Nitriles in Prebiotic Chemistry and Astrobiology

Abstract: Life appeared on Earth within a billion years of the planet’s formation. How? - no one knows. Theories regarding the origin of life involve reactions of molecules predicted to have existed on early Earth in what is called prebiotic chemistry. In this thesis, I use computational methods to investigate hypotheses in prebiotic chemistry and astrobiology. With computational chemistry, it is possible to predict the thermodynamics and kinetics of chemical processes. The long-term goal of this line of research is furthering our understanding of the origin of life. The first part of this thesis is devoted to hydrogen cyanide (HCN) chemistry. HCN is believed to have been present on early Earth. Molecular building blocks of DNA, RNA, and proteins have been detected among HCN reaction products. However, because of HCN’s reactivity, the molecule forms numerous other compounds as well. One such set of proposed reaction products are HCN-derived polymers - a diverse group of structures which have been proposed to form in many ways. Here I present a thermodynamic landscape of HCN-derived molecules and polymers. Using the thermodynamic map, some hypothesized reaction pathways are proven to be unfeasible. Polyaminoimidazole is estimated to be one of the most stable polymers, while the nucleobase adenine is computed as the most stable of all studied structures. We also investigate the first steps in the formation of two proposed HCN reaction products: diaminomaleonitrile and polyimine. Our results reveal that all studied competing reactions have similar activation barriers. These results open for the possibility of a diverse beginning to HCN oligomerization under kinetic control. The estimated timescale of HCN oligomerization suggests that reactions in low-temperature environments as cold as 200 K could occur within thousands of years. I discuss the implications of the predicted reaction rates for HCN chemistry in astrochemical environments like comets and Saturn’s moon Titan. The second part of the thesis investigates an astrobiological hypothesis: the possibility for cryogenically operable membranes in the seas of Titan. It is concluded that a previously suggested polarity-inverted membrane made from acrylonitrile, a so-called azotosome, cannot spontaneously self-assemble and is therefore unlikely to exist on Titan.