Experimental identification of aminomethanol (NH2CH2OH)—the key intermediate in the Strecker Synthesis

The Strecker Synthesis of (a)chiral α-amino acids from simple organic compounds, such as ammonia (NH3), aldehydes (RCHO), and hydrogen cyanide (HCN) has been recognized as a viable route to amino acids on primordial earth. However, preparation and isolation of the simplest hemiaminal intermediate – the aminomethanol (NH2CH2OH)– formed in the Strecker Synthesis to even the simplest amino acid glycine (H2NCH2COOH) has been elusive. Here, we report the identification of aminomethanol prepared in low-temperature methylamine (CH3NH2) – oxygen (O2) ices upon exposure to energetic electrons. Isomer-selective photoionization time-of-flight mass spectrometry (PI-ReTOF-MS) facilitated the gas phase detection of aminomethanol during the temperature program desorption (TPD) phase of the reaction products. The preparation and observation of the key transient aminomethanol changes our perception of the synthetic pathways to amino acids and the unexpected kinetic stability in extreme environments.

5/ While it is experimentally interesting and laudable to have formed and identified aminomethanol, the meaning in an interstellar context is less clear, and more should be made of this. For example, interstellar ices should have substantial water content, while alternative C-H bonds for insertion by O singlet-D should also be prevalent in methane and methanol, and arguably others. We also do not know what the O2 composition of interstellar ices actually should be, based on observations (although this is not the case for cometary ice). It could, in fact, be small; smaller even than that of NH2CH3, which has also not yet been detected in the solid phase in the ISM and has only recently been detected in the gas phase in rather small quantities. How might all of this impact the importance of the findings? 6/ How applicable is Strecker synthesis in a purely solid-phase context? 7/ How does the mechanism tested here compare with other lab studies, e.g. that of Elsila et al. (2007, ApJ, 600, 911), who used H2O:CH3OH:HCN:NH3 mixtures to produce amino acids? Although they considered a slightly different mechanism, their isotopic labeling results indicated that Strecker synthesis should be less important than radical reaction processes. 8/ The identification of aminomethanol rests strongly on the calculations, which mainly take a back seat in the text. Are there any alternative estimates or measurements of any of the ionization energies calculated here? Is there any alternative verification that can now or in future be used to back up the calculations?
If all of the above points are addressed, I feel that the manuscript would be ready for publication.
Reviewer #3 (Remarks to the Author): The present communication entitled "Gas-phase identification of aminomethanol (NH2CH2OH) -the key intermediate in the Strecker synthesis", S.K. Singh et al shows an experimental work on the formation of NH2CH2OH induced by keV electrons of NH2CH3:O2 ice. Coupling VUV photoionization time of flight spectrometer and theoretical calculations (IE of different isomers), they have identified NH2CH2OH isomers desorbing from irradiated ice. This work relies on well establish procedure in Kaiser's laboratory to investigate formation of different molecules in irradiated ices.
A weak aspect of the paper is that the investigation of NH2CH2OH formation is not a novelty from a conceptual point of view. In the literature, one can find several studies dedicated to investigate the formation of aminomethanol experimentally and theoretically (several references arein the present article). For example it has been shown that NH2CH2OH can be formed in H2O:NH3:H2CO ice ( F Duvernay et ApJ 791 (2014) 75 ). It would be worth mention why in particular NH2CH3:O2 is particularly interesting to investigate aminomethanol formation.
Another point, the whole procedure of isomer identification relies on theoretical calculations. Authors considered error limits due theoretical uncertainties and experimental conditions for benchmark systems. It would be worth also to compare with experimental IE values of discussed isomers (if available).
One can find also inconsistency between the text in "Mass spectrometry" paragraph and Fig 2. It has been stated the data recorded at 9.50 reveal several products at m/z 45, 47 58 and 60. This is visible in Fig. 2b (obtained for 9.10 eV if we follow a caption)? Figure 2 is nice to follow globally dynamics of appearing and disappearing m/z species as a function of temperature. However, the present figure does not allow to read masses in details. Lines to guide eyes should be added or it would be better to show a cut for 210 K -intensity as a function of m/z for all three ices NH2CH3:O2, Nd2CD3:O2 and NH2CH3:18O2. A short comment of high mass formation would enrich the article.
It would be also worth to clarify broadening of the TPD profile at m/z =49. It has been mentioned that is due fragmentation of masses m/z= 58 (C2H6N2) and 80 (CH418O2) ( e.g. loss of fragments of m/z 9 and 31?, respectively). This fragmentation is caused by which factor? Moreover, why we do not observe similar broadening for other ices?
The authors present their original discovery of aminomethanol synthesized during the electron irradiation of methylamineoxygen ices. Unambiguous detection of aminomethanol is achieved by a unique combination of two standard techniques: temperature program desorption and photoionization time of flight mass spectrometry. Comparison of the sublimation temperature dependence of the mass spectra for irradiated ices containing either the isotopologue CD3ND2 -O2, or CH3NH2 -^{18}O2, or aminomethanoloxygen ice demonstrates the unambiguous detection of aminomethanol that is clearly produced by electron-molecule interactions in the simple ice mixture.
Using standard quantum chemistry tools, the authors briefly discuss two reaction mechanisms, either of which may be responsible for the production of aminomethanol: the electron-impact dissociation of O2, producing an excited oxygen atom that reacts with methylamine to form aminomethanol in the icy matrix; or the unimolecular dissociation of methylamine in the ice, releasing a hydrogen atom, that combines with on oxygen atom before recombining with the aminomethyl radical. Both mechanisms seem feasible, and perhaps future electron scattering calculations, as well as molecular dynamics simulations will shed more light on the relative likelihood of each mechanism.
This work is of a broad interest to the scientific community due to the fundamental importance of this synthetic pathway in the abiotic/prebiotic synthesis of simple amino acids. The paper is very clearly written, and the conclusions are fully-supported by the experimental results. I recommend publication in Nature Communications without further revision.
This study considers the formation of aminomethanol in mixed O2/NH2CH3 laboratory ices, through electron bombardment. This process is intended to reproduce the effects of cosmic ray impingement on interstellar ices and/or solar system bodies. The electron-induced dissociation of O2 results in the production of singlet-D oxygen, which has been shown elsewhere to be capable of insertion into C-H bonds; insertion into NH2CH3 is found here to produce aminomethanol (NH2CH2OH). Aminomethanol is an intermediary in the Strecker synthesis of glycine and/or other amino acids, and its possible presence in interstellar space could therefore enable the production of amino acids either in situ, or following delivery to planetary surfaces. But its production in the laboratory has proved to be very challenging. The present work cleverly uses photoionization mass spectrometry during temperature programmed desorption of the post irradiation ices to detect the presence of aminomethanol in the gas phase. The work makes use of calculated ionization potentials to distinguish between the different CH5NO isomers. I believe that this is an important and interesting enough study to be published in Nature Communications. In fact, I believe it somewhat undersells the ingenuity of the successful technique presented here, providing as it does the evidence of the laboratory production of this elusive molecule. A little more could be added to the text to put the implications of the work in context for the non-expert reader (see below), as well as to explain more clearly how important the result may be for amino acid production in pre-solar or early solar system environments.

