In a previous post I issued a challenge that was issued prior to my talk to the Wellington Astronomical Society: can you work out how homochirality arose in life? To remind you, chirality is what causes handedness. If you have gloves, your left hand has its glove and the right hand its, and one cannot really replace the other. Homo chirality means there is one only form of handedness, thus in your body, sugars are D sugars (right handed) while all your amino acids are L, or left handed. The problem is, when you synthesis any of these through any conceivable route given the nature of the starting materials, which have no chirality, you get an equal mix of D and L. On the other hand, if you synthesize the molecules through a chiral entity, chirality remains. Think of using a left-handed glove. If you use it as a mold for a plaster cast, you will keep making casts of left hands, not right hands.
How did nature select one lot and neglect the others? The real reason for asking this, though, was not to do with chirality. Most people can get through life without stopping to worry about why their proteins are made from L amino acids. Space travellers landing on another planet might, though, because if you landed on a planet where all the amino acids were D, then you could not eat their food and be nourished. However we are here. No, the real reason was, this is a chance to show how to develop a theory.
Everyone develops theories, for example, “Who trashed the letterbox?” is an example I gave in my first ebook, which was about developing theories. The book was mainly about scientific theories, so don’t rush out and buy it unless science really interests you, but that point is valid about life. If you look at the web, you can find many places where people theorize on political matters. That would be very good for democracy, if they did it properly, but not so good if the methodology is very bad. Most simply jump to the first conclusion their prejudices lead to, and if that is the way we intend to run our democracy, then we are in trouble. The reason I picked on this issue of chirality is that it is easy, and it is unlikely to run into prejudiced anger and hence can be considered dispassionately.
There are numerous scientific papers devoted to the question of how homochirality arose: they consider the weak force (which does not apply to chemistry anywhere else); materials adsorbed on special clays (without asking how the material can get off again, or why another clay won’t give the complementary material); polarized light (why is there not the opposite result with oppositely polarized light); and even an assertion there is a weak preference in meteorites.
I believe the answer is strangely simple when instead of starting at the beginning with a mixture of both forms, you stop worrying about how it happened, and start asking why it happened? Why would emerging life discard half of the resources available to it? After all, if it did, why did not some other form use both? By using both, it would have twice the amount of resource, so it should be able to survive better, and should prevail.
The obvious answer is that life chose one form because it had to, so where is homochirality so important? The answer is reproduction. What happens is reproduction is governed by nucleic acids that can form a double helix, or duplex. If you have a strand, complementary nucleobases get absorbed on the strand, and if all the bases can link through the phosphate esters, they form their own helix. When that strand is complete, the strands can separate, and the process starts again. That is the essence of reproduction. Now, the problem is in joining those phosphate esters because the appropriate parts have to be in the right place. The new strand has to have the same degree of twist, in the same direction. This is where the chirality comes in. To get a regular twist, or pitch to the helix, all the ribose units have to have the same handedness. Think of making a bolt, and a nut to fit it. If the bolt has right hand thread, then suddenly lurches every now and again into left hand thread, how can you make a nut to fit it?
If a sugar came in with the opposite chirality, the twist would be wrong, the ends would not match up, and the base could not join the strand. It would then go away and nothing would happen until the correct pitch to the helix could be supplied, and that is with the correct chirality of the ribose. At first, strands with any mix could occur, but duplexes would only form with one chirality, and when one came along, since it could reproduce and the others could not, inevitably it must prevail.
Why does that go out to all the other molecules? Because they are made either directly or indirectly from RNA molecules. (RNA is the generator of enzymes.) Accordingly, everything that comes from the chiral RNA will also carry the appropriate chirality.
Was that so difficult to conceive?