. “Further analyzes and theoretical investigations of ANs and other prebiotic molecules related to polysaccharides and nucleobases will provide new insights into the chemical evolution of the molecules, and thus the origin of life,” concludes Dr. Shoji.