Response:
We thank the reviewer for appreciating our study and the techniques used here. We have now discussed the significance of our results in the context of amino acid production in pre solar or early solar system environments in the manuscript.

Relevant changes in the manuscript (on pages 7-8):
The following paragraphs are added in the discussion section.
"The present study also shows evidence of the formation of aminomethanol in astrophysical-like conditions. The methylamine (CH 3 NH 2 )-oxygen (O 2 ) ice mixture can be appraised as a model astrophysical ice-analog (ISM). Primary amines such as methylamine (CH 3 NH 2 ) and ethylamine (C 2 H 5 NH 2 ) were observed in ISM towards SgrB2 as well as on comet 67 p/Churyumov-Gerasimenko. [36][37][38] Amines are also contemplated to play a critical role in the prebiotic chemistry of early Earth 39,40  It is my feeling that the same experiment conducted using an ice mixture more representative of interstellar ices might not produce enormous quantities of aminomethanol,so the influence of the Strecker mechanism in feeding off of aminomethanol formed in precisely this way might not be so great (amino acetonitrile could be formed through other mechanisms, for example).

Response:
The chemistry occurring in the interstellar medium is very complex. A given molecular entity could have multiple formation pathways in an actual or realistic interstellar ice mixture. Therefore, to understand the energetic feasibility and mechanisms of these processes, we need to first investigate simple model interstellar ice analogs before these studies can be extended to more complex systems.

