Got into an interesting discussion on why life-molecules on Earth are of one chirality (handedness)—and lo! meteorites from elsewhere in the solar system that bring in such molecules are likewise -handed.
Heard anything on this?
Discussing chirality…and other big words.
by CJ | Sep 30, 2014 | Journal | 19 comments
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Homochirality seems to be a big flashing sign saying, “Produced by nano-scale processes”—which up to current cutting-edge tech means “biological”. Wikipedia points then to the Murchison meteorite, but it’s hard to distinguish “initial reports” from “what is currently known about the meteorite”.
Of course there might be a large-scale process that would produce a single chirality. Since the different chiralities often bend polarized light in opposite directions (which is how they were discovered—and the plot gimmick behind Dorothy L. Sayers’s The Documents in the Case) might it be possible for a photosensitive process to be triggered by polarized light and therefore produce more of one isomer than the other? I dunno; maybe someone with a better background in chemistry could say.
Chirality seems to be a characteristic
of a molecule involving a carbon atom chain: “Organic compounds, molecules created around a chain of carbon atom (more commonly known as carbon backbone), play an essential role in the chemistry of life. These molecules derive their importance from the energy they carry, mainly in a form of potential energy between atomic molecules. Since such potential force can be widely affected due to changes in atomic placement, it is important to understand the concept of an isomer, a molecule sharing same atomic make up as another but differing in structural arrangements. This article will be devoted to a specific isomers called stereoisomers and its property of chirality (Figure 1)..”
http://chemwiki.ucdavis.edu/Organic_Chemistry/Chirality/Chirality_and_Stereoisomers
Which I suppose would answer the begging question of ‘why organics are this way and not everything else, too?’
Chirality in and of itself doesn’t change the potential energy of a molecule. That potential energy has more to do with the physical geometry of molecules. Are the six-member rings that carbon forms in a “chair” or “boat” configuration? Would the normally energy-favored “chair” configuration bring two similarly charged functional groups close enough to each other that they repel, making the “boat” configuration have a lower potential energy? Are there double (or triple) bonds in different locations of the atom, leading to more or less stress? Single-bonded carbon (really, any atom that likes a tetrahedral bonding geometry) likes bond angles of 109.5 degrees. Double bonded carbon likes bond angles of 120 degrees. A flat 6-member ring (like graphite has) has bond angles of 120 degrees, which is why cyclohexane “deforms” into the chair or boat configurations, and you only get flat rings if you have double bonds in the ring, like benzene or graphite. Reducing the number of members in the ring to five, or four, increases the stress. Substituting a non-carbon atom into the ring (like a nitrogen) can also increase the stress. A lot of organic chem can be thought of as geometry and bond stress problems, and it makes a lot more sense that way than as a list of reactions to memorize.
As noted below, any element that bonds tetrahedrally can give chiral centers. There are chiral silicon compounds.
Finally, there are certain chemical reactions which produce one chirality preferentially, again due to molecular geometry and accessibility of reaction sites. I need to go digging through my organic textbook for an example, though, and it’s late on the east coast. I hope this post was coherent and understandable!
Yes, it was, but if we’re talking life then for the most part unsubstituted cyclohexane conformers thermodynamic differences (7.1 kcal/mol, (my Morrison & Boyd was close to hand)) are irrelevant. In so far as cyclohexane forms a subunit of a more complex molecule, its shape will be dictated by the way the molecule as a whole is shaped from electrostatic attraction and repulsions, and the sizes of the substitutions.
I’m not certain of chiral silicon molecules. Logically they should be possible, but they would be less stable than carbon centers, perhaps too unstable to do much with.
I didn’t get along too well with large bio-molecules, academically-speaking, and ended up switching to a physics major where I could go play with photons instead. 🙂 Thank you for the reminder about their behavior, of course the larger structure would dominate.
The quick Google Scholar search I did turned up chiral silanes (or I wouldn’t have made that claim), but they primarily appear to be curiosities in the lab.
An interesting question one step further, can life exist without a chiral preference?
