Larry Moran has a post up about a new paper that claims to help solve the 'problem' of the origin of biological chirality by showing that amino acids forming in space are not an equal mix of the two 'mirror' isomers but slightly biased towards the L isomers used by living organisms for protein synthesis. [NON-RACEMIC AMINO ACID PRODUCTION BY ULTRAVIOLET IRRADIATION OF ACHIRAL INTERSTELLAR ICE ANALOGS WITH CIRCULARLY POLARIZED LIGHT]. I was going to write a post arguing that the use of only one isomer isn't a problem in need of explanation, but then realized that I'd written such a post several years ago. I haven't seen my argument being made anywhere else, so I'm reposting it below.
Is the origin of biological chirality a no-brainer?
One argument invoked by creationists for a magical origin of life is that the observed homochirality (definition below) of biological molecules could not have arisen without divine intervention. Creationists are notorious for their sloppy reasoning, but I'm a bit puzzled that scientists also see this as a big problem. Here's an example from the Answers in Genesis people, and the blue box in this article is an example from PLoS Biology.
The fundamental argument seems to be that some special forces or factors are needed to explain how and/or why the first living things used only D-sugars and only L-amino acids. It's often claimed that this would not have happened unless the starting abiotic materials had an excess of one enantiomer (definition below) over the other.
I think people have fallen into the error of assuming that, at the molecular level, enantiomers are much more similar to each other than to other related molecules. But it shouldn't be any harder for an asymmetric reactant or catalyst to distinguish D-glucose from L-glucose than from either enantiomer of fructose or galactose. Or to distinguish L-leucine from D-leucine than from D- or L-isoleucine. They may contain the same atoms but they all have entirely different shapes, and so they are all entirely different molecules.
(Here's the same point made about words. The words pacer and recap are palindromes, but most readers have no more trouble telling them apart than telling either from caper.)
Chemists discovered chirality when investigating the bulk properties of pure crystals and solutions. Chemical synthesis of a chiral molecule from simpler non-chiral precursors usually produces equal amounts of both enantiomers, whose identical physical properties make them very difficult to separate. Chemists thus view enantiomers as almost-identical molecules, differing only in their 'handedness'.
But chemists are late arrivals on the evolutionary scene, and the first self-replicating entities would usually have interacted with individual molecules in complex mixtures. Because all but the simplest of biologically relevant molecules are asymmetric, most inter-molecular interactions would always have been between asymmetric participants, each no more likely to confuse their partner with its enantiomer than with any other molecule. The fact that crystals of D-glucose and L-glucose have the same bulk properties (solubility, melting temperature) would have been irrelevant.
We don't need to fuss with defining 'life', but can simply think about the origin of entities capable of evolving by natural selection (having heritable variation causing differential reproduction). Any molecule complex enough to have heritable variation would certainly have been complex enough to be asymmetric. To such molecules, discrimination between enantiomers wouldn't have been any more of a problem than discriminating between other possible reactants.
Although this now seems so obvious to me, I only realized it a couple of years ago. Until then I unquestioningly accepted the general teaching that the origin of biological chirality was a big problem. Am I missing something?
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Definitions of enantiomer and homochirality: Like letters in words, the same combinations of atoms can be put together in different arrangements. Sometimes the arrangements can be mirror-images of each other. Chemists call molecules that contain the same atoms connected in mirror-symmetry arrangements enantiomers of each other. Enantiomers are distinct molecules (like the words speed and deeps), but pure preparations of these chiral molecules have the same bulk physical properties. The molecules found in organisms are usually homochiral: only one of the two enantiomers is used. Our metabolism uses only the D enantiomers of sugars and only the L enantiomers of amino acids.
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