Aminomethanol is a well-known intermediate of the Strecker process. Various theoretical studies
have demonstrated the involvement of aminomethanol in Strecker synthesis of glycine in astrophysical-like conditions (Rimola, A., et al. Phys. Chem. Chem. Phys. 12, 5285-5294, (2010); Koch, D. M., et al. J. Phys. Chem. C 112, 2972-2980, (2008). Our laboratory experiments suggest that aminomethanol could generate from amines on ice grains. The aminomethanol formed in proto-stellar cores or protoplanetary disks could be subsequently incorporated inside comets and meteorites, (Danger, G., et al. Astrophys. J. 756, 11, (2012)) which are sources of extraterrestrial amino acids on early earth. Aminomethanol (OHCH 2 NH 2 ) could eventually produce amino acids on comets and meteorites via aminoacetonitrile following the Strecker route.

"The present experimental study supports the theoretical investigations 23,49 that have demonstrated the involvement of aminomethanol in Strecker synthesis of glycine in astrophysical environments."
The Strecker mechanism should also arguably take place in the liquid phase, at least during the hydrolysis stage, so interstellar production in this way may not be so plausible. This should be commented on. Cometary, meteoritic or early Earth conditions may nevertheless lend themselves to amino acid production in this way.
Response: Strecker synthesis could also occur in solid-phase. Several theoretical investigations have been performed, which suggest that the Strecker process could happen in the absence of liquid water. For instance, quantum chemical calculations performed by Rimola et al. show that a Strecker-type reaction to form glycine is energetically feasible on solid-water ice grains (Rimola, A., et al. Phys. Chem. Chem. Phys. 12, 5285-5294, (2010)). The formation of acetonitrile precursor of glycine on an icy grain mantle via addition of hydrogen cyanide (HCN) to dehydrogenated form of aminomethanol has also been shown through calculations (Danger, G., et al. Astrophys. J. 756, 11, (2012)).

Relevant changes in the manuscript (on page 8): We have added the following sentences
"It is important to note here that the Strecker process could initiate even in the absence of liquid water. Quantum chemical calculations performed by Rimola et al. show that a Strecker-type reaction to form glycine is energetically feasible on solid-water ice grains. 23 The formation of 5 acetonitrile precursor of glycine on an icy grain mantle via addition of hydrogen cyanide (HCN) to dehydrogenated form of aminomethanol has also been shown through calculations. 49 " Besides the above, I list a selection of comments, questions and suggestions: 1/ Considering who may be reading this work, I suggest that the title indicate clearly that the identification of aminomethanol has taken place in an experimental setting (rather than in space).

Response:
We have modified the title to "Experimental Identification of Aminomethanol (NH 2 CH 2 OH) -The Key Intermediate in the Strecker Synthesis".

Relevant changes in the manuscript (on page 1):
The title of the article is changed to "Experimental Identification of Aminomethanol (NH 2 CH 2 OH) -The Key Intermediate in the Strecker Synthesis".

Response:
The ice deposition angle is around 20° with respect to the normal of the substrate.   (Bergner, J. B., et al. Astrophys. J. 845, 29, (2017)). Formation of COMs by the oxygen insertion method does not require diffusion of heavy molecular species indicating that the oxygen insertion reactions will be more facile at temperatures lower than 30 K. We agree with the reviewer that alternative reactions of oxygen such as insertion of O ( 1 D) atom at the C-H bonds of methane and methanol should be prevalent in the ISM. However, that does not imply that the probability of oxygen reacting with amines to form aminomethanol (OHCH 2 NH 2 ) will be negligible.

Relevant changes in the manuscript (on page 9): We have added the following sentences
Furthermore, aminomethanol undergoes unimolecular decomposition to form methanimine (CH 2 NH) in the presence of water. In a water-rich ice, the surrounding water molecules could act as a catalyst in the dehydration process of aminomethanol (Rimola, A., et al. Phys. Chem. Chem. Phys. 12, 5285-5294, (2010)); this could also be the reason for the failure in detecting aminomethanol in a water-rich interstellar environment. Here, we choose a non-aqueous ice mixture to avoid the dissociation process of aminomethanol in the ice matrix.