Excellent. I’m certainly no chemist, but I roomed for five years (not in a Holiday Inn) with somebody who was deep into Organic. I was (during our second year) working on Lucretius’ (ancient Roman) De Natura, in which he has atoms with uniquely shaped hooks which only fit certain other atoms (I thought it really a pretty good ‘other’ view of valence)— used to drive my roomie nuts, because I would try to get her to explain the modern theory so I could compare them, but she really did not want to hear about hooks… I also lucked into one of the best chem profs in the state, who amused himself by teaching grunt chemistry, so he’d trot out equipment to show off to us that, consequently, *I* knew about, but my roomie, actually in the field, had never met. So I owe him and those arguments with my roommate for giving me grounding that lets me go on learning in the field…I’m always fascinated by the subject.
When I was in college Chem studies, the presumption always was that all right-handed molecules would work just as well as all left-handed ones we use. And it was presumed that there were equal chances life could have started down either path with equal probability. (Presuming we discount Panspermia, of course.) However, I’ve never run across anything that says, yea nor nay. I think we have to consider it unanswered. On the other hand, they have since found symmetry violations in QM, so it may be that all right-handed doesn’t work. In short, “dunno”. 😉
The following is a link to an article about the weak-nuclear force causing asymmetry. http://www.scientificamerican.com/article/weak-nuclear-force-shown-to-give-asymmetry-to-biochemistry-of-life/
If the question is, “Why carbon?”, on the other hand, that answer is easier. First of all, we need an atom that will make 4 separate covalent bonds. An atom like, say, sodium that only has one electron it might share can’t be the base of a chiral molecule.
It is left as an exercise for the interested student to discover why the geometry of less than four bonds won’t make mirror images.
But, secondly, a large atom, say lead, that is in the same family as carbon doesn’t make covalent bonds because its four outer electrons are too far from the nucleus (Inverse Square Law), and are to some extent “shielded” from the positive charge of the nucleus by the filled intervening electron shells.
So the best combination of the opposing forces is carbon: smallest of the atoms that have four valence electrons.
Couldn’t silicon? The problem is thermodynamics. The “shielding” effect makes those covalent bonds weaker, and it’s tough to do the synthesis without tearing apart the molecule you’re trying to make. So, possible? Yes, theoretically. In the natural world, it remains to be seen.
Here’s another thought—that cosmic rays are involved. http://phys.org/news/2014-09-dna-right-handed-helix.html
Maybe the whole universe just gangs up on left-handed carbon chains.
Let’s see if the experiment and it’s interpretation is confirmed.
While the DNA helix is right-handed, living amino acids are left-handed.
In any event, the discovery of stereochemistry and chirality by Louis Pasteur was arguably one of the greatest advances in Chemistry.
Another reason why we left-handers feel abused…….;)
I believe that Issac Azimov wrote a short story which revolved around how to tell someone in another universe the difference between left and right. I don’t remember of how it was resolved.
http://phys.org/news/2014-10-handedness-life.html
I question it’s mention of 19 amino acids. IIRC there are actually 22 “living” amino acids. Two are not used by normal eukaryote nuclear synthesis, but in the slightly different syntheses of, is it mitochondria or archaebacteria, I forget. Ahhh, close! See below:
“There are more than 300 amino acids found in nature of which only twenty, known as the standard amino acids, are the building blocks for protein. Only green plants and most microbes are able to synthesize all of the 20 standard amino acids that are needed by all living species. Mammals can only synthesize ten of the twenty standard amino acids. The other amino acids, valine, methionine, leucine, isoleucine, phenylalanine, lysine, threonine and tryptophan for adults and histidine, and arginine for babies are obtained through diet.” Wikipedia – Biosynthesis
“They [amino acids] include the 23 proteinogenic (“protein-building”) amino acids, which combine into peptide chains (“polypeptides”) to form the building-blocks of a vast array of proteins. These are all L-stereoisomers (“left-handed” isomers), although a few D-amino acids (“right-handed”) occur in bacterial envelopes and some antibiotics. Twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as “standard” amino acids. The other three (“non-standard” or “non-canonical”) are selenocysteine (present in many noneukaryotes as well as most eukaryotes, but not coded directly by DNA), pyrrolysine (found only in some archea and one bacterium) and N-formylmethionine (which is often the initial amino acid of proteins in bacteria, mitochondria, and chloroplasts).” Wikipedia – Amino acid
My personal take on it is similar to the Big Bang. We may never know exactly how it got started, but once it did everything else followed, i.e. was inevitable.
So why do we have chirality, bilateral symmetry? See this article that turned up in “Science Daily”today.
When is a clam like an egg?
Humpty Dumpty sat on a wall…
New research on the subject of chirality we were discussing here.