The origin of life has always been a great subject of research, with many theorizing that the “molecules of life”—nucleic acids, proteins, carbohydrates, and lipids—were introduced to pre-biotic Earth by a comet or meteorite that struck its surface [1]. Early or pre-biotic Earth defines a period about 4.5 billion years ago, before life existed due to the extreme environment on the planet [2]. Proteins are made up of amino acids, which are organic compounds generally characterized by a carboxylic acid group (COOH), an amine group (NH₂), and a specific “R” group, as shown in Figure 1A. While many exist, only 20 of them serve as the building blocks of proteins in organisms. Synthesis of amino acids requires the presence of certain molecules, such as H₂O, NH₃, CH₄, and HCN, which have been found in interstellar medium (ISM) [3].

Interstellar medium is the matter and radiation that exists in galaxies between star systems. ISM is organized into molecular clouds, which are areas dense in chemical species (especially hydrogen and helium), where star formation reactions could occur [4]. Molecular clouds are further divided into various types, depending on conditions such as temperature and density. One such type, dense clouds, are characterized by a temperature of 15–20 K and a molecular density of 10³-10⁶ cm^-3 [5]. While the temperatures are low, energy sources such as cosmic rays and soft X-rays have been detected [6].

Considering amino acids are of great biological importance, there has been great interest in verifying their existence, as there is a possibility that they have extraterrestrial origins [8,9]. Indeed, studies have shown that some amino acids can be generated in conditions matching those of interstellar dense clouds [7]. Since glycine is the simplest of all amino acids (shown in Figure 1B), much research has been done to verify its presence in ISM.

There have been many attempts to verify glycine’s presence in interstellar clouds in recent years, the most famous being glycine’s “detection” in 2003, based on the observation of 27 spectral lines associated with glycine from three different molecular clouds [8-13]. However, the results of this study were disputed after analysis of the data showed that the spectral data could have belonged to a number of different molecules that were structurally similar to glycine, such as formic acid and acetic acid [9]. Furthermore, all of the spectral lines from the three clouds only had three lines in common, which further weakened the evidence for glycine’s detection presented in the study [9]. Therefore, glycine’s presence in ISM has yet to be confirmed. However, it must be acknowledged that there are difficulties associated with glycine’s detection, specifically regarding its weak lines in rotational spectrums [8].

Over a decade ago, NASA probe Stardust obtained cometary samples showing promising data [14]. Analysis of these samples showed organic matter rich in oxygen and nitrogen, while presence of deuterium and N-15 suggested that at least some of the matter had interstellar origins, most likely of dense clouds and/or protostellar nebulae. Additionally, a statistically significant amount of glycine, relatively higher than those in controls, was found in the comet sample returned by Stardust [14]. Further analysis of glycine present in the comet confirmed its detection and interstellar origin based on the stable carbon isotopic ratio. The study also ruled out the possibility of contamination of glycine since it was found on the aerogel (comet-exposed) side of the sample [15]. Overall, the presence of glycine in the comet not only confirms glycine’s existence in ISM, but also illustrates the possibility of safe delivery of glycine to Earth, additionally strengthening the theory of interstellar origin of life.

Although glycine and various other molecules have been detected in comets, the mechanism for the formation of gas-phase glycine in dense clouds is unclear, although attempts have been made in numerous studies [16-22]. There is a considerable amount of research in the mechanism for glycine’s formation and in precursors that could be of interest for its synthesis. A five-membered heterocyclic compound, hydantoin (glycolylurea), as shown in Figure 2, has been proposed to be a possible precursor. It is known to occur from prebiotic molecules (such as urea and glycolic acid) under ISM conditions and was also detected in carbonaceous chrondites [23]. However, hydantoin has not yet been detected in ISM [19,24].

Although numerous studies have been conducted to find and verify interstellar glycine, its possible precursors and formation mechanism in various ISM media remain unknown. This study proposes a new reaction for the formation of interstellar glycine in gas phase under dense cloud conditions, based on the reaction mechanism shown in Figure 3. The proposed reaction uses small molecules (HCN, CO, H₂O, and H₂) known to be abundant in ISM [4,25], and is theoretically calculated to be exothermic in ISM, which suggests the possible feasibility of this reaction occurring in dense clouds.