Formation pathways of proteins in space


 Until now, experiments demonstrated the possible formation of relatively small prebiotic molecules under typical space conditions. We demonstrated experimentally that condensation of atomic carbon on the surface of cold solid particles (cosmic dust) leads to the formation of monomeric fragments of polyglycine. These fragments polymerize effectively producing polypeptides. The chemistry involves three of the most common species (CO, C, and NH3) present in star-forming molecular clouds. It proceeds via a novel pathway that skips the stage of amino acids formation in protein synthesis and is effective even at low temperatures without irradiation or presence of water. Therefore, the amount of proteins formed in space through this process could be quite substantial. The delivery of proteins to rocky planets in the habitable zone might be an important element for the origin of life.

The origin of life has always been one of the most intriguing questions throughout human 22 history. Biomolecules delivered to early Earth by asteroids or comets during the period of heavy 23 bombardment about four billion years ago have been proposed to play a role in the origin of life 24 1,2 . Similar processes may apply to rocky exoplanets. Analysis of meteoritic materials led to the 25 identification of amino acids, sugars, and nucleobases, among other complex organic molecules 26 of extraterrestrial origin 3 . The amino acid glycine has been discovered in comets 4 . The widely 27 accepted hypothesis of the formation of organic molecules in space assumes their synthesis in ice 28 mantels covering the refractory dust grains present in space 5 . At later stages when asteroids are 29 formed from this dust, the chemistry in liquid water may also enhance the molecular complexity 30 6 . Experimentally, the formation of a variety of amino acids, and even their dimers as well as 31 other organic molecules, was detected after energetic processing of different molecular ices 7,8 .

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These experiments are comparable to Urey-Miller-type experiments performed at elevated 33 temperatures 9 . The non-energetic pathway was also found for different small organic molecules 34 10,11 and even glycine 12,13 . However, till now, only very small fragments of biopolymers were 35 found to be formed in these experiments. 36 The formation of peptides and proteins is usually considered as a polymerization process of 37 amino acids. This process requires the condensation of amino acids accompanied by the loss of 38 water. This is a thermodynamically unfavourable process and, therefore, it proceeds at high 39 temperatures under the assistance of catalysts or requires energetic processing of the material 14 .
40 Therefore, the prebiotic synthesis of peptides is thought to occur in two steps, each of which has 41 a low probability 15 . Instead of first synthesizing amino acids in order to subsequently break them 42 down for the polymerization process, we suggest a very efficient and direct formation of the 43 monomeric fragments of peptides and its further polymerization. This reaction between NH 3 , C, 44 and CO occurs on the cold dust grains, without external energy. Quantum chemical calculations 45 predict that the CO + C + NH 3 → NH 2 CH=CO reaction is barrierless 16 . The polymerization 46 of NH 2 CH=CO results in the formation of peptide chains. In contrast to the polymerization of the 47 amino acids, the polymerization of NH 2 CH=CO is a much simpler process.

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To test this reaction pathway experimentally, we performed co-deposition of these species on the 49 surface of both Si and KBr substrates cooled to 10K and placed inside an ultra-high vacuum 50 (UHV) chamber. Low-energy carbon atoms were generated by a dedicated atomic carbon source 51 17 . The background pressure inside the vacuum chamber (1 × 10 -10 mbar) and the temperature of 52 the substrate (10 K) allowed us to mimic dense molecular cloud conditions.

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Infrared absorption spectra of the ice produced after the co-deposition of CO + NH 3 and CO + C  and CO + C + NH 3 (black).

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The deposition of all three reactants (CO, C, NH 3 ) on the substrate at 10 K reveals new IR 62 absorption bands, which were not observed in any experiment involving only two reactants.

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Therefore, these bands have to be assigned to a product formed by all three reactants. Moreover, 64 we observed only negligible amounts of the residue at 300 K in the experiments involving only 65 two reactants. Thus, we conclude that during the deposition at 10 K, the reaction involves all 66 three reactants, and exactly the product of this reaction is required for the formation of the non-67 volatile products at 300 K. CO). The last one is the same as the product of the reaction C + NH 3 . Therefore, the reaction of 80 three reactants results in the formation of the NH 2 CH=CO product at 10 K. The formation of this 81 molecule is also confirmed later during the temperature-programmed desorption (TPD) by 82 monitoring the ions with the masses of 57 and 56 u (see Figure S4). At the same time, a number 83 of C atoms could react with either CO or NH 3 only. It could lead to the formation of some 84 number of CCO as well as H 2 CNH and NH 2 CH 2 NH 2 molecules.

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Chemical transformation during temperature rise 86 After the deposition, the substrate was heated with a rate of 2 K per min. The IR spectra 87 measured at important temperatures are shown in Figure 3. bands C=O bands decreased significantly. With the mass spectrometer we did not detect any 97 considerable desorption from the substrate besides CO and NH 3 gases. Therefore, the 98 disappearance of the IR bands is rather due to the chemical transformation than the evaporation 99 of the corresponding material. The decrease in the intensity of bands in the 2800 -3050 cm -1 100 range is a common indication of the glycine polymerization 19 . This takes place because the 101 or NH 2 groups of glycine are transformed into NH groups present in peptides. Additionally, 102 the C=O stretching vibration in NH 2 CH=CO disappears and we observe the formation of the 103 amide I band at 1668 cm -1 CO stretching and amide II at 1560 cm -1 , which are characteristic for 104 the peptide bond. After complete ammonia evaporation, no considerable changes in the IR 105 spectra were observed. Therefore, the material present on the substrate after warming up to 106 300 K (R300K) should have been formed at low temperature during ammonia evaporation.   The IR spectrum of R300K is shown in Figure 4. It highlights the absorption features 119 characteristic for the peptide bond. As can be seen, the spectra of R300K resemble the spectra of calculations show that NHCH 2 CO as well as NH 2 CHCO-NHCH 2 CO molecules are not stable.

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They fragment by abstraction of the CO molecule. Therefore, the calculations predict that the  Ex situ mass spectrum of R300K is shown in Figure 5. We could identify a series of mass peaks, Therefore, the dust, from which planets, asteroids, and comets will be formed at later stages, has 180 to be formed in the ISM due to the accretion of gaseous species. The C atoms could be converted 181 to the molecular form before the accretion. However, this is a relatively long process 27 .

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Therefore, a notable portion of carbonaceous dust is expected to be formed by the accretion of C 183 atoms. This leads to the formation of organics 28 . As revealed by the current study, a notable considerably increase a chance that the molecules required for the chemistry leading to 199 abiogenesis will be formed. Therefore, the low-temperature chemistry that happens between the