Crossposted at Politicook.net
Chirality comes from the Greek root χειρος, meaning "of the hand". (Many thanks to reader codairem who not only helped me with the spelling, but the noun case. I appreciate the assistance, because I strive to be as correct as possible here. Sorry that it took this long to correct, but I had to attend to some other things). Most people outside of us chemistry wanks do not know that many organic molecules have the property of "handedness", meaning that they come in two varieties, a "right-handed" and a "left-handed" one. Actually, it is bit more complex, and this will become clear in a few paragraphs.
First, let us define what chirality is. Take a look at your hands. They are pretty much the same, both with backs and palms and, for most of us, five fingers each. Now, at first blush one would think that they are essentially identical to each other, except for any accidental scar or the dominant one being just a little larger than the other.
Breaking: This diary is being hijacked by, well Translator.
I just got my absentee ballot today (Saturday) and filled it out this evening after my stint on What's for Dinner. Here it is in its entirety. If illegible, please let me know.
Because of the geometry of the document, I had to break it down into four pictures. Here is the top one:
It is the instructions and the fill in for voting for President and Vice-President. You will note that on the right, I just left the unopposed candidates one blank. The important one in this picture is that I filled in Obama and Biden. But look at the choices we have in Arkansas.
Here is the picture.
Here is the second half of the front page. Note that I voted against the Blue Dogs, Pryor and Ross, without endangering our caucus. Even if the Greens win, they will caucus with the Democrats. I wrote both Pryor and Ross after the FISA debacle that I would never vote for either of them again. Nice to be able to do so without supporting Republicans. Here is the image:
Those votes, whilst important, are not the only things of the ballot. The Gov race is two years from now, but some other things are more important. Please look at the reverse side of the ballot and how I voted.
The first one has to do with allowing the State legislature more freedom, and the way I look at it, the less the better, so I voted "NO".
The one just below has to do with allowing the State legislature to meet more often, and, for the reasons above, again I voted "NO". The less time to make laws, the better, I think.
The middle one is an amendment to authorize a lottery, run by either the state or by an independent, not for profit, entity, and to use all of the proceeds after administrative payouts and prizes for scholorships. One of the phrases indicates that those monies have nothing to do with apprpriated funds and can not affect them. I voted "YES", because there have been, and will be, many other much more onerous ones, like the one a couple a years ago that would have privatized it to only one set of corporatists.
The first one on the right side would make illegal for a couple who are not married either to adopt or to foster children. This is the next swipe against gay marriage in my state, since the anti-gay marriage one passed a couple of years ago. Of course I voted "NO".
The one under it has to do with the State getting more control on the waters of Arkansas, and I do not reject that out of hand, but the authority to expend $60,000,000 sort of puked me off of it. There are more important things, so I voted "NO".
Now to the real diary. I hope that no one was offended by this little tirade. My voting record is not a secret.
Not so. If they were identical, you could twist your wrists and rotate your arms such that they appeared the same. But you can not, even if you are a contortionist. Hands are mirror images of each other, but because of their spatial arrangement, they are non-superimposible mirror images of each other.
A body is chiral if it is not superimposible with its mirror image. Take a sphere, for example. The mirror image of a sphere is superimposible with the original, so it is not chiral. (The rules say that you can manipulate the images in any way by rotation in any plane, but you can not change the image itself). A square is also achiral. But hands are different. They are constructed such that it is impossible to superimpose one and its mirror image. Such mirror images are termed enantiomers when molecules are involved.
Molecules are like that, too. The most important class are organic compounds with a carbon singly bonded to four different atoms or groups, but many other examples exist. Here is a picture of a simple chiral molecule:
For the three dimensionally challenged, get two packing peanuts or other small pieces of plastic foam and eight toothpicks. Take markers and color pairs of toothpicks so that you have four colors of toothpicks, then push them into the foam so that the arrangement looks sort of like an old fashioned jack with which kids used to play. Then do the same for the second piece of foam to make a mirror image of the first one. Try as you may, it is not possible to twist or turn the second one to superimpose it with the first one.
Here is a picture of a pair that I made with scraps of packing material and four toothpicks (I cut mine in half) They do not look very good, but in person are better:
Here is a picture of pretty much the same thing using my fancy molecular model set that Mrs. Translator bought for me many years ago when I started graduate school. You would not believe how many times this set helped me understand things, like the unique properties of DABCO. (Tips to anyone who knows what this is).
OK, please do not go to sleep yet, because you should be proud that you just created molecular models. A lot of this is done by computer graphics these days, but even we chemists like to have something that we can hold and rotate to get a better feel for things.
So, why is this important? The answer is, because of life itself.
The proteins and sugars in our bodies are chiral. There was an episode of Star Trek where Spock was thrown into a mirror image universe and the proteins in food were unusable, so he had to convert them to the correct configuration to survive. Chiral molecules react with achiral ones identically for the most part, but very differently with other chiral molecules.
Here is an example that everyone just about knows. Do you ever use caraway seeds for flavoring? They smell the way they do because the "smelly" molecule is carvone. Do you like spearmint? Carvone is the "smelly" compound it it, too. What gives?
It turns out that the proteins that form the scent receptors in our nose are chiral, too. The carvone from caraway is "S" carvone, and the one from spearmint is "L" carvone, and the two interact with the receptors differently, giving rise to different perceptions. Here is a picture of the two different carvones:
The dashed lines mean, from thick to thin, that the group pointed towards the back of the plane of the computer screen, and the wedges mean that the group is coming out of the plane of the screen, towards you.
The "S" designation is from the Latin sinister, meaning "left" and the "D" is from the Latin dexter, meaning "right". No wonder why the wingnuts think of the political left as evil, the Latin word says so. This is the convention that we chemists use to define which configuration is present. Interesting, in some severe Muslim traditions, the left hand is considered to be unclean. I do not know how much that has to do with Arabic tradition as opposed to Roman influence, and any historian reading this is encouraged to clear up the area.
It is possible for a molecule to contain more than one carbon bonded to four different things (an asymmetric carbon), and indeed that happens often. The acid from grapes, tartaric acid, is a good example. It was actually Louis Pasteur who figured this out, sorting mirror image crystals of cream of tarter by hand with only a magnifier and forceps.
Compounds with two different chiral centers are called diastereomers. I mention this only because one my students, who was struggling with the concept of chirality, answered a quiz question by saying disastromers. I suppose for many this topic does seem to be disastrous. I gave her full credit, by the way.
Complex molecules can have many chiral centers, and proteins and polysaccharides (see a previous post for them) can have millions. All proteins are composed of amino acids, and on our planet almost all of them are of only one chirality. The others just do not fit together with them, so the cause of Mr. Spock's near starvation.
I am not going to beat this to death tonight, but there are two other concepts that bear mentioning. Chiral compounds interact with light differently. The implication is that light itself is chiral, otherwise it would not interact at all or interact the same with either enantiomer. At the risk of being wankish, you can tell the difference between caraway and spearmint oil just by passing light that is polarized through it. Polarized light has been filtered through the same material that Polaroid sunglasses are made, and it filters out light except for only one small set of vibrations.
When you shine that light through caraway, you will find that it has been rotated by 61 degrees with respect to the vertical. That is like taking two pairs of Polaroid sunglasses and rotating them that amount and trying to read through them. But is you pass it through spearmint, it is -61 degrees. So light is chiral.
I could go on, but would rather respond to comments and questions. Remember, this is an open science thread, so please do not limit your thoughts to this subject only.