"Follow the water" is our basic strategy in searching for life in the universe. The universality of water as the solvent for living systems is usually justified by arguing that water supports the rich organic chemistry that seeds life, but alternative chemistries are possible in other organic solvents. Here, other, essential criteria for life that have not been sufficiently considered so far, will be discussed

Pohorille, Andrew

Pratt, Lawrence R.

NASA Technical Reports Server

Water as a solvent for life 
Andrew Pohorille and Lawrence R. Pratt 
NASA-Ames Research Center 
 
“Follow the water” is our basic strategy in searching for life in the universe. The 
universality of water as the solvent for living systems is usually justified by arguing that 
water supports the rich organic chemistry that seeds life, but alternative chemistries are 
possible in other organic solvents. Here, other, essential criteria for life that have not been 
sufficiently considered so far, will be discussed.  
 
Life is based on non-covalent interactions. They might be either specific (enzyme-
substrate interactions, selective ion transport) or nonspecific (lipid-lipid or lipid-protein 
interactions). Their strength needs to be properly tuned, and this is mediated by the 
solvent. If interactions are too weak, there might be undesired response to natural 
fluctuations of physical and chemical parameters. If they are too strong it could impede 
kinetics and energetics of cellular processes. Thus, the solvent must allow for balancing 
these interactions, which provides strong constraints for life [1]. 
 
Among many solvents, water exhibits a remarkable trait that it promotes both 
solvophobic and solvophilic interactions. Solvophobic (hydrophobic in the case of water) 
interactions are necessary for self-organization of matter. They are responsible, among 
others, for the formation of membranes and protein folding. The diversity of structures 
supported by hydrophobic interactions is the hallmark of terrestrial life responsible for its 
diversity, evolution and the ability to survive environmental changes. Solvophilic 
interactions, in turn, are needed to ensure solubility of polar species. Water offers a large 
temperature domain of stable liquid and the characteristic hydrophobic effects [2-5] are a 
consequence of the temperature insensitivity of essential properties of its liquid state 
[1,4]. Water, however, might not be the only liquid with these properties. Properties of 
water and other pure liquids or their mixtures that have a high dielectric constant and 
simultaneously support self-organization will be compared. Properties that appear to be 
unfavorable to life (e.g. its chemical activity against polymerization reactions) will be 
discussed and the requirements for alternatives to water to support life in space will be 
summarized.  
 
References: 
1.  Pohorille, A. and Pratt L.R. (2012) Orig Life Evol Biosph. 42, 405-9. 
2. Pohorille, A. and Pratt, L. R. (1990) J. Am. Chem. Soc. 112, 5066-5074. 
3. Pratt, L. R.  and Pohorille, A. (1992) Proc. Natl. Acad. Sci., 89, 2995-2999. 
4. Hummer, G., Garde, S., Garcia, A. E., Pohorille, A. and Pratt, L. R. (1996) Proc. Natl. 
Acad. Sci. USA, 93, 8951-8955. 
5. Pratt, L. R. and Pohorille, A. (2002) Chem. Rev. 102: 2671-2692. 
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Water as a Solvent for Life

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