Prebiotic Precursors of the Primordial RNA World in Space: Detection of NH₂OH

Life likely appeared about 3.8 billion years ago, 700 Myr after the formation of the Earth (Mojzsis et al. 1996). However, the processes that allowed the transition from chemistry to biology are still uncertain. Present-day life is mainly based on three biopolymers: RNA and DNA nucleic acids, which store genetic information, and proteins, responsible for most metabolic activities. One of the main challenges for understanding the origin of life is the “chicken and egg” paradox: nucleic acids need catalytic proteins for their replication, while the biosynthesis of proteins requires the prior presence of genetic information coded in nucleic acids. To solve this problem, Gilbert (1986) proposed the hypothesis of a primordial RNA world, in which RNA molecules could have performed the functions that were carried out by DNA and proteins in the subsequent DNA/RNA/protein world.

One of the weaknesses of the RNA world hypothesis was that ribonucleotides (RNA monomers) seemed unlikely to be synthesized under plausible prebiotic conditions (Atkins et al. 2011; Ruiz-Mirazo et al. 2014). However, this possibility has recently gained support thanks to prebiotic chemistry experiments that have shown that ribonucleotides can be formed from relatively simple organic molecules such as cyanoacetylene (HC₃N), cyanamide (NH₂CN), glycolaldehyde (CH₂OHCHO), urea (NH₂CONH₂) and/or hydroxylamine (NH₂OH; see Powner 2009; Patel 2015; Becker et al. 2016, 2019).

Interestingly, except for NH₂OH, all these RNA molecular precursors have been detected in the interstellar medium (ISM; Turner 1971; Turner et al. 1975; Hollis et al. 2000; Belloche et al. 2019). As recently shown by Becker et al. (2019), NH₂OH is a key precursor in the unified synthesis of both pyrimidine and purine ribonucleotides. However, observational searches of NH₂OH in the ISM have so far been unsuccessful (Pulliam et al. 2012; McGuire et al. 2015; Ligterink et al. 2018). This is a puzzling result since NH₂OH is expected to form efficiently on dust grains according to chemical models and laboratory experiments (Garrod et al. 2008; Zheng & Kaiser 2010; Fedoseev et al. 2012; Garrod 2013; He et al. 2015).

In this Letter, we present the first detection of NH₂OH in the ISM toward the quiescent molecular cloud G+0.693-0.027 (G+0.693, hereafter) located in the Sgr B2 complex in the Galactic Center. This cloud shows very rich chemistry in complex organic molecules (Martín et al. 2008; Requena-Torres et al. 2008; Rivilla et al. 2018, 2019; Zeng et al. 2018) ,including urea (Jiménez-Serra et al. 2020), another key ingredient in the pathway to ribonucleotides proposed by Becker et al. (2019). All this makes G+0.693 the perfect target to search for NH₂OH in the ISM.

We have carried out a high-sensitivity spectral survey at 3, 2, and 1 mm toward the G+0.693 molecular cloud using the IRAM 30 m telescope. The IRAM 30 m observations were performed in three observing runs during 2019: April 10–16, August 13–19, and December 11–15 (projects numbers 172-18, 018-19 and 133-19). We used the broadband Eight MIxer Receiver and the fast Fourier transform spectrometers in FTS200 mode, which provided a channel width of ∼200 kHz, i.e., a velocity resolution of ∼0.3−0.85 km s⁻¹. We smoothed the spectra to resolutions of 1.6, 1.4, and 4.0 km s⁻¹ for the 3, 2, and 1 mm data, respectively, high enough to sample the observed line widths of ∼15−25 km s⁻¹, and to provide similar rms noise (2.6−3.8 mK), and optimal line visualization. We stress that the results of our analysis (described in Section 3) are independent of the spectral resolution. Each frequency setup was observed several times, with the central frequency shifted by 20–100 MHz in order to identify possible spectral features resulting from unsuppressed image side-band emission. The image-band lines, identified by comparing the same frequency range observed during two different spectral setups, were eliminated during the reduction of the data. The covered spectral ranges were 71.76−116.72 GHz, 124.77−175.5 GHz, 199.8−238.29 GHz, 252.52−260.30 GHz, and 268.2−275.98 GHz. The observations were centered at α(J2000.0) = 17^h47^m22^s, δ(J2000.0) = −28°21′27″. The position switching mode was used in all observations, with the off position located at (−885″, 290″) from the source position. The intensity of the spectra is given in , as the molecular emission toward G+0.693 is relatively extended over the beam (Requena-Torres et al. 2006; Rivilla et al. 2018).

