The November Chemistry World had an article on homochirality, with the question, "How did it evolve?" Clearly a problem, because the article did not really offer a solution. The problem is, the biogenetic chemicals should have been formed in both D and L forms equally. So why do we have D sugars and L amino acids? First, as the article points out, for all we know throughout the Universe there are an equal number of worlds supporting this choice as have chosen the other option. There is no reason to believe that D sugars are somehow superior, and certain red algae have polysaccharides based on alternating D and L galactose, so there is nothing that prevents the opposite form. So, how did homochirality evolve? The article offers a good survey of the guesses as to how an initial preference would feed on itself, but the problem then is, why was there an initial preference? In most cases, any means of obtaining a preference would appear to be too small to make any significant difference.
In my ebook, Planetary Formation and Biogenesis, I suggest there are two better questions. The first is, why did homochirality evolve? The second, and more important, is, why choose ribose, and having done that, why the furanose form? I think the answer to the last one is important. It is possible to make duplexes out of a number of pyranose pentoses, including ribose, and all of them have a slightly stronger association energy than the ribofuranose. My suggestion is that the furanose form does something the pyranose form does not do, in which case the reason for choosing ribose is clear, even though ribose is one of the least likely sugars to be formed from a synthesis that would offer a mixture: it alone has a reasonable amount of furanose form in solution. So the question then is, why prefer furanose?
The first step towards RNA in biogenesis is to join a purine or pyrimidine to ribose. This is a simple condensation reaction, but it does not work very well for purines, and not at all for pyrimidines, in aqueous solution. The condensation reaction is thermal, so there has to be a means of heating it more strongly, or alternatively, providing more vibrational energy at the reactive site. The formation of the phosphate ester at C-5 is also a condensation reaction. We know that both reactions go photochemically for adenine, ribose and phosphate, and while this is unlikely because adenine only absorbs photons at about 250 nm or less, I suggested there could be a different mechanism: absorption of visible light by something like a porphyrin and subsequent thermal energy transfer. If so, the reason for the furanose is the only form that will get to a phosphate ester, because it alone is flexible enough to transfer the vibrational energy to C-5.
If so, then the origin of homochirality is reasonably obvious. The RNA form condenses photochemically, until the RNA polymers get long enough to act as ribozymes. Once they do that, they can depolymerize as well as catalyse polymerization. For a while, anything might be formed, but once a homochiral polymer strand is formed, it can form a helix that will act as a template for a double helix. Once it does that, if the duplex separates, we have two templates. It needs the duplex to reproduce, and the duplex will not form if the strands have mixed chirality. Once reproduction starts, whatever structure was selected will predominate. If you need homochirality to reproduce, and if, once you get reproduction that form will predominate, then surely homochirality is inevitable.
This will be my last post here for 2015, so may I wish readers a very merry Christmas, and a successful 2016.