Relevant changes in the manuscript (on pages 7-8):
We have added the following paragraphs to discuss the significance of our results in the context of the interstellar medium.
"Primary amines such as methylamine (CH 3 NH 2 ) and ethylamine (C 2 H 5 NH 2 ) were observed in ISM towards SgrB2 as well as on comet 67 p/Churyumov-Gerasimenko. [36][37][38] Amines are also contemplated to play a critical role in the prebiotic chemistry of early Earth 39,40  difficulties and probably low abundance in the gas phase of ISM. 45 It has been suggested that the poor abundance of oxygen in the gas phase could be due to the condensation of molecular oxygen (O 2 ) onto the interstellar grains. 46

Relevant changes in the manuscript (on page 3):
We have added the following sentences "Furthermore, the rate of unimolecular decomposition of aminomethanol (OHCH 2 NH 2 ) to methanimine (CH 2 NH) will significantly reduce in a non-aqueous medium containing reactants methylamine and oxygen. In a water-rich environment, the surrounding water molecules could act as a catalyst in the dehydration process of aminomethanol 23 and therefore could obscure its detection." 6/ How applicable is Strecker synthesis in a purely solid-phase context?
Response: As mentioned above, Strecker synthesis could occur in solid-phase. Several theoretical investigations have been performed, which suggest that Strecker's process could happen in the absence of liquid water.

Relevant changes in the manuscript (on page 8): We have added the following sentences
"It is important to note here that the Strecker process could initiate even in the absence of liquid water. Quantum chemical calculations performed by Rimola et al. show that a Strecker-type reaction to form glycine is energetically feasible on solid-water ice grains. Formation of acetonitrile and aminomethanol precursors of glycine on an icy grain mantle has also been demonstrated through calculations." 7/ How does the mechanism tested here compare with other lab studies, e.g. that of Elsila et al. (2007, ApJ, 600, 911), who used H2O:CH3OH:HCN:NH3 mixtures to produce amino acids?
Although they considered a slightly different mechanism, their isotopic labeling results indicated that Strecker synthesis should be less important than radical reaction processes.

Response:
The purpose of this study is to (1) investigate the formation of the aminomethanola crucial intermediate of Streckers process in well-defined and controlled laboratory simulation experiments, (2) produce the target molecule in sufficient abundance to enable its detection in the gas phase. As mentioned before, aminomethanol is very labile and easily decomposes to imine in the presence of water. Neighboring water molecules act as a catalyst in the dehydration process of aminomethanol and therefore could obscure its detection. Unlike previous laboratory studies, here we choose a non-aqueous ice mixture to avoid the unimolecular decomposition of aminomethanol in the ice matrix. In our experiments, the aminomethanol (OHCH 2 NH 2 ) could either form via O( 1 D) atom insertion into the C-H bond of methylamine or radical-radical recombination of hydroxy and •CH 2 NH 2 radicals. The former pathway could be dominant as it is energetically more favorable.
8/ The identification of aminomethanol rests strongly on the calculations, which mainly take a back seat in the text. Are there any alternative estimates or measurements of any of the ionization energies calculated here? Is there any alternative verification that can now or in future be used to back up the calculations?
Response: Previous work (Turner et al. ChemPhysChem, 2021, 22, 985-994) at this level of theory (CCSD(T)/CBS+ZPE) has been able to match experiment to within 0.05 eV in all known or even 0.01 eV in many cases. Hence, the IEs are similarly accurate here.
If all of the above points are addressed, I feel that the manuscript would be ready for publication.