The identification of the molecular lines was performed using version 2020 June 19 of the Spectral Line Identification and Modeling (SLIM) tool within the MADCUBA package⁸ (Martín et al. 2019), which uses the spectroscopic entries from the Cologne Database for Molecular Spectroscopy (CDMS, Endres et al. 2016) and the Jet Propulsion Laboratory (JPL; Pickett et al. 1998), and generates the synthetic spectra under the assumption of local thermodynamic equilibrium (LTE) conditions. The spectroscopic information of NH₂OH (CDMS entry 33503, from 2003 March) was obtained from Tsunekawa (1972) and Morino et al. (2000). NH₂OH is nearly a prolate symmetric top molecule, in which the principal a- and c-axes are in a plane of symmetry. Both a-type (ΔK_a = 0) and c-type (ΔK_c = 0) transitions are allowed, but only the former are expected to be detected in the ISM since the projected dipole moment along the a-axis is about 10 times larger (Tsunekawa 1972). The partition function of the ground vibrational state used in the analysis, obtained from the CDMS entry, does not have a substantial contribution from the lowest-lying vibrational states (with energies >555 K, Morino et al. 2000) due to the low excitation temperatures of the molecular emission toward G+0.693 (T_ex ∼ 5–20 K; Requena-Torres et al. 2008; Zeng et al. 2018; Rivilla et al. 2019).

We list in Table 1 all the a-type transitions of NH₂OH that fall in the observed data: 15 lines of the rotational transitions J = 2−1, 3−2, and 4−3. We note that transitions from higher energy levels (J_u > 5) have low predicted intensities due to the low excitation temperatures of G+0.693. The fitted line profiles of NH₂OH (see details of the fit below) are shown with a red solid line in the left panels of Figure 1. Five of the targeted NH₂OH transitions (1 at 3 mm, 3 at 2 mm, and 1 at 1 mm, indicated with an asterisk in Figure 1), appear either unblended or only slightly blended. The other transitions, which are blended with brighter lines from other molecules, have predicted intensities consistent with the observed spectra.

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Table 1.  List of Observed Transitions of NH₂OH

Frequency	Transition	log I	Sul × μ2	log Aul	gu	Eu	Blendinga
(GHz)	()	(nm2 MHz)	(D2)	(s−1)		(K)
100.683580	21,2−11,1	−4.8754	0.52036	−5.9078	5	15.2	blended with CH3OCHO
100.748230	20,2−10,1	−4.7384	0.69380	−5.7821	5	7.2	unblended
100.807620	21,1−11,0	−4.8743	0.52041	−5.9062	5	15.2	blended with CH3COOH and s-C2H5CHO
151.020701	31,3−21,2	−4.2821	0.92509	−5.2759	7	22.4	unblended
151.101986	32,2−22,1	−4.5203	0.57820	−5.4793	7	46.3	blended with s-C2H5CHO
151.102324	22,0	−4.5203	0.57820	−5.4793	7	46.3	blended with s-C2H5CHO
151.117668	20,2	−4.2189	1.04068	−5.2239	7	14.5	slightly blended with s-C2H5CHOb
151.207007	31,2−21,1	−4.2810	0.92520	−5.2742	7	22.5	slightly blended with unidentified line
201.352578	31,3	−3.8964	1.30100	−4.8621	9	32.1	blended with CH3SH and HDCO
201.435072	33,1	−4.3192	0.60710	−5.1926	9	95.8	blended with CH3OH
201.435072	43,1−33,0	−4.3192	0.60710	−5.1926	9	95.8	blended with CH3OH
201.460663	3	−4.0274	1.04082	−4.9584	9	56.0	blended with CH3OH
201.461598	3	−4.0274	1.04081	−4.9583	9	24.1	blended with CH3OH
201.481718	3	−3.8563	1.38777	−4.8333	9	32.1	slightly blended with unidentified line
201.600715	3	−3.8954	1.30089	−4.8606	9	15.6	blended with H2C18O and CH3CONH2

Notes. We provide the parameters from CDMS catalog entry 33503: frequencies, quantum numbers, base 10 logarithm of the integrated intensity at 300 K (log I), and upper state degeneracy (g_u). We also show the values of S_ul × μ², the base 10 logarithm of the Einstein coefficients (log A_ul), and the energy of the upper levels (E_u). The last column gives the information about the species with transitions blending with some of the NH₂OH lines.

