Interstellar Proton-Mediated Sugar Formation in Isolated Gas-Phase Systems: A Quantum Chemical Study

Sugars, essential prebiotic building blocks, have been hypothesized to form within the interstellar medium (ISM). However, their synthesis under ISM conditions–characterized by extremely low temperatures and pressures–remains elusive. By means of first-principles calculations, we investigated the role of proton catalysis in key sugar formation reactions: (i) self-oligomerization of formaldehyde via formose pathway, (ii) D-aldose isomerization, and (iii) D-aldose cyclization. Our calculations assumed static conditions in the gas phase, neglecting the potential influence of water ice. The results reveal that proton catalysis significantly lowers the activation energy barriers at each iterative step of the formose pathway compared to uncatalyzed pathways. However, most of these reactions remain energetically inaccessible, despite the presence of an assisting molecule such as water or ammonia. Notably, once D-aldoses are formed, proton-mediated pathways efficiently facilitate the synthesis of D-ketoses and cyclic D-ribose, offering insights into selective sugar formation in the ISM. To further support experimental characterization, we analyzed the vibrational spectra of key species. C=O stretching and O-H⋅⋅⋅O intramolecular vibration serve as key spectroscopic markers for identifying sugars and related compounds. These findings highlight both the catalytic potential and inherent limitations of interstellar protons in gas-phase sugar formation, emphasizing the need for further investigation into alternative reaction pathways, catalysts, and energy sources essential for sugar synthesis under ISM conditions.