In the previous post, I argued that reproduction had to start with RNA, but that leaves the obvious question, why not something else? The use of purines and pyrimidines to transfer energy arises simply because the purines and pyrimidines are the easiest to form, given the earliest atmosphere almost certainly was rich in ammonia, hydrogen cyanide, cyanocetylene, and urea would soon be formed. Some may argue with the “easily formed”, however leaving a sample of ammonium cyanide and urea to its own devices will get nucleobases. Cytosine is a little more difficult, but with available cyanoacetylene, it is reasonably likely. The important point is that if you accept my mechanism for how rocky planets form, these chemicals are going to be prolific. I shall justify that later. The important thing about these chemicals is that they lead to the formation of multiple hydrogen bonds only with their partners. As explained in the last post, there is no alternative to hydrogen bonds for transferring information, and these are the only chemicals that can provide accuracy under abiogenic conditions.
The polymer linking agent is phosphate, so why phosphate? Phosphoric acid has three hydrogen atoms that are available for substitution, i.e.it can form three functions. Two are to form esters and as I noted previously, the third is to provide solubility. The solubility is important because if there was not anionic repulsion, the strands would bundle together and reproduction would not work. The strands would also not provide catalysis, which occurs because a strand can fold around a cation like magnesium and form the shapes that seem to be needed. The good news is that unlike in enzymes, it can rearrange the magnesium and thus get different effects. Of course, enzymes are hugely more effective, but an enzyme generally only does one thing.
The polymer forms esters by phosphate bonding to a sugar. Think of the reaction as
P – OH + HO – C -> P – O – C + H2O (1)
where P is the phosphorus of a phosphate or phosphoric acid, and C is the carbon atom of a sugar. Note that this reaction is reversible, but at room temperature the bonds are quite stable. These ester bonds are very strong, which is important because you do not want your carefully prepared polymer to randomly fall to bits. On the other hand, it must be able to be disrupted or substituted and not be essentially fixed, as would happen if proteins were used for information transfer. The reason is, life is evolving by random trials, and it is important that since many of these trials will be unproductive, there has to be a way to recover an many of the valuable chemicals as possible for further trials, and also to unclutter the system so that something that conveys advantages does not get lost in the morass of failures or otherwise useless stuff. Only phosphate offers these properties. In principle, you might argue for arsenate, but its bonds are weaker, thus less reliable, and worse, arsenic reacts with hydrogen sulphide (common around fumaroles which as we shall see are necessary sites) to form insoluble sulphides. These are the very pretty yellow layers in geothermal areas. No other element will do.
There are a variety of other sugars that if used to link nucleobases to phosphate will form duplexes, so the question then is, why weren’t they used? The ability to catalyse its own scission is the first of two reasons why ribose is so important. Once the strands get long enough to fold around themselves, catalysis starts, and one of the possible catalytic reactions is the promotion of the remaining OH group on the ribose to help water send the reaction (1) into reverse, which would break a link in the polymer chain. Deoxyribose does not have such a free hydroxyl and hence does not have this option, which is why DNA ended up being the information transfer chemical once a life form that had something worth keeping had emerged. What this means is that RNA has the opportunity to mutate, which is a big help in getting evolution going, and when it is broken, the bits remain available for further tries in some rearranged form.
The question then is, how do you form the phosphate ester? You mix phosphate and the sugar in solution and – oops, nothing happens. Reaction (1) is so slow at ambient temperature that you could sit there indefinitely, however, if you heat it, it does proceed. However, the rate of a reaction like this depends on the product of the concentrations on each side, with such a product on the right-hand side determining the rate of the reaction going from right to left. If you look at (1), it probably occurs to you that in aqueous solution, the concentration of water is far greater than the concentration of sugar. You will see people say that life could start around black smokers, but when you check, at the temperatures they require for the forward reaction to go it requires the concentration of water to be less than about 2%. Good luck getting that at the bottom of the ocean. You may protest that nevertheless there is life there, devouring emerging nutrients. True, although the ocean acts as a cooling bath, and the life forms have evolved protective systems. There are no such things when life is getting started. Life has moved to be close to black smokers but it did not start there.
What we need is a more precise way of delivering the required energy to the reaction site. So far, one and only one method has been found to make such initial linkages, and that is photochemical. If adenine is irradiated with light in the presence of ribose and phosphate, you get AMP, and even ATP. We now see why only ribose was chosen. AMP, and for that matter, RNA, link the nucleobases and phosphate through the ribofuranose form. Such sugars can exist in two forms: a furanose (a five-membered ring involving an oxygen atom linked to C1 of the sugar) and a pyranose form (the six-membered equivalent.). Now the first important point about a sugar is it cannot transmit electronic effects arising from the nucleobase absorbing a photon. However, it can transmit mechanical vibrational energy, and this is where the furanose becomes important. While the pyranose form is always rigid, the furanose form is flexible. The reason ribofuranose can form the links, in my opinion, is it can transmit and focus the mechanical energy to the free C-5 which will vibrate vigorously like the end of a whip and form the phosphate ester. Ribose is important because it is the only sugar with a reasonable amount of furanose form in aqueous solution. It is also worth noting that in the original experiments, no phosphate ester was formed from the pyranose form. As the furanose is used, the equilibrium ensures pyranose maintains the furanose/pyranose ratio.
That leaves open the question, how are the polymers formed? It appears that provided you can get the mers embedded in a lipid micelle or vesicle (the most primitive form of the cell wall), leaving these in the sun on a hot rock to dry them out leads to polymers of about 80 units in an hour. This is the first reason why life probably started around geothermal vents on land. Plenty of hot rocks around, with water splashes to replenish the supply of mers, and sunlight to form them. The second reason will be in the following post.
The title statement can now be answered. Life must start with RNA because it is the only agent that can lead to biological reproduction without external assistance. I started the last post indicating I would show what sort of planets might harbour life. The series is nearly there, but some might like to try the last step for themselves.