^aDetailed information about the species producing some blending is presented in Appendix A (Table A1). ^bThe analysis of s-propanal (s-C₂H₅CHO) is presented in Appendix B.

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To properly evaluate the line contamination by other molecules we have searched for more than 300 different molecules in our data set, including all the species detected previously toward G+0.693 (e.g., Requena-Torres et al. 2006, 2008; Rivilla et al. 2018, 2019; Zeng et al. 2018; Jiménez-Serra et al. 2020), and those detected so far in the ISM (McGuire 2018).⁹ We have fitted the emission of all the detected molecules, and produced the total LTE spectra, which is shown with a solid blue line in the left panels of Figure 1. We list in the last column of Table 1 the molecules with transitions that produce blending with some of the NH₂OH lines. More detailed information about all the identified molecular transitions that contribute to the line emission in the spectral ranges shown in Figure 1 is presented in Appendix A.

To highlight the emission from NH₂OH, we have computed the residual spectra obtained by subtracting from the observed spectra the predicted LTE contribution for all the identified molecular species but NH₂OH. The resulting residual spectra, shown in the right panels of Figure 1, clearly exhibit the spectral line profiles that match the LTE predictions for NH₂OH in red solid lines (see below), supporting the identification of NH₂OH.¹⁰

To derive the physical parameters that explain the NH₂OH emission, we used the SLIM-AUTOFIT tool of MADCUBA, which provides the best nonlinear least-squares LTE fit to the data using the Levenberg–Marquardt algorithm. The fitted parameters are molecular column density (N), excitation temperature (T_ex), central velocity (v_LSR), and FWHM of the Gaussian profiles. SLIM considers the solution of the radiative transfer as described in Equation (4) of Martín et al. (2019). For the modeling of G+0.693, where no continuum emission is detected (e.g., Ginsburg et al. 2016), we consider T_c = 0, and the cosmic background temperature of T_bg = 2.73 K. The fit to the NH₂OH profiles was performed by also considering the total contribution of the LTE emission from all the other identified molecules (blue line in Figure 1). We fixed the line width to 15 km s⁻¹, which reproduces the profiles of the two strongest completely unblended transitions, 2_0,2−1_0,1 and 3_1,3−2_1,2 (Figure 1). This value is consistent with the typical molecular line widths found toward G+0.693, which span between 14 and 34 km s⁻¹ (e.g., Requena-Torres et al. 2008; Zeng et al. 2018). We considered typical values of T_ex found in G+0.693 in the range 5–20 K (see e.g., Zeng et al. 2018). Low values (5–10 K) underestimate the observed intensities of the NH₂OH transitions at 2 and 1 mm, while values in the range 15−20 K are able to reproduce well all the transitions. We thus fixed T_ex = 15 K¹¹ to perform the fit, noting that if the assumed fixed value is 20 K the derived column density would only vary by a factor of ∼1.1. We then performed AUTOFIT, leaving column density and v_LSR as free parameters. The resulting best LTE fit is shown by the red curve in Figure 1, and the derived physical parameters are presented in Table 2. To derive the final uncertainty of the column density we added quadratically the error derived from the fitting algorithm, and 20% from the typical flux density uncertainty of the IRAM 30 m telescope. We obtain a column density of (2.8 ± 0.6)×10¹³ cm⁻², which translates to a molecular abundance with respect to molecular hydrogen of (2.1 ± 0.9) × 10⁻¹⁰.

Table 2.  Derived Physical Parameters of NH₂OH and Other Molecular Species That Might be Chemically Related

Molecule	N	Tex	vLSR	FWHM	Abundancea	Reference
	(×1013 cm−2)	(K)	(km s−1)	(km s−1)	(×10−10)
NH2OH	2.8 ± 0.6	15	68.6 ± 0.8	15	2.1 ± 0.9	1
HNO	5.8 ± 1.3	3.7 ± 0.2	66.4 ± 0.4	20.5 ± 0.9	4.3 ± 1.8	1
N2O	48 ± 11	18 ± 2	64.7 ± 0.8	21 ± 2	36 ± 15	1
NO	3600 ± 200	4.1 ± 0.4	68 ± 1	21 ± 1	2700 ± 700	2
NO2	<29	10	69	20	<22	1
HONO	<0.2	10	69	20	<0.16	1
HNO3	<0.5	10	69	20	<0.33	1

Notes.

^aWe adopted = (1.4 ± 0.3) × 10²³ cm⁻², from Martín et al. (2008).

References. (1) This work, (2) Zeng et al. (2018).

