The prebiotic origins of biopolymers and metabolic co-factors are key questions in Origins of Life studies. In a simple warm-little-pond model, using a drying phase to produce a urea-enriched solution, we present a prebiotic synthetic path for the simultaneous formation of neopterins and tetrahydroneopterins, along with purine nucleosides. We show that, in the presence of ribose and in a formylating environment consisting of urea, ammonium formate, and water (UAFW), the formation of neopterins from pyrimidine precursors is robust, while the simultaneous formation of guanosine requires a significantly higher ribose concentration. Furthermore, these reactions provide a tetrahydropterin-pterin redox pair. This model suggests a prebiotic link in the origin of purine nucleosides and pterin cofactors that provides a possible deep prebiotic temporal connection for the emergence of nucleic acids and metabolic cofactors.

Context. Formaldehyde is a potential biogenic precursor involved in prebiotic chemical evolution. The cold conditions of the interstellar medium (ISM) allow H 2 CO to be reactive, playing a significant role as a chemical intermediate in formation pathways leading to interstellar complex organic molecules (iCOMs). However, gas-phase molecular formation mechanisms in cold regions of the ISM are poorly understood. Aims. We computationally determine the most favored gas-phase molecular formation mechanisms at local thermodynamic equilibrium (LTE) conditions that can produce the detected amounts of H 2 CO in diffuse molecular clouds (DMCs), in dark, cold, and dense molecular clouds (DCDMCs), and in three regions of circumstellar envelopes of low-mass protostars (CELMPs). Methods. The potential energy surfaces, thermodynamic functions, and single-point energies for transition states were calculated at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory and basis sets. Molecular thermodynamics and related partition functions were obtained by applying the Maxwell-Boltzmann quantum statistics theory from energies computed at CCSD(T)-F12/cc-pVTZ-F12 with corrections for zero-point energy. A literature review on detected abundances of reactants helped us to propose the most favorable formation routes. Results. The most probable reactions that produce H 2 CO in cold astrophysical regions are:DCDMCs, and •CH 3 + •O( 3 P) → 1 H 2 CO + •H in region III, •CH 3 +•O( 1 D) → 1 H 2 CO + •H in region II, and 1 CH 2 + • 3 O 2 → 1 H 2 CO + •O( 3 P) in region I belonging to CELMPs. Conclusions. Quantum chemical calculations suggest that the principal carbonaceous precursors of H 2 CO in cold regions for the gas-phase are CH 2 (a 1 A 1 ), and •CH 2 (X 3 B 1 ) combined with •O 2 ( 3 Σ g ) and •CH 3 ( 2 A " ) + •O( 3 P) / O( 1 D). Reactions based on more complex reagents yield less effective thermodynamics in the gas-phase H 2 CO molecular formation.

Context. Cyanamide (NH 2 CN) and its tautomer carbodiimide (NHCHN) are believed to have been key precursors of purines and pyrimidines during abiogenesis on primitive Earth. The detection of guanine and cytosine in meteorites and comets provides evidence of their nonterrestrial formation. Although NH 2 CN has been found in several molecular clouds, NHCHN has only been detected in Sgr B2(N). Their possible molecular formation mechanisms in the gas phase and therefore their respective molecular precursors remain an open subject of investigation. Aims. The main objective of this paper is to determine which reactions can produce NH 2 CN and HNCNH in the amounts observed under the astrophysical conditions of Sgr B2(N). The determination of their most likely precursors could serve to provide new insights into possible routes to purine and pyrimidine synthesis, and by extension to nucleosides, under the astrophysical conditions of dense molecular clouds. Methods. Initially, we proposed 120 reaction mechanisms, 60 being dedicated to NH 2 CN formation and the remaining 60 to HNCNH. These mechanisms were constructed using 25 chemical species that were identified in outer space. We calculated the molecular energies of reactants and products at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory, and defined the values of thermodynamic functions using the Maxwell-Boltzmann statistical quantum theory. Via an extensive literature review on the abundances of reactants and products in Sgr B2(N), in addition to a detailed kinetic study for a range of 20-300 K, we identify the most likely reaction mechanisms for both cyanamides of those proposed previously and presently.Results. From the 120 analyzed reactions, only nine for NH 2 CN and four for HNCNH could thermodynamically account for their synthesis in Sgr B2(N). The kinetic portion of our study shows that Ra60 (CH 3 NH 2 + •CN → NH 2 CN + •CH 3 ), with a modified Arrhenius expression of k T = 1.22 x 10 −9 ( T 300 ) −0.038 exp − ( −147.34 T ) cm 3 mol −1 s −1 , is the most efficient reaction at low temperatures (<60 K). Above 60 K, no reaction with known reagents in Sgr B2(N) is efficient enough. In this way, Ra37-2 (•HNCN + •NH 2 → NH 2 CN + 3 NH ) appears to be the most likely candidate, showing a modified Arrhenius constant of k T = 2.51 x 10 −11 ( T 300 ) −32.18 exp − ( −1.332 T ) cm 3 mol −1 s −1 . In the case of carbodiimide production, Rb18 (•H 2 NC + •NH 2 → HNCNH + •H) is the most efficient reaction, fitting a rate constant of k T = 4.70 x 10 −13 ( 300 T ) −3.24 exp − ( 36.28 T ) cm 3 mol −1 s −1 in Sgr B2(N). Conclusions. The detected gas-phase abundances of cyanamide (NH 2 CN) in Sgr B2(N) can be explained as: Ra60 (•CN + •CH 3 NH 2 ) from 20-60 K; Ra5: (•CN + •NH 2 ) from 60-120 K; and Ra37-2 (•HNCN + •NH 2 ) from 120-300 K. The carbodiimide (HNCNH) synthesis could proceed via Rb18 (•H 2 NC + •NH 2 ). Moreover, the presence of •HNCN and •H 2 NC in Sgr B2(N) are predicted here, making them viable candidates for future astronomical observations. The fores...