The Origin of Biological Homochirality

Abstract

The fact that sugars, amino acids, and the biological polymers they construct exist exclusively in one of two possible mirror-image forms has fascinated scientists and laymen alike for more than a century. Yet, it was only in the late 20th century that experimental studies began to probe how biological homochirality, a signature of life, arose from a prebiotic world that presumably contained equal amounts of both mirror-image forms of these molecules. This review discusses experimental studies aimed at understanding how chemical reactions, physical processes, or a combination of both may provide prebiotically relevant mechanisms for the enrichment of one form of a chiral molecule over the other to allow for the emergence of biological homochirality.

Figure 1.

(A) Mirror-image molecules of alanine, a proteinogenic amino acid, and ribose, a pentose sugar. (B) Right-handed α-helix structures of a peptide from amino acids (left) and RNA from ribonucleotides (right).

Figure 2.

Autocatalytic amplification of enantiomeric excess in the Soai alkylation of pyrimidyl aldehydes.

Figure 3.

Prebiotically plausible reactions showing enantioenrichment of either sugars or amino acids.

Figure 4.

Blackmond/Brown model (left) and product enantiomeric excess as a function of fraction conversion of substrate (right) predicted from reaction rate profiles fit to the kinetic model (blue lines) and corroborated by analysis of experimental enantiomeric excess (ee) data.

Figure 5.

Equilibrium between chiral crystals and their saturated solution phases for systems comprised of an unequal number of d and l molecules. (Right) Racemic compound, in which heterochiral interactions are stronger than homochiral interactions; (left) conglomerate, in which homochiral interactions dominate.

Figure 6.

Effect of stirring on rates of primary (A) versus secondary (B) nucleation in the “Eve crystal” model of conglomerate-forming NaClO₃.

Figure 7.

Evolution of solid phase homochirality based on dissolution and reaccretion of molecules from small crystals (shown in blue) of one enantiomer onto larger crystals (shown in purple) of the other enantiomer, through the conduit of solution phase interconversion between enantiomers.

Figure 8.

Cosolvate enriches the solution phase enantiomeric excess by suppressing the solutility of the racemic compound. (Right) Crystal structure of LD proline with CH₃Cl.