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To gain insight into the interstellar chemistry and synthesis routes of NH₂OH in the ISM, we have also analyzed the emission from chemically related species. We detected the nitroxyl radical (HNO) and nitrous oxide (N₂O). The observed spectra along with the derived best LTE fits for these two molecules, reported for the first time in G+0.693 in this Letter, are presented in Appendix C. Other species, such as HONO, NO₂, and HNO₃, were not detected. The derived physical parameters for all these species are reported in Table 2.

4.1. Formation of NH₂OH in the ISM

4.2. The Role of NH₂OH in Prebiotic Chemistry

We acknowledge the anonymous reviewers for their careful reading of the manuscript and their useful comments. This work is based on observations carried out under projects number 172-18, 018-19, and 133-19 with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). We thank the IRAM-30 m staff for the precious help during the different observing runs. V.M.R. acknowledges support from the Comunidad de Madrid through the Atracción de Talento Investigador Senior Grant (COOL: Cosmic Origins Of Life; 2019-T1/TIC-15379), and from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 664931. I.J.-S. and J.M.-P. have received partial support from the Spanish FEDER (project number ESP2017-86582-C4-1-R), the Ministry of Science and Innovation through project number PID2019-105552RB-C41, and State Research Agency (AEI) through project number MDM-2017-0737 Unidad de Excelencia María de Maeztu−Centro de Astrobiología (INTA-CSIC).

We list in Table A1 the identified molecular transitions of the molecules that contribute to the spectra shown in Figure 1. We have searched for more than 300 different molecules in our spectral survey, including all the species detected previously toward G+0.693 (e.g., Requena-Torres et al. 2006, 2008; Rivilla et al. 2018, 2019; Zeng et al. 2018; Jiménez-Serra et al. 2020), and those detected so far in the ISM¹³ (McGuire 2018). We show in Table A1 transitions with line intensities mK.

Table A1.  Transitions of the Molecules That Contribute to the Spectra Shown in Figure 1

Molecule	Freq.
	(GHz)	(mK)
3 mm—NH2OH J = 2−1
CH3OCHO	100.6815450	93.3
CH3OCHO	100.6833680	93.4
NH2OH	100.6835800	6.1
CH3OCHO	100.6931270	1.22
HCCCHO	100.7169300	18.4
CH3OCHO	100.734805	1.1
NH2OH	100.7482300	12.1
NH2OH	100.8076200	6.1
CH3COOH	100.8119422	1.52
s-C2H5CHO	100.8149861	6.3
s-C2H5CHO	100.8149861	6.3
2 mm—NH2OH J = 3−2
CH3OCH3	150.992109	1.8
CH3OCH3	150.9921790	2.8
s-C2H5CHO	150.9952334	1.5
CH3OCH3	150.9953910	7.4
s-C2H5CHO	150.9954363	1.5
CH3OCH3	150.9954363	1.5
CH3OCH3	150.9986390	4.6
NH2OH	151.0207010	11.6
c-C3H2	151.0391730	3.2
g-C2H5OH	151.0641006	2.2
s-C2H5CHO	151.0653970	1.1
s-C2H5CHO	151.0654207	1.1
s-C2H5CHO	151.0831633	1.1
s-C2H5CHO	151.0831861	1.1
s-C2H5CHO	151.0970518	1.1
s-C2H5CHO	151.0970740	1.1
NH2OH	151.1019860	2.2
NH2OH	151.1023240	2.2
s-C2H5CHO	151.1148181	1.1
s-C2H5CHO	151.1148394	1.1
NH2OH	151.1176680	19.4
C2H5CN	151.1272640	3.5
HOCH2CN	151.1300319	1.0
CH3COOH	151.1620321	1.5
CH3CHO	151.1671882	18.4
t-HCOOH	151.1762475	81.4
NH2OH	151.2070070	11.6
HCOCH2OH	151.2430164	4.5
1 mm—NH2OH J = 4−3
HDCO	201.3413620	13.2
CH3SH	201.3440150	14.6
NH2OH	201.3525783	13.6
t-C2H5OH	201.4372959	24.3
CH3OH	201.4454903	77.0
NH2OH	201.4606626	3.3
NH2OH	201.4615977	3.3
NH2OH	201.4817175	21.6
NH2OH	201.6007146	13.6
H2C18O	201.6142649	21.4
CH3CONH2	201.6210000	1.7

Note.  We present the name of the species, the frequency of the transition, and the line intensity I_L.

