Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

How Homochirality Arose

So, my theory challenge, with three weeks to think about it, got no responses. Perhaps nobody is reading these posts. Perhaps nobody cares about theory. That would be ugly. Perhaps the problems were too hard. Really? Anyway, first, a review of where science is at the moment:  My argument is that none of this review answers the question, but it does give a very large number of references. Given that there was this much activity that failed, maybe this challenge was unnecessasrily hard, but let me give you my proposal on how homochirality occurred.

The way to form theories is to ask questions, and in this case ask why did nature choose to be homochiral, given that it wasted half its resources. Why would not some other life form use both, and gain competitive advantage? The obvious answer is that nature chose homochirality because it had to, i.e. if it did not become homochiral, there would be no life. Now, most of what life requires does not demand homochirality. Sources of chemicals could in principle be of any chirality, light is not chiral, energy transport (ATP) depends on the tripolyphosphate, however there is one part where chirality is critical: reproduction. Reproduction occurs when a strand of nucleic acid allows its complement to form as a second strand, where it forms a duplex (double helix). When the duplex separates later, both single strands can grow further new strands, which in turn can form two new duplexes. Note that the helical nature is imposed by the chirality of C-4 on the ribose. The single strand does not have to form a helix, but the two strands, to be intertwined, must both form a helix with the same pitch.

The second strand does not grow by itself. What must happen is that the second strand forms by the complementary bases, with 5-phosphated ribose attached, form hydrogen bonds with their complementary base on the nucleic acid strand. It is now loosely attached by the few hydrogen bonds, and either the required 3-hydroxyl is close to a 5'-phosphate or it is not. If it is, then the ester bond can form, given an impulse from somewhere to overcome the activation energy. If the ribose chirality is correct, esters can form; if it is not, the two sites never come close enough, no ester is possible, and the base eventually wanders off and sooner or later the correct chirality will appear and the duplex grows. Think of a nut and bolt - you cannot make this work if every now and again the thread changes from left handed to right handed pitch. If there is a wrong chirality on the first strand, no duplex can form either, and the impulse required to bring the groups together is now also the impulse required to unravel the duplex.

RNA strands can form loops held together by magnesium ions and these emerging ribozymes can act as catalysts, and these can hydrolyse exposed RNA strands. It may be that it can preferentially solvolyse parts where the pitch chages. Some work is required to validate that piece of speculation, neveretheless, the duplex is at a lower energy than two single strands, so eventually we expect a double helix to form, especially if errors in the chain can be solvolysed.

Once you have a reproducing chiral molecule that can act as a catalyst, then it uses all the resources more effectively than any other option, and when it catalyses syntheses, it synthesises chiral entities. Thus it is RNA that is critical for homochirality; it is the only molecule that can arise naturally, sort itself out, then reproduce. Reproduction ensures that it prevails. Whether it chooses D for sugars and L for amino acids would be pure chance on this interpretation, and it would be predicted that half of alien life would choose the other.

Is that unnecessarily difficult?
Posted by Ian Miller on Mar 9, 2015 12:23 AM Europe/London

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