Non-Aqueous Environments for Cell Formation: Azotosomes in Cryogenic Environments

The role of water as a fundamental molecule for life has been challenged and debated repeatedly. Despite intense efforts and huge astronomical projects, there is no proof for the existence of a living organism or a life form outside the Earth. According to our perception, water is a requirement for the evolution of life because it comprises «70 wt% of the mass of a cell. However, water in liquid form is rare in other planetary objects. Alternative solvation systems such as liquid methane, which is abundant, could be an alternative host environment and potential solvent medium, but cryogenic temperatures would be needed. In such conditions, lipids and other hydrocarbon-based molecules would lose the mobility and flexibility they possess in the aquatic environments of the Earth.

Could self-organised vesicles, in a manner similar to liposomes, be formed and function in non-aquatic environments? In these environments, more extreme pressure and temperature conditions occur than those in present-day Earth.

To debate this concept further, Stevenson and co-workers demonstrated through theoretical studies that stable cryogenic membranes could be formed from molecules that have been observed in the atmosphere of Saturn’s moon: Titan. The latter has an atmosphere composed of liquid methane close to its surface. According to observation made by Cassini [43], in the upper atmosphere of Titan there are several nitrogen-containing small molecules in abundance. Stevenson and Clancy proposed that certain organic nitrogen compounds can function in liquid methane at very low temperatures [44]. Molecules resembling liposomes are now defined as ‘azotosomes’ due to the presence of nitrogen (derived from the Greek word azoto). The authors employed molecular simulations to show that, in a cryogenic solvent, azotosomes may have elasticity equal to that of common lipid bilayers in water at room temperature.

It is the presence of liquid molecules in other planetary systems, an

example of the liquid methane solutions that sparked the interest in azotosomes. They have been predicted to have similar mechanical properties to those of conventional cell membranes. Figure 1.10 shows a proposed aggregated assembly of azotosomes and a comparison between the interaction among the phospholipids and nitrogen-based surfactants in a liquid methane environment. Experiments with such systems require cryogenic temperatures. Hence, up until now, studies can be only purely theoretical and are supported by extensive studies on molecular dynamics to demonstrate the crucial role of the nitrogen group. In recent work by Stevenson and co-workers, simulations showed that by comparing the azotosome-simulated membranes of hexane- and acetonitrile-based bilayers, in the absence of a nitrogen group, the layer appeared to be thick, brittle and without flexibility, thereby demonstrating the importance of polar head groups. HCN could not self-assemble in an azotosome due to its very small size.

A vesicle is considered to have a diameter o

Figure 1.10 A vesicle is considered to have a diameter of 0.9 nm (90 A). Representation of an assembly of liposomes in a polar solvent (a) and the azotosomes (b) in a non-polar solvent. A drawing of an azotosome vesicle can be seen in the work of Stevenson and co-workers [44]

The abundance in the atmosphere of Titan of various nitrogenbearing compounds is HCN (200 ppm), cyanoacetylene (40 ppm), acrylonitrile (10 ppm), cyanoallene (4 ppm), acetonitrile (3 ppm), 2,4-pentadiynenitrile (1 ppm), and propanenitrile (0.5 ppm). These molecules and compounds may act as alternative solvents in extraterrestrial cryogenic conditions. The structure of Titan’s atmosphere is shown in Figure 1.11.

A diagrammatic representation of the atmosphere of Titan with respect to different pressures and temperatures is shown. These observations are based on data from Voyager 1

Figure 1.11 A diagrammatic representation of the atmosphere of Titan with respect to different pressures and temperatures is shown. These observations are based on data from Voyager 1

 
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