Life’s Building Blocks Have a Handedness and a Cosmic Connection
Despite the seemingly chaotic and unhospitable environment represented by space, life’s most fundamental building blocks can still emerge. Through spacecraft missions and sample returns, amino acids and sugars have been found in carbonaceous meteorites. On Earth, these molecules show a preference for a particular handedness - which are versions of a molecule that are mirror images of each other (see Figure 1). Whereas most abiotic reactions give an even split between left- and right-handed molecules, the compounds in carbonaceous meteorites are biased toward a particular handedness. But not just any particular handedness. As noted by Glavin et al. (2019), amino acids are biased toward left-handed variants and sugar alcohols are biased toward the right-handed versions, which are the same set of handedness as used by life!
In 2019, Glavin et al. released a paper in Chemical Reviews summarizing the current state of knowledge of molecules that have a handedness to them (Figure 1), which is a feature called chirality. Chiral molecules include life’s most important building blocks, amino acids, which are almost exclusively left-handed. Amino acids are the primary constituents of proteins. These have important structural roles and a special class of proteins called enzymes are even involved in crucial chemical reactions by increasing their rates. Another set of chiral compounds relevant to life are sugars. Life uses mostly right-handed sugars. Sugars make up nucleic acids so they are found in RNA and DNA; they can act as coenzymes in some metabolic reactions; they have important structural roles in cells; and, of course, they are a source of energy for life.
Figure 1. Above is a pair of molecules that are non-superimposable mirror images of one another. Pairs like this are called enantiomers. This molecule here is a generic amino acid where the R group represents some chemical group that is not already represented (-COOH, -NH2, -H).
The pairs of mirror-imaged molecules are called enantiomers and many hypotheses have been proposed as to how abiotic reactions could have led to a bias in one enantiomer over another. Abiotic reactions are ones that do not depend on life interacting with other chemical species. Abiotic reactions are dependent on other sources such as light and temperature, which are the types of reactions in space. Glavin et al. (2019) note that the prevailing hypothesis is that circularly-polarized light (CPL), which is a chiral form of light, could have biased the amino acids and sugar alcohols. However, there are many more hypotheses that have been proposed (Figure 2).
Figure 2. A modified diagram from Meierhenrich (2008) edited to show some other symmetry breaking hypotheses since the book’s publication. Since 2008, Globus et al. (2021) suggested that polarized muons could have led to enantioselective mutagenesis, which in turn could have allowed more favorable mutations to a lineage of proto-organisms or prebiotic chemistry that was using the “correct” chirality. The CISS effect, which has utilized a strong magnetic field and large, flat magnetite surfaces, has been able to produce strong biases. However, it remains to be seen if it can operate in an astrophysically or prebiotically relevant environment. In my own research, I am studying to see whether circularly polarized light could have indirectly influenced the chirality of amino acids and sugar alcohols by first biasing a chiral inorganic intermediate, namely, clinoenstatite.
What makes CPL so compelling? One of the biggest reasons is that it has already been detected in interstellar space around star-forming regions in the Orion nebula. Since then, experimental evidence has suggested that CPL in the ultraviolet region can lead to an enantiomeric bias in various amino acids. Estimates and experimental results for other proposed mechanisms have not been able to surpass the enantiomeric excess predicted by CPL except for the CISS effect (Ozturk et al., 2023). CISS stands for chiral-induced spin selectivity, and it can produce a bias of up to 60%. However, it remains to be seen if it can act in an astrophysically or prebiotically relevant environment. A lot of work remains to be done to see which of the mechanisms (Figure 2) are correct or to see if there is another mechanism that has not been thought of just yet!
This article made use of the following publications:
Glavin et al. 2019, Chemical Reviews, 120(11), 4660-4689
