Discovery of Ethanolamine in space

Introduction

We all know cells are the basic building block of all living organisms. To be even more precise, the cell membrane plays a vital role in the various processes occurring in the cell. It protects the cell, acts as a protective wall and controls the entry and exit of materials. If we further zoom in on the cell membrane, we will find bilayer phospholipids. These phospholipids are usually made up of a hydrophilic head group (like serine, ethanolamine [EtA] or choline) attached to two hydrophobic tails (hydrocarbon chains derived from fatty acid).

In simple words, the cell membrane is made up of phospholipid, which itself is made up of various organic molecules. But our centre of attraction here is only EtA (ethanolamine) because researchers recently discovered EtA in space!!

More about EtA :

EtA is the simplest one and the second most abundant phospholipid in the cell membrane. EtA is also the direct precursor of glycine (the simplest amino acid), which plays an essential role in the origin of life on Earth. The origin of phospholipids on Earth is still unknown and researchers are working on it. Various theories suggest that phospholipids could be synthesised under prebiotic conditions. Many believe that the building blocks of phospholipids were delivered from space through meteorites and comets. Laboratory experiments clearly showed that a significant portion of organic molecules on comets or meteorites can actually survive the atmospheric frictional heat and impact on the ground. 

Previously, researchers discovered various molecules from meteorites including fatty acids, alcohols and phosphonic acids. EtA has been found in Almahata Sitta meteorite, yet its origin is unknown. A possible answer could be the thermal decomposition of amino acids. However, it is very difficult to have suitable conditions for any molecule to undergo decomposition on comets or meteorites. Another theory suggests that EtA was initially synthesised in the interstellar medium and the recent detections of EtA in space (interstellar medium) clearly support the theory. 

Detection in ISM :

Researchers discovered EtA inside a molecular cloud G+0.693 located in Sagittarius B2 of the Galactic Center. SgrB2 (Sagittarius B2) is the largest molecular cloud in the galactic centre and G+0.693 is one of the most chemically rich reservoirs of molecules in our galaxy. Researchers used high-sensitivity spectral surveys to detect the EtA. A special tool is called SLIM (Spectral Line Identification and Modeling). The final result of the survey gives a density of 1.51×10¹³ EtA per cm sq. Researchers were observing G+0.693 from the Yebes Observatory located in Guadalajara, Spain using a 40 m telescope. The surveys were made using IRAM 30 m telescope. The receiver they used for surveys was connected to a 16 Fourier transform spectrometer with a spectral coverage of 2.5 GHz. To get the precise results, researchers had to cover a spectral range of 31 GHz to 50 GHz. 

They pointed the telescope towards SgrB2 for about a month. These observations were part of project 20A008, conducted during the month of February 2020. Researchers had to check the focus and pointing of both telescopes every 1-2 hours.     

Formation of EtA :

The exact path of formation of EtA in ISM is still unknown to the researchers. In fact, not only the EtA, researchers are working to trace the origin and formation of various organic molecules found in the ISM, especially in the regions like SgrB2.

One of the possible paths of formation of EtA is through the hydrogenation chain of HN=C=CO. Researchers have traced a small fraction of HCCO (ketenyl) in the ISM of clouds like L483, Lupus-1A and even in G+0.693. The addition of N to the ketenyl will result in HNCCO. Another way this HNCCO can be formed is from ketene (H₂CCO). The reaction of ketene with imine radical NH will result in the liberation of the H₂ molecule and will form HNCCO. This path is more preferably to the previous one because ketene is abundant in G+0.693, about 20 times more than EtA.

H₂CCO + NH → HNCCO

Now, the further hydrogenation of HNCCO will yield NH₂CHCO. Apart from this hydrogenation reaction, researchers predicted another path through which NH₂CHCO might have formed. The reaction between NH₃, CO and atomic C can also form NH₂CHCO. Although the three-body reactions are always less efficient than the two-body reactions, the abundance of atomic C in the clouds of G+0.693 made researchers predict this path as a more efficient one. The two paths to form NH₂CHCO are shown below.

1. HNCCO + 2H → NH₂CHCO (Hydrogenation)

2. NH₃ + CO + C → NH₂CHCO

The next step in the formation of EtA is the hydrogenation of NH₂CHCO. Further hydrogenation will yield aminoacetaldehyde (NH₂CH₂CHO). The reaction is shown below,

NH₂CH₂CO + 2H → NH₂CH₂CHO

But this is not the only path. When researchers detected the presence of NH₂CH₂ in G+0.693, they predicted yet another path to form NH₂CH₂CHO. The reaction of NH₂CH₂ with CO will form an intermediate product NH₂CH₂CO. Its further hydrogenation will again form aminoacetaldehyde. The reactions are shown below.

NH₂CH₂ + CO → NH₂CH₂CO + H (hydrogenation) → NH₂CH₂CHO

And finally, the further hydrogenation of NH₂CH₂CHO will give our required product, i.e the EtA.

NH₂CH₂CHO + 2H → NH₂CH₂CH₂OH

The detection of EtA makes aminoacetaldehyde (NH₂CH₂CHO) the next potential target to study for astronomers and researchers. This is because, if you want to create EtA from any of the above paths you will need to create NH₂CH₂CHO at one point. Without forming this intermediate product you cannot move further for the formation of EtA. Thus, the final general path, as predicted by researchers that led to the formation of EtA in ISM is shown below.

HNCCO + 2H → NH₂CHCO + 2H → NH₂CH₂CHO + 2H → NH₂CH₂CH₂OH

Conclusion:

The organic matters like EtA were delivered to Earth through meteorites like Almahata Sitta, as discussed above. Once EtA reached the Earth's surface, it formed phospholipids under early Earth conditions. The earlier cells were made up of simpler fatty acids or alcohols. However, the molecules like EtA replaced the alcohol groups with more complex molecules like phospholipids. This eventually created precursor molecules to form RNA-like molecules, required for replicative and metabolic processes. This theory can imply not only the origin of life on Earth but also on other habitable planets or moons within the solar system or anywhere else in the Universe.