Another Mechanism to Explain Origin of Homochirality Questioned
One of my teenage daughters is left-handed. And nobody in our family wants to sit next to her when we eat at the table. It’s not because she has bad manners. It’s because her left arm bumps into the right arm of the person seated next to her. When it comes to meal time, left-handed and right-handed people don’t go well together.
The same is true for life. Left-handed and right-handed molecules don’t mix well. The building blocks of proteins (amino acids), DNA (deoxyribose), and RNA (ribose) have to be exclusively in left-handed (amino acids) and right-handed (deoxyribose and ribose) configurations—a condition called homochirality—for life’s chemistry to be possible.
Because of its central importance, origin-of-life investigators focus a lot of attention on trying to account for the origin of homochirality, so far without much success. (In fact, I discussed this problem a few weeks ago.) New work*, however, has led a team of scientists to propose yet another mechanism to account for the genesis of homochirality. As with other proposals, this idea turns out to be unrealistic upon careful consideration.
To appreciate the discovery, I need to provide some background. For convenience, some of what follows is repeated from the earlier mentioned post on the origin of homochirality.
Some molecules are mirror images of each other. Molecular mirror images result when four different chemical constituents bind to a central carbon atom. (The central carbon atom is called the chiral carbon.) These chemical groups are oriented in space in one of two possible arrangements that turn out to be reflections of each other. As mirror images, these compounds cannot be overlaid on one another so that all the chemical groups coincide in space. Because they can’t be superposed, molecular mirror images (called enantiomers) are distinct chemical entities.
Some of the compounds that play key roles as life’s building blocks, such as amino acids, and the sugars deoxyribose and ribose are chiral compounds.
It turns out that the amino acids that comprise proteins and the sugars that are part of the constituents of DNA and RNA have uniform chirality, a condition biochemists call homochirality. In other words, all the amino acids in proteins have the same chirality. And all the sugars in DNA and RNA have identical chirality as well.
Homochirality is a strict requirement for life. Chirality dictates the three-dimensional positioning of chemical groups in space. And the spatial location of the chemical moieties (equal parts) plays an essential role in the interactions that stabilize the three-dimensional structure of proteins. (A protein’s structure determines its function.) As a case in point, for some proteins the incorporation of even one amino acid of the opposite mirror image into its backbone will disrupt the protein’s structure, and hence, function. Additionally, as Hugh Ross and I point out in Origins of Life, laboratory experiments demonstrate that the “wrong” enantiomeric form of a nucleotide inhibits the formation of DNA and RNA assembly.
Homochirality and the Origin of Life
In order to adequately explain the spontaneous emergence of life, origin-of-life researchers have to account for the origin of homochirality. This is no easy feat. While numerous proposed explanations for the genesis of homochirality have been advanced, none seem compelling and most are riddled with problems. (For a detailed discussion of some of the difficulties researchers encounter in their attempts to explain the origin of homochirality see Origins of Life.)
Part of this challenge stems from the fact that chemical reactions which generate chiral compounds from achiral starting materials produce a 50:50 mixture of both mirror images. (This type of mixture is called racemic.) In other words, chemical processes, as a rule of thumb, do not yield homochiral products—unless a chiral excess already exists at the outset for one of the reactants or the reaction catalyst.
Does Crystallization Lead to Homochirality?
A research team from Columbia University, headed up by Ronald Breslow, believes it can explain how homochirality emerged. These investigators have previously demonstrated that when amino acids crystallize out of solution in which there is a slight excess of one enantiomer, the ensuing crystal possesses a 50:50 mixture of the two enantiomers. On the other hand, the liquid phase becomes enriched with the enantiomer that initially was in slight excess. They have shown that a slight chiral excess of about 1% can quickly become amplified to about 90% after two rounds of crystallization. The reason for this enrichment stems from the reduced solubility of amino acid complexes formed when left-handed and right-handed versions combine compared to complexes formed from left-handed and left-handed forms (or right-handed and right-handed forms). The difference in solubility will cause the crystal to exclude the enantiomer that is initially in excess.
Based on this behavior, the Columbia team proposes the following scenario to explain how homochirality originated. First, they note that in meteorites like Murchison, a slight chiral excess has been detected for some amino acids recovered from the meteorite. Then they argue that meteorites delivering amino acids to early Earth would seed the oceans with a slight chiral excess of amino acids. As the oceans waters washed onto ancient shorelines and water evaporated, amino acid crystals would form, leaving behind an even greater chiral excess in the waters that returned to the oceans. Eventually, the amino acids in the oceans would be populated with nearly 100% of one enantiomer at the expense of the other. This end result, they claim, becomes the birth of homochirality as this chiral excess gets transferred to molecules taking part in the origin-of-life process.
At first glance this scenario seems quite reasonable. Closer examination, however, exposes a fundamental problem: chiral excess in Earth’s oceans will not promote homochirality in life molecules, but, in fact, detracts from it.
To illustrate, consider the reactions between amino acids to make peptides (small protein chains). Amino acids will not react with each other in water to form peptides. (In water the reverse reaction, in which peptides break down into the constitutive amino acids, is favored.) Origin-of-life researchers posit that this difficulty can be overcome if ocean waters deposit amino acids onto ancient shorelines. As the water evaporates, the reaction between amino acids becomes more likely. Additionally, the minerals on the shore can serve as catalysts promoting the reaction.
Herein lies the difficulty. According to the mechanism proposed by the Columbia chemists, the amino acids deposited on the shore will be a racemic mixture, displaying little if any chiral excess. The chirally enriched amino acids will be diluted out in the ocean waters and unable to react to form peptides.
In reality, the mechanism proposed by the Columbia scientists would inhibit the birth of homochirality, not promote it. Therefore, the homochirality problem still represents an “elbow-in-the-side” of chemical evolutionary explanations for the origin of life.
*This study made science news headlines when first published. I discussed the scientific and biblical implications of this research on the April 9, 2008 edition of our new podcast, RTB’s Science News Flash. This podcast offers a unique Christian perspective on headline-grabbing discoveries. A free subscription to this podcast is available through iTunes.