Reviewer #3 (Remarks to the Author):
The present communication entitled "Gas-phase identification of aminomethanol (NH2CH2OH) the key intermediate in the Strecker synthesis", S.K. Singh et al shows an experimental work on the formation of NH2CH2OH induced by keV electrons of NH2CH3:O2 ice. Coupling VUV photoionization time of flight spectrometer and theoretical calculations (IE of different isomers), they have identified NH2CH2OH isomers desorbing from irradiated ice. This work relies on well establish procedure in Kaiser's laboratory to investigate formation of different molecules in irradiated ices.
A weak aspect of the paper is that the investigation of NH2CH2OH formation is not a novelty from a conceptual point of view. In the literature, one can find several studies dedicated to investigate the formation of aminomethanol experimentally and theoretically (several references are in the present article). For example it has been shown that NH 2 CH 2 OH can be formed in H2O:NH3:H2CO ice ( F Duvernay et ApJ 791 (2014) ).
Furthermore, the mass spectra measured by the authors through electron impact ionization technique at 70 eV caused extensive fragmentation of the parent ion. In addition, the electron impact ionization mass spectrometry cannot be used to distinguish the structural isomers and determine the true structure of the products. Therefore, the assignment of aminomethanol in these articles is not affirmative. This has been mentioned in the introduction section of the manuscript on page 3.
Here, we have employed isomer-selective photoionization technique to detect the aminomethanol in methylamine and oxygen ice mixture. This technique has been very effective in determining the nature of the complex organic molecules in multicomponent ice mixtures.
It would be worth mention why in particular, NH2CH3:O2 is particularly interesting to investigate aminomethanol formation.
Response: Aminomethanol easily decomposes to methanimine (CH 2 NH) in an aqueous environment. In a water-rich environment, the surrounding water molecules could act as a catalyst in the dehydration process of aminomethanol (Rimola, A., et al. Phys. Chem. Chem. Phys. 12, 5285-5294, (2010)) and could obscure its detection. Therefore, unlike previous laboratory studies, here we choose a non-aqueous ice mixture to avoid the unimolecular decomposition of aminomethanol in the ice matrix.

Relevant changes in the manuscript (on page 3):
We have added the following sentences "Furthermore, the rate of unimolecular decomposition of aminomethanol (OHCH 2 NH 2 ) to methanimine (CH 2 NH) will significantly reduce in a non-aqueous medium containing reactants methylamine and oxygen. In a water-rich environment, the surrounding water molecules could act as a catalyst in the dehydration process of aminomethanol 23 and therefore could obscure its detection." Another point, the whole procedure of isomer identification relies on theoretical calculations.
Authors considered error limits due theoretical uncertainties and experimental conditions for benchmark systems. It would be worth also to compare with experimental IE values of discussed isomers (if available).
Response: While we agree with the reviewer that having such data on hand for these molecules would be beneficial, since they are not available, we must rely on benchmarks; this is a well-13 established approach in computational and experimental physical chemistry, when experimental data are absent. As given in our response to Reviewer 2, Comment 8, previous benchmarks show that this computational approach has exceptional experimental fidelity. The molecules explored herein provide no questionable results or behaviors leading us to question the extension of these methods to the present systems.
One can find also inconsistency between the text in "Mass spectrometry" paragraph and Fig 2. It has been stated the data recorded at 9.50 reveal several products at m/z 45, 47 58 and 60. This is visible in Fig. 2b (obtained for 9.10 eV if we follow a caption)? Figure 2 is nice to follow globally dynamics of appearing and disappearing m/z species as a function of temperature. However, the present figure does not allow to read masses in details. Lines to guide eyes should be added or it would be better to show a cut for 210 K -intensity as a function of m/z for all three ices NH2CH3:O2, Nd2CD3:O2 and NH2CH3:18O2. A short comment of high mass formation would enrich the article.
Response: Ion counts at m/z ratios of 45, 47, 58, and 60 are also observed in the spectrum measured at 9.50 eV. The graphs in Figure 2 are now replotted in 2D with grid lines to clearly show the observed masses.
Higher molecular weights products are observed at m/z = 58, 59, 60, 61 74, and 90. Signals at m/z = 60 (C 2 H 8 N 2 ), 59 (C 3 H 9 N) and 58 (C 2 H 6 N 2 ) were previously observed in processed pure methylamine ice and can be tentatively assigned to isomers of ethylene diamine, N-methylethanamine and N-methylformimidamide (. A possible formation pathway of ethylene diamine could follow a radical-radical recombination of two CH 2 NH 2 radicals. N-methyl-ethanamine could generate via recombination of •CH 3 and •CH 2 NHCH 3 radicals. While Nmethylformimidamide could generate via dehydrogentation of ethylene diamine (C 2 H 8 N 2 ).
Oxygen could insert at the C-H and/or N-H bonds of N-methylformimidamide to generate products observed at m/z = 74 (C 2 H 6 N 2 O) and 90 (C 2 H 6 N 2 O 2 ). It important to note here that mass signals 47, 61, 74 and 90 were not observed in pure methylamine ices.