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The syn-conformer of propanal (s-C₂H₅CHO) has been identified toward the G+0.693 molecular cloud. The analysis has been done using the SLIM-MADCUBA tool, following the same procedure described in Section 3. We use the CDMS entry 58505 (version 2018 January). We fixed v_LSR to 69 km s⁻¹ and FWHM to 21 km s⁻¹, and obtained T_ex = 12.0 ± 0.8 K, and N = (7.4 ± 1.5) × 10¹³ cm⁻², which translates into an abundance with respect to H₂ of (5 ± 2) × 10⁻¹⁰. The best LTE fit for selected transitions (listed in Table B1) is shown in Figure B1 with a red line. The s-C₂H₅CHO transitions that contribute to the spectra shown in Figure 1 are listed in Table A1.

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Table B1.  Selected Transitions of s-C₂H₅CHO Detected toward G+0.693

Frequency	Transition	log Aul	Eu
(GHz)		(s−1)	(K)
77.1819857	71,7 E	−5.1418	17.4
77.1819857	81,8−71,7 A	−5.1418	17.4
77.9521729	70,7 E	−5.1275	17.3
77.9521729	80,8−70,7 A	−5.1275	17.3
87.0228499	80,8 E	−4.9808	21.5
87.0228499	90,9−80,8 A	−4.9808	21.5
88.5156329	72,5 E	−4.9810	21.0
88.5156329	82,6−72,5 A	−4.9810	21.0
88.5476678	33,1−22,0 E	−5.0515	8.0
88.5485859	33,1−22,0 A	−5.0116	8.0
95.2964249	91,9 E	−4.8656	26.1
95.2964249	100,10−91,9 A	−4.8656	26.1
95.7920583	91,9 E	−4.8531	26.1
95.7920583	101,10−91,9 A	−4.8531	26.1
96.1200037	90,9 E	−4.8484	26.1
96.1200037	100,10−90,9 A	−4.8484	26.1
99.3945076	32,2 E	−4.9654	10.0
99.3945076	43,1−32,2 A	−4.9643	10.0
105.2870083	91,8 E	−4.7369	29.1
105.2870083	101,9−91,8 A	−4.7369	29.1

Note. We provide the frequencies of the transitions, quantum numbers, the base 10 logarithm of the Einstein coefficients (A_ul), and the energy of the upper levels (E_u).

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We report here the detection of the nitroxyl radical (HNO) and nitrous oxide (N₂O) toward G+0.693. We show in Figures C1 and C2 the detected transitions of HNO and N₂O, respectively, which are listed in Table C1. The analysis have been done using the SLIM-MADCUBA tool, following the same procedure described in Section 3. The derived parameters from the fits (N, T_ex, FWHM and v_LSR) are presented in Table 2.

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Table C1.  Transitions of HNO (JPL Entry 31005, version 1996 February) and N₂O (JPL entry 44004, version 2014 August) Detected toward G+0.693

Molecule	Frequency	Transitiona	log Aul	Eu
	(GHz)		(s−1)	(K)
HNO	81.47749	10,1−00,0 F = 2–1	−5.65235	3.9
HNO	81.47749	10,1−00,0 F = 1–1	−5.65240	3.9
HNO	81.47749	10,1−00,0 F = 0–1	−5.65240	3.9
HNO	162.937949	20,2−10,1 F = 3–2	−4.67021	11.7
HNO	162.937949	20,2−10,1 F = 2–1	−4.79519	11.7
HNO	162.937949	20,2−10,1 F = 1–0	−4.92554	11.7
HNO	162.937949	20,2−10,1 F = 1–1	−5.05044	11.7
HNO	162.937949	20,2−10,1 F = 2–2	−5.27229	11.7
HNO	162.937949	20,2−10,1 F = 1–2	−6.22654	11.7
N2O	75.369224	3−2	−7.25771	7.2
N2Ob	100.49174	4−3	−6.86711	12.1
N2O	125.61369	5−4	−6.56660	18.1
N2O	150.735046	6−5	−6.32247	25.3
N2O	200.975333	8−7	−6.32247	43.4
N2Oc	226.0940229	9−8	−6.32247	54.3

Notes. We provide the frequencies of the transitions, quantum numbers, the base 10 logarithm of the Einstein coefficients (), and the energy of the upper levels (E_u).

^aThe format for the quantum numbers is for HNO and J for N₂O. ^bTransition contaminated by CH₃OCHO (100.490682 GHz) and aGg-(CH₂OH)₂ (100.4906121 GHz). ^cTransition contaminated by CH₃SH (226.093108 GHz).

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