Homochirality, a Puzzle in Biology that Remains Unsolved - Evolution News & Views

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Homochirality, a Puzzle in Biology that Remains Unsolved

One of the big chemical conundrums in biology is figuring out how nature selected only "left-handed" amino acids and "right-handed" sugars for our bodies, even though synthesizing these compounds gives a mixtures of both the right- and left-handed ones.

By way of a mini-chemistry review, carbon atoms bond in a 3-dimentional, tetrahedral, fashion. So methane, CH4, looks like a carbon with four hydrogen atoms bonded around the carbon such that the hydrogen atoms are the points of a tetrahedron. If there are different things bonded to the carbon atom, then there are two possible bonding orientations around this carbon.

One of those orientations is called left-handed and the other is called right-handed. Just like your left and right hands, these molecules are mirror images of each other. When a molecule has this mirror-image property, it is called a "chiral" molecule from the Greek word for "hand." (See an ENV article, here, for more on chiral molecules and homochirality in nature.)

Usually, when you do a reaction in the lab that will give you a chiral molecule, you end up with a 50/50 mix of left- and right-handed molecules. For the most part, these molecules are chemically equivalent. So when we find that nature only uses L-amino acids (left-handed) and D-sugars (right-handed) in organisms, there is something odd about this.

Again, laboratory syntheses of amino acids and sugars typically yield the expected 50/50 mixture. This means if we assume a purely chemical origin of life, then there must be some mechanism that selects one molecule over its mirror-image counterpart. Unfortunately, for Darwinists anyway, scientists have yet to identify a physical mechanism for selecting one chiral molecule over another.

Or have they?

A Nature Communications article is being touted as bringing us one step closer to solving the chirality mystery. In this article, the researchers sought to determine if a fundamental property of chirality and crystallization could be discovered by analyzing the behavior of a bunch of simple, microscopic, geometric figures (equilateral triangles) at various densities.

Basically they are looking at the flow and orientation of these 2-dimensional, microscopic triangles on a surface to see how they orient themselves. The idea is to find out what might cause achiral (that is, "not" chiral) molecules to form a chiral macrostructure.

The authors used equilateral triangles (achiral shapes) to study the possible emergence of chirality. The researchers chose triangles because they are the simplest of the geometric shapes, and relatively few studies have been done with them. These are 2-dimensional triangles, not 3-dimenstional shapes such as a tetrahedron or a trigonal pyramid.

Using microlithography techniques, the authors studied what combination of geometry, density, and Brownian motion can cause achiral triangles to order themselves into a chiral macrostructure. They eventually found localized chiral centers, or localized ordering. They explain this localized ordering by demonstrating that at a certain density, the shapes prefer order because it gives them more "wiggle room" than when they are in a very dense, disordered state. In technical terms, this is entropy-driven ordering.

By way of analogy, think of a very crowded room where everyone is standing. It is not organized, and people are all squashed together. At some point, the number of people in the room (the density) becomes so great that an individual would actually have more room to move his arms or legs or just look around if everyone were standing in lines, or in an orderly fashion, rather than all pushed together.

On a molecular scale, this "wiggle room" is one type of entropy, and molecules tend to act in such a way as to maximize entropy. The authors of the research found the particular density of equilateral triangles that would cause them to spontaneously order themselves to maximize their "wiggle room."

While this finding might interest those who wish to explore entropy-driven crystal structures, it does not seem to have any application beyond the exact system that the researchers were studying. In other words, what does this have to do with 3-dimensional molecules? Two-dimensional properties do not necessarily translate into three-dimensional properties. As the authors point out, even properties between two different 2D shapes do not overlap:

Entropy-driven ordering in dense multi-particle systems can be strongly influenced by particle shape.

[...]

Although interesting and rich, the observed phases of squares and pentagons do not provide any clear predictive power for the phase behaviour of dense 2D systems of regular triangles [emphasis added].

Thus there is no reason to assume that a system of 2D regular triangles translates into a system of 3D molecules. Even if we assume a molecule is a tetrahedral shape (not all are, and protein shapes are even more complex), since the behavior of other 2D shapes do not translate, why would we assume this research works for molecular structures?

Yet in their conclusion the authors state:

The emergence of two types of triatic liquid crystal phases, including one that exhibits LCSB [local chiral symmetry breaking], indicates that the subtle combination of geometry and entropy may combine to produce yet further surprises for other shapes, whether in two or three dimensions.
Certainly, with a given set of parameters, we might see different behavior. That's obvious. And as these authors demonstrate, by doing experiments designed with certain parameters and geometric constraints, we may see different behavior than what is observed with different parameters, which would be an argument for design, perhaps. Even so, this experiment's applicability is very limited.

Lastly, this demonstration, while it may have some application for understanding ideas about a certain type of crystallization, provides no solution to the enigma of homochirality in biology. While there are regions of local chirality in the triangle system, the authors state that "the systems as a whole is a racemic mixture of small chiral domains." Which is exactly what we see in the lab, a 50/50 mix of the left-handed and right-handed molecules. (Racemic means a mixture of left- and right-handed molecules.)

Furthermore, looking down on a 2D plane, we see a particular handedness, but from underneath the plane, that handedness switches (it is a mirror image). If we consider all of the planes of symmetry, something important when dealing with molecular structures, we see that in three dimensions this is still an achiral system.

It seems that the "problem" of homochirality in nature is still a mystery. Thus far, there is still no naturalistic explanation for nature's preferences for L-amino acids and D-sugars.


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