Continuous UV irradiation played an inconclusive role in selecting the canonical genetic alphabet

An abundance of heterocyclic molecules analogous to canonical nucleobases (i.e., adenine, thymine, guanine, cytosine, and uracil) used as the genetic alphabet in DNA and RNA could have been present in various prebiotic environments on early Earth. However, extant biology generally uses the canonical nucleobases and nucleosides as the genetic alphabet, and not any number of possible noncanonical counterparts. Ultraviolet (UV) irradiation was likely abundant on the surface of the early Earth and has been suggested to play a role in the selection of the canonical nucleobases and nucleosides as the genetic alphabet of life. Previous studies examining the photophysics of various canonical and noncanonical nucleobases and nucleosides have suggested that the canonical molecules have shorter excited-state lifetimes and may therefore be less prone to photodamage. Here, we evaluate the photostabilities of various canonical and noncanonical nucleobases and nucleosides under continuous UV irradiation at λ = 254 nm to determine their rates of degradation and compare how photostable they are. Additionally, we estimate the relative photostabilities of these molecules under a plausible early Earth UV environment. We find that the canonical nucleobases and nucleosides span nearly the entire range of observed rates of photodegradation. These results indicate that the selective factors for the genetic alphabet may be more complex than purely due to selection by UV photostability.