Okay, okay… time to get this over with. Why do I bother? Anyway, to re-iterate again, this is the article I’m going through.
The Problem of Sugar
In this section we’re changing track a little. It’s moving away from directly messing up reactivity trends in amino acids to something even more bizarre – the assertion that the presence of reducing sugars will impede the formation of RNA or protein chains.
Okay, so lets go through this one more time. Sugars are a source of energy for us (indeed, all cellular living creatures). They are oxidised through reaction with atmospheric oxygen to carbon dioxide and water vapour, and the resulting liberation of chemical energy – via a few convoluted steps that aren’t worth going into – is used to power us. As a result, sugars hang around in solution in our bodies all the time. We can nicely store away an excess as fatty tissue (plants store it away as cellulose), but at some point we need to get sugars into solution where they can react. That this process doesn’t immediately kill us by destroying our metabolism, ability for DNA to replicate or RNA to form, or prevent peptide chains forming is pretty compelling evidence that this particular creationist claim isn’t the insurmountable chemical barrier it claims to be. It’s, in other words, false. Like, liar-liar-pants-on-fire kind of false.
Remember, the only thing that our living cells do differently to chemical reactivity in the “outside” world is provide suitable catalysts for chemical reactions. We do not miraculously reverse stability or reactivity trends – enzymes can control the chemistry by altering the rate of selected reactions but they do not work physics-defying miracles. Therefore any chemistry going on in a living cell can be considered as a good proxy for how a hypothetical “primordial soup” would work. That our own cells contain a mixture of sugars, amino acids and nucleic acid bases, but don’t stop functioning, is a clear indication that such a mixture won’t be an immediate problem in this “soup”.
The major problem with McCombs’ assertion here is how absolute it is. It’s a sub-par middle-school understanding of chemistry – it treats reactivity as an all-or-nothing approach. It’s pretending that if you mix components together, you get 100% yield or 0% yield, that you get a reaction or no reaction. This isn’t the case in chemistry. Actual chemistry contains complex equilibria; even if [A] + [B] goes to [C], this doesn’t mean there will be no [A] and [B] left in solution at all. An equilibrium would be set up where we have a certain amount of products and a certain amount of reactant left – all because of energy and stability. For example (this is a fairly straightforward physical chemistry experiment I’ve done with undergraduates), in the decomposition of ammonium carbonate we get an equilibrium between ammonium carbonate and gaseous carbon dioxide and ammonia above it – if we don’t remove those gases (recall how McCombs abused Le Chatilier’s Principle in a previous part) then that equilibrium will stay the same. The quantity of gas will not increase forever, the quantity of solid will not decrease forever. It will eventually stabilise.
Okay, that’s a fairly unrelated example – but it demonstrates the principle. When McCombs asserts that “If amino acids (to form proteins) and sugars (to form nucleotides) were present in that soup, they would instantly react with each other, thereby removing both components from the mixture” he is not only over-stating the reactivity of those products (which is obvious because this reaction doesn’t immediately kill us by preventing all the life-supporting biochemical reactions he is talking about), but assuming that they will react completely, and totally, and not have any form of back reaction. The presence of a reducing agent only sets up an equilibrium – it is other conditions that say how far it will go. And again, because such an inhibition doesn’t occur in our cells, we can safely assume that it’s not an inhibition in a hypothetical “primordial soup”.
The Problem of Chirality
Chirality is part of a fascinating form of isomerism known as stereoisomerism. It’s basically where chemical structures in three-dimensions form mirror images. In the case of organic chemistry this is where you have a carbon atom surrounded by four different groups. It means they can be re-arranged to be mirror images of each other, but could never be perfectly superimposed. Importantly, they are chemically identical – that is, their reactivity is exactly the same, their spectroscopic properties are exactly the same, there is no way we can tell them apart through chemical means (except there is, but I’ll get to that). The core analogy of this is hands – and therefore is sometimes colloquially known as “handedness”. Take this diagram I thieved from Wikipedia:
In fact, because it’s such a classic example, those molecules are amino acids. Amino acids display such stereochemistry, and the naturally occurring ones all have the same stereochemistry. They’re all of one particular “handedness”. This is important because proteins, as three-dimensional structures, are altered if suddenly the amino acids have different chiralities. In this sense, McComb’s point is valid as if they were a mixture – called a racemic (pronounced “rass-eem-ic”, not “race-mike”) mixture – the vital three dimensional structure of proteins and peptide chains would fail. It’s convenient, therefore, that they’re not like this and come as stereochemically pure substances.
Remember when I said above that there is a way to chemically distinguish them? Well, you can; with another stereochemical centre – which is what we call a diastereomer. Going back to the hands analogy, have you’ll notice that shaking someone’s right hand with your right hand (as you do) feels comfortable, natural, and matches. But what about opposites? Seriously, try it right now; shake someone’s left hand with your right hand, or try to shake your own hand. You’ll notice it feels very different. Uncomfortable, even. If you consider that “comfort” as an analogy with energy and stability, then you’ll quickly realise how things become stereochemically pure – we react with another stereochemical centre, forming something that may be more or less stable.* This change in stability is what causes separation, and what causes us to have stereochemically pure amino acids because it’s a principle that continues whether it’s the biosynthesis of them in our cells or the formation of protein chains. One combination of stereo-centres will work, the others won’t – and natural selection or chemical stability does it for us.
* There is a complication I won’t go into in too much detail here – what if both centres are opposite? E.g., you shaking someone’s left hand with your right hand, and shaking someone’s right hand with your left. In this case we’ve formed a new stereoisomer that is still a mirror image and therefore chemically indistinct. The environments in our bodies are chiral, so the amino acids are chiral – so the chirality is preserved from generation to generation. There are a few interesting theories as to why one form generates over another in nature – such as circularly polarised light influencing the reactivity, or alternatively we can say it was just chance and that chirality was preserved by natural selection, just as how naturally selecting a coin toss can produce “pure” results even if the coin alone would suggest a 50:50 heads:tails – but that isn’t McComb’s objection. He’s objecting based on the fact that chirality merely exists and so racemic mixtures of amino acids would make inferior proteins (solved due to the existence of diastereomers), not its origin as one form or another, which is a different question.
Problems? What problems?
This frustrates me, because these objections are based on a very hard science -both in the sense of “hard and soft” science and the fact that it’s a difficult subject to get to know. Like many creationist claims, these are all very simple sounding objections arranged in a big list to make it seem impressive. But they take a lot of time to unpack, explain, and demonstrate why they’re wrong. If you want to know more, consider enrolling in a chemistry course, flick through Wikipedia, buy a chemistry set, or stare at yourself and think “wow, there’s some atoms in there doing some seriously convoluted stuff and I’m the result” – not “Goddidit”.