Physics, Earth & Space
Tagish Lake Meteorite Does Not Solve the Homochirality Problem
Amino acids, the building blocks of proteins, share a puzzling feature. All amino acids used in making proteins are “left-handed” amino acids.
In chemistry, we call molecules “left-handed” or “right-handed” based on how things attach to a carbon atom. Imagine the palm of your hand is a carbon atom, and your fingers are different molecules attached to the carbon (e.g., a hydrogen, an amine, a carboxylic acid). Notice that both of your hands have the same “things” attached to it, but in a different order such that your hands are mirror images of each other. My right and left hands both have a pinky finger, a thumb, and an index finger, but the arrangement is different. The same thing happens to carbon atoms that have different things attached to it; you can get molecules that are mirror images of each other. These mirror images are mostly chemically equivalent. (See here and here for more on homochirality.)
The interesting part of this is, when we try to make amino acids in the lab, we always end up with a 50/50 mixture of right- and left-handed amino acids. But when scientists try to construct proteins from this 50/50 mixture, the proteins do not function properly. Right-handed amino acids do not work. So the big question is: How did nature make only left-handed amino acids? Or was there a 50/50 mixture and nature somehow isolated only the left-handed ones to make proteins?
Scientists from the Goddard Astrobiology Analytical Laboratory believe their recent studies on a Tagish Lake (British Columbia) meteorite sample may provide an answer to this question. (See here for the press release. The research paper is still in press). They found that the Tagish Lake sample has an excess of left-handed aspartic acid, one of the 20 amino acids used in protein construction. They also noted, however, that the sample contains only a slight left-handed excess (8% greater) of alanine, another one of those 20 amino acids. They, therefore, conclude that the amino acids on the meteorite sample are not from biological contamination from Earth, but were formed in space. Furthermore, the excess left-handed aspartic acid must have formed and been isolated somehow inside the meteorite, as opposed to being exposed to certain types of radiation that may cause a slight excess of one hand of amino acids over another.
The Tagish Lake meteorite has many unique properties that have made its classification difficult. It has been a subject of interest since January 2000 when it landed. For a look at some of the unique features of the Tagish Lake meteorite, see here. Importantly, the meteorite contained many more organic compounds than just amino acids.
The Goddard Astrobiology Analytical Laboratory researchers conclude that the amino acids must have formed within the meteorite, and the left-handed excess of aspartic acid is likely due to crystallization that occurs in the presence of water. Alanine, on the other hand, does not crystallize in the same way. This is why there was only a left-handed excess of aspartic acid but a minimal excess of alanine. They speculate that perhaps the early Earth formed left-handed amino acids in a similar way — by forming crystals in the presence of water.
The fact that a particular handedness can be isolated through crystallization is not news. Chemists do this in the lab when they want to isolate a left- or right-handed molecule. This only works if the molecule can form crystals that only contain one hand. Not all molecules form crystals and not all molecules form crystals that are purely left- or purely right-handed. Alanine is not a good candidate for isolating one form using crystallization because alanine crystals form from combinations of left- and right-handed molecules, giving the chemist crystals that are 50/50 left- and right-handed.
So what makes this finding so interesting? The researchers reason that because they have found a left-handed excess of aspartic acid that formed naturally, they speculate that nature could do this on Earth or perhaps the early Earth was seeded from amino acids that traveled on a meteorite similar to this one.
Unfortunately, this does not quite solve the left-handed (or “homochirality”) problem. You see, in order to form left-handed crystals of aspartic acid, there needs to be a slight abundance of left-handed molecules in the first place. In other words, crystals do not form from a true 50/50 mixture. There must be a slight excess of left-handed molecules to form pure left-handed crystals. Given the alanine observation (an 8% excess of left-handed molecules), we might conclude that something caused a slight excess, such as polarized light interacting with the amino acids. This is a possibility, but of the amino acids found on the meteorite, only aspartic acid and alanine are mentioned, meaning that whatever caused the slight excess did not affect all of the amino acids.
Another glaring problem is, while crystallization may be an explanation for how some left-handed amino acids were isolated in nature, it does not explain all of the amino acids. Not all of the amino acids form homochiral crystals. So were there two different mechanisms that happened to isolate only left-handed versions of all 20 of the predominant amino acids? This seems to be a bit of a stretch, and does not solve the mystery of why nature prefers left-handed amino acids.
The NASA report ends, as much origin-of-life research ends, with more speculation and storytelling than actual findings or viable conclusions:
This process only amplifies a small excess that already exists. Perhaps a tiny initial left-hand excess was created by conditions in the solar nebula. For example, polarized ultraviolet light or other types of radiation from nearby stars might favor the creation of left-handed amino acids or the destruction of right-handed ones, according to the team. This initial left-hand excess could then get amplified in asteroids by processes like crystallization. Impacts from asteroids and meteorites could deliver this material to Earth, and left-handed amino acids might have been incorporated into emerging life due to their greater abundance, according to the team. Also, similar enrichments of left-handed amino acids by crystallization could have occurred on Earth in ancient sediments that had water flowing through them, such as the bottoms of rivers, lakes, or seas, according to the team.” [emphasis added]
All of these statements are speculative and do not necessarily follow from the actual findings in the meteorite. The meteorite shows that aspartic acid, which we already know forms homochiral crystals, formed homochiral crystals while alanine, which we know forms crystals with a 50/50 composition, had a slight left-handed excess. This says nothing about how these amino acids could have formed on the early Earth.
Furthermore, the Tagish Lake sample contained multiple organic compounds, not just amino acids, so if these meteorites provide a naturalistic example of possible origin-of-life mechanisms, we need to consider the presence of other compounds that nature apparently did not “choose” to employ. These findings, therefore, do not help us understand why nature builds proteins from 20 particular organic compounds that are specifically left-handed or where those 20 amino acids came from.