Chirality is explained as the property of molecules that are non-superimposable mirror images of each other, just like a left and right hand. Using alanine as an example, the hosts show how the same atoms bonded in the same way can form two distinct 3D structures (left-handed and right-handed versions) that cannot be rotated into one another. They note that not all molecules are chiral (glycerol and butyric acid are given as achiral examples), and that chirality matters biologically because enzymes are shape-specific, much like a glove only fits one hand. The carvone molecule is used to show how mirror-image forms of the same compound can smell completely different, spearmint versus caraway, despite having identical atoms and bonds.
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The chemist explains that ribose and deoxyribose each have a specific handedness, which is why RNA and DNA form their spiral structures.
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The flat 2D chemical diagram of a molecule like alanine does not reveal its handedness; only a 3D structural view can distinguish the left and right-handed (D and L) forms.
937 words · auto-generated from the episode video
6:24handedness. So we've talked a lot about handedness, and it's not just an analogy. So what does it mean for a molecule to be left or right handed? Yeah, we went into detail with our Yang Mills episode, but we'll do it again. So let's hold our hands. This is my left hand, this is my right hand. There is no transformation that I can do to my right hand, like rotating wise, that will give me my left hand. Even if I rotate it, right? Sure, they meet up to form like a prayer hand or a clap, but these are two separate things,
7:05right? Because in this case, the hand is facing me, the thumb is to the left, but the hand is facing me, my palm is facing me, and here, the back of my hand is facing me, the front is facing the camera. So even though the thumb is pointing in the same direction, these are not the same, right? So it requires this 3D point of view to really understand this. And the chemistry world has a word for this, it's called chirality from the Greek word forehand. And so we would say that our hands are chiral, okay? Now let's look at some of the building blocks of life itself. This is alanine, it's an amino acid. Sometimes you can buy it in a powder if you wanna like get swole or whatever, right?
7:47And on the left hand side, we see the chemical diagram of this thing. There's a carbon at the center, it's an amino acid, so it's got an amine group and NH2 to its left, a carboxyl group to its right, that's a COOH, and then the R group, which is the thing that changes for every single amino acid, in this case, it's just a methyl group, CH3. If you look at that diagram itself, you don't find anything funny. But if you look at the 3D picture, you find that there are two versions, okay? There's the left-handed alanine and the right-handed alanine, which in chemistry, they call D-alanine, L-alanine is the left-hand version. And these are chiral molecules because
8:31just to how with our hand, there's no way to rotate one hand to make it the other. There's no way to rotate the right-handed enantiomer, which is the right-handed version of this molecule, to make it the left. It's the same atoms, it's the same bonds even, right? The carbons are bonded to the oxygen in exactly the same way, and the nitrogen in exactly the same way. But in one case, the oxygen is coming out of the page, and in the other case, the oxygen is going into the page. Just like how our hands, in one case, the palm is facing me, but in the other case, the back of my hand is facing me, right? This is what we talk about when we're talking about mirror image molecules, and why they're
9:13chemically alike in some environments, but in other environments, you can actually distinguish them. And that last detail is what matters. If you look at the left-hand side, the flat diagram does not give you the information. You need the 3D information to distinguish between the left and the right-handed version, okay? Now, not everything in life is chiral. You can have achiral molecules. For example, there is the glycerol molecule, which is just carbons with a bunch of OH groups attached. That is achiral. If I take a mirror image of that, it's the same. Fatty acids like butyric acid, again, if I take a mirror image of that, it's gonna be exactly
9:54the same. Glucose, on the other hand, is chiral, so it's ribose. That's why actually DNA and RNA have a particular handedness, because ribonucleic acid in RNA, we only use a particular handedness, and that's what causes the spiral. Deoxyribose, again, has a particular handedness, in this case, right-handed, which is why DNA is right-handed. In biology, you have a mixture of both, but this has crucial implications for biochemistry in general, because imagine trying to fit a right-handed glove into your left hand.
10:35It's just not going to work, right? And this has implications in biochemistry, because the glove, you can imagine, is an enzyme that has a particular handedness, and it is expecting the same handedness of whatever thing that it's going to bind to, right? If you give it the other handedness, things are gonna get funny. One of the big implications of this is, for example, the left-handed carvone, which is the odor that we get from spearmint, right? We've got a photo of that, where we show the left-handed version of carvone is how we get spearmint.
11:15That's the odor that we get from, you know, when we smell mint. The right-handed version of the same molecule, our receptors in our nose get confused. They start smelling something completely different. It's the odor of caraway. The same chemistry, the same, I shouldn't say the same chemistry. The chemist will get mad. The same atoms, the same arrangement of atoms, but in the 3D world, they look entirely different, because there are mirror images. That has an effect on how we actually sense this molecule. These two forms, they differ in activity, metabolism, side effects.
11:56Sometimes both are useful, sometimes not. The point is that their behavior can be measured, and you can infer it by looking at their activity. This is incredible. I think the next natural question here, now that we kind of understand this idea of handness,
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