Reaction Routes for Experimentally Observed Intermediates in the Prebiotic Formation of Nucleobases under High-Temperature Conditions

Enantiodetermining processes in the synthesis of alanine, serine, and isovaline

In this study, quantum chemical calculations were used to explore the synthesis of three chiral α-amino acids, specifically alanine, serine, and isovaline, from reactants found in interstellar space. Our focus is on the crucial step in the synthesis pathway that determines the chirality of the amino acids. The results indicate that in the case of alanine, the determination of enantiomer is primarily influenced by the direction of the collision of molecules or functional groups, which leads to the formation of a chirality center in a crucial intermediate. However, contrary to chemical expectations, the enantiodetermining/enantioselection step for serine and isovaline synthesis occurs prior to the creation of a chirality center.

Photophysical Characterization of Isoguanine in a Prebiotic-Like Environment

It is intriguing how a mixture of organic molecules survived the prebiotic UV fluxes and evolved into the actual genetic building blocks. Scientists are trying to shed light on this issue by synthesizing nucleic acid monomers and their analogues under prebiotic Era-like conditions and by exploring their excited state dynamics. To further add to this important body of knowledge, this study discloses new insights into the photophysical properties of protonated isoguanine, an isomorph of guanine, using steady-state and femtosecond broadband transient absorption spectroscopies, and quantum mechanical calculations. Protonated isoguanine decays in ultrafast time scales following 292 nm excitation, consistently with the barrierless paths connecting the bright S₁ (ππ*) state with different internal conversion funnels. Complementary calculations for neutral isoguanine predict similar photophysical properties. These results demonstrate that protonated isoguanine can be considered photostable in contrast to protonated guanine, which exhibits 40-fold longer excited state lifetimes.

Self-catalytic mechanism of prebiotic reactions: from formamide to pterins and guanine

Reaction pathway of prebiotic reactions for formation of the pteridines: pterin, xanthopterine, isoxanthopterine and leucopterine, as well as the purine nucleobase guanine from pure formamide are presented. In these reactions, formamide or its tautomer, formimidic acid, play the role of proton-carrying catalyst. All required raw materials, such as hydrogen cyanide, ammonia, water, formic acid, urea, 2-aminomalononitrile, glyoxal, glyoxylic acid and oxalic acid needed in the self-catalyzed reactions are obtained by partial decomposition of formamide. We show that the prebiotic formation of nucleobases and pterins is closely linked and they probably coexisted at the beginning of chemical evolution.

Radical Scavenging Activity of Natural-Based Cassaine Diterpenoid Amides and Amines

The radical scavenging capacities of four new cassaine diterpenoid amides including 3β-hydroxydinorerythrosuamide (1), 3β-acetoxydinorerythrosuamide (2), 3β-tigloyloxydinorerythrosuamide (3), and 6α-hydroxydinorcassamide (4) present in leaf extract and four new cassaine diterpenoid amines namely erythroformine A (5), erythroformine B (6), 6α-hydroxy-nor-cassamine (7), and nor-erythrosuamine (8) recently identified in the extract of the bark of Erythrophleum fordii were elucidated using density functional theory (DFT) method. Different thermochemical properties characterizing antioxidant potential including bond dissociation enthalpy (BDE), proton affinity (PA), and adiabatic ionization potential (IP) were calculated at the B3LYP/6-311G(d,p) level of theory. Scavenging reaction mechanisms of cassaine diterpenes toward HOO^• radical including formal hydrogen transfer (FHT; either hydrogen atom transfer (HAT) or proton coupled electron transfer (PCET)), radical adduct formation (RAF), single electron transfer (SET), and proton transfer (PT) were studied in the gas phase, water, and benzene. The potential energy profiles and kinetic calculations for the FHT and RAF reactions were calculated at 298.15 K. The results showed that all the studied compounds present strong antioxidant activity via HAT mechanism with BDEs varying from 69.4 to 77.9 kcal/mol. While solvents have only a slight effect on HAT and RAF mechanisms, SET and PT reactions are likely to occur in polar media. Among the studied compounds, 3 is the most reactive one both for HAT (H-abstraction at C7, Δ H -11.3 kcal/mol) and for RAF (radical addition at C13 = C15 bond, Δ H -5.2 kcal/mol) reactions. The rate constants of these reactions are also comparable with several referenced antioxidants such as Trolox and ascorbic acid.