The Chemicals of life
It has often been said to me that the Miller experiments in the 1950' were flawed. They may be right, but that doesn't mean that the chemistry of the origins of life has stayed still.
So, what has been discovered? Well, it all depends on the atmosphere. The early atmosphere was reducing (it had no free oxygen), but just how reducing it was is a matter of debate still.
It is important to consider the probability of the following reactions happening. The ones shown are all ones that occur easily with applied heat. This is a very quick summary of the details in 'Biochemistry' 2nd Edition by Lehninger. 1975 from Worth Publishers. If you are interested in more detail, or if you think this sounds too easy, contact me and I will supply greater detail.
The first organic chemicals
The first important chemical is Hydrogen Cyanide (HCN). It is formed from:
2CH4 + N2 à 2HCN +H20
C02 + NH3à HCN + H20
Now that we have HCN, this will react to form Cyanoacetlyne, Cyanamide and various Nitriles.
HCN and Ammonia (NH3) will react to form many common amino acids and a large amount of Adenine, which is important for nucleic acids of course, but also for polyphospates, used by organisms for energy. Polyphosphates and polyphosphate esters also form readily, which help in the formation of polypetides.
Peptide bonds (bonds that make up proteins) form by a condensing reaction (removing the components of water from two peptides). If there is a lot of water around, this is obviously difficult, and the only way it can happen is if there is a condensing agent. In modern organisms, condensation reactions are performed with the catalyst Dicyclohexyl Carbodiimide. This obviously wasn't available, but one form of this was available, with the reaction sequence:
HCN à Cyanimide à Cyanoguanidine à Crabodimiide derivative
Work has also been done with the clay Morillonite, this catalyses the same condensing reactions.
The interesting point about the polypeptides that are formed is that they are not random. Given different amino acids in the starting mixture the same polypeptides form.
The problem is that no single polypeptide will last for long, but with the continual production of new ones, an equilibrium could be reached.
The following are easily formed: Purines/Pyridimines/ribose and 2-Deoxyribose. Adenosine and Deoxyadenosine also detected.
Ethyl Metaphosphate formed from HCN, and this leads to AMP, ADP and ATP.
Using the energy available from these polyphosphates and adding heat, links are formed and a backbone template is made. This naturally forms an inverse template in exactly the same way that DNA does, but it is less stable than DNA. Depending on whether L or D stereoisomers are used will determine whether left or right handed amino acids used. There does not seem to be any advantage to using one over the other, so the use of left handed amino acids in modern organisms could be just chance.
From a more recent source:
Ryboszymes were discovered in 1983. These are catalysts made of RNA and further work showed that from random oligonucleotides a catalyst could be isolated which joined the oligoneucleotides together.
"Quite recently Szostak found even stronger evidence that an RNA molecule produced by prebiotic chemistry could have carried out RNA replication on the early earth. He started by creating a pool of random oligonucleotides, to approximate the random production presumed to have occurred some four billion years ago. From that pool he was able to isolate a catalyst that could join together oligonucleotides. Equally important, the catalyst could draw energy for the reaction from a triphosphate group (three joined phosphates), the very same group that now fuels most biochemical reactions in living systems, including nucleic acid replication."Orgel on the Origin of Life (accessed 4/9/04)
This is still a long way from the start of life, for instance, I have yet to learn of any asymmetrical nucleic acids being formed, but it is the first steps towards that goal. It means that this Gap for God is getting smaller.
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