After Miller and Urey's experiments in the 1950s formed amino acids, scientists spent decades trying to form sugars. Finally, in 2009, the Sutherland laboratory found that a mix of sunlight, hydrogen cyanide, and hydrogen sulfide kick off a process that leads to sugars and potentially all the other organic compounds required for life - cyanosulfidic chemistry. [1]
Because life is based on the combination of carbon atoms, John Sutherland realized that the key to early life is the combination of carbon atoms into molecules. Carbon atoms will combine if one carbon molecule would prefer to donate an election (nucleophile) and another carbon molecule would prefer to accept an electron (electrophile). Sutherland’s good nucleophile is formaldehyde (H2CO), and his good electrophile is hydrogen cyanide ion (CN-). He showed that this chemistry can begin with the production of formaldehyde from hydrogen sulfide and hydrogen cyanide, two likely compounds on an early earth with a strongly reducing atmosphere. The first step in the process is that UV light ionizes hydrogen sulfide (H2S) and forms bisulfide, which releases a proton (H) and an electron. The proton and electron combine with hydrogen cyanide (HCN). This happens twice to form formaldehyde amine. The formaldehyde amine combines with water to form formaldehyde. The formaldehyde then combines with hydrogen cyanide ion to form a two- carbon sugar. The two-carbon sugar combines again with hydrogen cyanide to form a three-carbon sugar. The steps are outlined in the following list.
Process a. Produce formaldehyde from hydrogen sulfide, hydrogen cyanide, and water.
1. Hydrogen sulfide is ionized by UV light to provide hydrogen ions and electrons:
H2S + UV light --> HS- + H+ --> HS + e- + H+
2. The hydrogen ions (H+) and electrons (e-) join with hydrogen cyanide and form formaldehyde amine.
HCN + H+ + H+ --> CH3N
3. Formadehyde amine is hydrolyzed by water, releases ammonia, and forms formaldehyde
CH3N + H2O – NH3 --> CH2O
The next step is to form a two-carbon sugar from formaldehyde and hydrogen cyanide ion (CN-), which would be available in water because hydrogen cyanide dissolves in water. HCN --> CN-
Process b. Combine formaldehyde, hydrogen cyanide ion, and water to form a two-carbon sugar.
4. Formaldehyde combines with hydrogen cyanide and forms a two-carbon chain called glyconitrile
CH2O + HCN --> C2H3NO
5. Hydrolysis (water) removes the nitrogen-hydrogen group and forms glycoaldehyde, a two-carbon sugar. Support for the natural formation of glycoaldehyde has been found in space. [2]
C2H3NO + H2O – NH --> C2H4O2 Two-carbon sugar, glycoaldehyde
Process c. Combine hydrogen cyanide with the two-carbon sugar to form a three-carbon sugar (just as it combined with formaldehyde to form the two carbon sugar)
6. The two-carbon sugar combines with hydrogen cyanide and then water to form glyceraldehyde, a three-carbon sugar.
C2H4O2 + HCN --> C3H5O2N
C3H5O2N + H2O – NH --> C3H6O3 Three carbon sugar, glyceraldehyde
Two-carbon and three-carbon sugars are the building blocks of the molecules of life: lipids, proteins, carbohydrates, and nucleic acids. However, combining them into the five-carbon sugars in nucleic acids is problematic because the two carbon and three-carbon sugars would need to form in different ponds with different chemistries. One of the pools contains calcium cyanimide (CaNCN), but the other contains sodium or potassium cyanide (Na/K)CN. The two chemistries could form from different comets with different impact temperatures. If streams from the two pools combined downstream, then the five-carbon sugars could form. [3] While this scenario is more complex than formation in a single pool, it is plausible.
Observations of the two-carbon sugar, glycoaldehyde, proves that glycoaldehyde naturally existed in space or early planetary environments. If glyceraldehyde (three-carbon sugar) forms by a similar process as glycoaldehyde, then it is reasonable to assume that glyceraldehyde also formed naturally on the early Earth.
Patel calculated the likelihood of the reactions in cyanosulfidic chemistry. [4] Beginning from glyceraldehyde, Patel evaluated the statistical likelihood that subsequent reactions would form the fundamental molecules of life: ribonucleic acids, lipids, and amino acids. [5] He showed that there are high yields for these reactions and processes. For example, he showed the likelihood of the reactions in the path to lipid precursors are between 30% to 60%. This is extremely important because one of the known problems with origin of life scenarios is that useless molecules are just as likely to form as molecules useful for life. Thus, Sutherland’s cyanosulfidic chemistry preferentially forms the molecules of life. [6] This is extremely important because there are billions of variations of organic molecules.
This video from the Sutherland lab (https://youtu.be/W4ER_OpqvG8) shows how the basic molecules of life might have formed in a primordial soup and formed the first protocells, which would have been extremely simple in comparison to modern cells. Although the Sutherland laboratory discovered much of the chemistry that led to the formation of the molecules of life, scientists are still making progress and filling in gaps. For example, scientists do not know how short chains of RNA formed from single RNA molecules. Once short chains of RNA form, scientists are able to show how they replicate, grow, and construct other molecules and RNA chains, which is the subject of the next section. Sutherland stated that cyanosulfidic chemistry only puts us at the “end of the beginning,” but it is a huge advance because scientists hadn’t even reached the “beginning” in the decades of research since Miller and Urey formed amino acids in a reactor in the 1950s.
[1] Sutherland, John et al. Studies on the origin of life – the end of the beginning. Nature Reviews: chemistry. (2017) 1(12): 1-7. 13
[2] Than, Ker (August 29, 2012). "Sugar Found In Space". National Geographic. Archived from the original on July 14, 2015. Retrieved August 31, 2012.
[3] Patel, B. H., Percivalle, C., Ritson, D. J., Duffy, C. D., & Sutherland, J. D. (2015). Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism. Nature Chemistry, 7(4), 301–307. http://doi.org/10.1038/nchem.2202
[4] Patel, Common origins
[5] Patel, Common origins
[6] Patel, Common origins