Formation of Glycerol through Hydrogenation of CO Ice under Prestellar Core Conditions

Abstract

Observational studies reveal that complex organic molecules (COMs) can be found in various objects associated with different star formation stages. The identification of COMs in prestellar cores, i.e., cold environments in which thermally induced chemistry can be excluded and radiolysis is limited by cosmic rays and cosmic-rayinduced UV photons, is particularly important as this stage sets up the initial chemical composition from which ultimately stars and planets evolve. Recent laboratory results demonstrate that molecules as complex as glycolaldehyde and ethylene glycol are efficiently formed on icy dust grains via nonenergetic atom addition reactions between accreting H atoms and CO molecules, a process that dominates surface chemistry during the "CO freeze-out stage" in dense cores. In the present study we demonstrate that a similar mechanism results in the formation of the biologically relevant molecule glycerol-HOCH 2 CH(OH)CH 2 OH-a three-carbon-bearing sugar alcohol necessary for the formation of membranes of modern living cells and organelles. Our experimental results are fully consistent with a suggested reaction scheme in which glycerol is formed along a chain of radical-radical and radical-molecule interactions between various reactive intermediates produced upon hydrogenation of CO ice or its hydrogenation products. The tentative identification of the chemically related simple sugar glyceraldehyde-HOCH 2 CH(OH)CHO-is discussed as well. These new laboratory findings indicate that the proposed reaction mechanism holds much potential to form even more complex sugar alcohols and simple sugars.

Reactive Desorption of CO Hydrogenation Products under Cold Pre-stellar Core Conditions

“…Non-overlapping (and generally weaker) IR absorption features of COMs, i.e., around 860 cm −1 for glycolaldehyde and ethylene glycol are not visible, yielding upper limits of the order of ∼5×10 13 molecules cm −2 s −1 , i.e., <3% of the consumed CO column density. It should be noted that previous studies on solid state COM formation by Chuang et al (2016Chuang et al ( , 2017 and Fedoseev et al (2017) were performed in co-deposition modus that are assumed to simulate interstellar ice conditions in a more representative way. In Figure 6, the obtained upper limits of N RD (C)/N consumed (C) ratios are summarized for two different time periods: from the start to the end (0 − 180 minutes) and from 60 − 180 minutes.…”

Formation of interstellar methanol ice prior to the heavy CO freeze-out stage

“…In UV-rich environments in the interstellar medium, the formation of CH 3 OH may thus be crucial to the formation of larger COMs. The studies by Chuang et al (2015Chuang et al ( , 2017 and Fedoseev et al (2017) show that CH 3 OH may be a key player in the formation of larger COMs, also for cold dense prestellar core conditions. The authors show that radicals derived from CH 3 OH via an abstraction process can recombine or combine with other radicals, resulting in the formation of COMs as large as glycerol, even at temperatures below 20 K and without the need for UV-induced radiation.…”

Hydrogen transfer reactions of interstellar complex organic molecules

“…In fact the H + CO reaction network has evolved into a network where carbon-carbon bonds can be formed via radical-radical reactions between the 'fundamental' radicals that are created as intermediates, i.e., HCO, CH 2 OH, and CH 3 O. Most of these reactions have been studied experimentally in various ways (Fedoseev et al 2015;Butscher et al 2015;Chuang et al 2016;Fedoseev et al 2017;Chuang et al 2017;Butscher et al 2017) and they have also been proposed by and are included in a number of astrochemical model studies (Garrod et al 2008;Woods et al 2012;Coutens et al 2018). Similar conclusions are also supported by observational work for specific species (Rivilla et al 2017;Li et al 2017).…”