Homochirality: Why Nature Never Makes Mirror Molecules

Molecules of biological origin always have a fixed handedness or chirality. For example you only ever see right handed sugars and left handed amino acids in nature. But why?

Credits:

Viking lander video - MASA

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[SteveMould]

This one was requested by quite a few of you. I hope it answers all your questions (it definitely doesn't).

[KhAnubis]

I‘m gonna be real with you, I‘m left handed too and I never once realized scissors could show exactly where they were cutting, I just thought you had to kind of hope for the best

[OnlyOnSindays]

1:07 made me fall of seat laughing. Imagine buying a bag of sugar and opening just to be like "Awe damn... I got the single molecule again.."

[KubuntuYou]

The real question here is, "does left handed sugar taste different from right handed sugar, and do Twix ™ bars use the correspondingly chiral sugar in their left vs right campaign?"

[sp_ce.]

Just realized it’s sometimes called “dextrose” because that means “right sugar”, but that means L-glucose (left handed) should be called “sinistrose”, which just sounds evil.

[imperturbableDreamer]

The homochirality of sugar is actually a question I asked at university and just got a shrug and "that's how it is" as an answer, which has bothered me for years.
I guess the answer remains the same: it's arbitrary all the way down. But having an idea of why it is arbitrary is somehow extremely reassuring, even if I have long since moved on from those lectures. Thank you.

[RubenSteeman]

I'm left-handed and learnt to cut with right-handed scissors as well. At primary school I was really good at cutting elaborate shapes in paper because I used my good, left hand to move the paper, instead of moving the scissors to form the shapes, like most people instinctively do.

[Friend_of_the_One-Eyed_Ladies]

Aviation snips (tin snips) come in 3 main flavors: left cut, right cut, and straight cut. Red, green, and yellow handles, in that order. You need all 3 types to do tin work. The left and right are mirror images of each other. The reason you need both left and right cut is so you can cut left or right curves. You can't cut a counterclockwise ("left") circle/curve with a right-cut snips, and vice versa. Right-handed people typically use the left-cut (red) snips most, but both left and right snips are in constant use.

[Cliff]

In some mirror universe, Matt Parker replaces the head of Steve Mould in a picture with Captain Kirk

[BlastingAgents]

9:29 just to clarify: The ribosome consists of both, proteins and RNA, whereby the catalytic site is made from RNA; so, strictly speaking, it’s not only tangled-up RNA.

[KurtCollier]

fun story- this video is how I learned that when we go looking for life on other planets we bring sweets, and even make an effort to get the type you might like.

[Mr_Mooo]

This has been the most interesting youtube video that I have seen on youtube in a long time. You are a great explainer with a great dry sense of humour.

[xavierjust]

Really nice video. However, both L- and D-limonene are naturally occurring. Fun fact, because of genetic reasons, some people can tell them appart, others don't.

[TallinuTV]

Awesome in depth explanation. That Mars sugar experiment, I realized they probably sent two containers with the left and right handed sugars separated, so they could tell if one of them was being depleted at a different rate than the other, to distinguish between forms of decomposition that would affect both equally and therefore not indicate presence of life.

[tjejojyj]

I love the peeling of Feynman’s onion. We got to the “we don’t know” layer later than I was expecting as there was a lot to absorb at each stage.

Well done.

[askthisscientician]

How to irritate a biologist: just say "RNA is the single strand version of DNA"

[SmokeandLights]

I've been thinking about this video a bit lately, in relation to a 4th dimension phenomenon. I saw a video that explained that a 4th dimensional being would be able to "flip" a 3 dimensional object, much like we can turn a piece of paper, leaving the object mirrored. Sadly, I cannot remember what it was called or what channel it was on.

The implications of that are pretty significant to life, if you take into account the concepts covered in this video. A 4th dimensional being could flip you over, and pretty much make you die very quickly, if your molecules were incompatible with the ones you need to survive.

[benzenering]

Great video! I think the concept of homochirality was explained very well. However, there are a few mistakes that I would like to address.

1) If we tried to make sugars in the lab (or other chiral molecules), we would not always end up making a 50/50 mixture of enantiomers. This might have been an intended simplification, but I think it is good to mention the truth for clarity. A lot of reactions do indeed produce a 50/50 mixture of enantiomers, but there are many examples of reactions where a 100% pure enantiomer is made. There are also some ways to bias the enantioselectivity of the reaction one way or the other, but it's too complicated to describe in a comment. Also, what's actually more common in organic chemistry is to be concerned with the changes in the chirality of individual stereocentres found in a single molecule. A stereocentre is essentially a carbon atom that has its own handedness, and a single molecule can have multiple of these 'handedness' centres. If two molecules have most of their stereocentres being identical, but some of these stereocentres differ, these molecules are referred to as diastereoisomers. It's actually a lot more common that reactions only change the stereochemistry of one or a couple of stereocentres in a molecule rather than all of them at once. In some reactions, we end up making a 50/50 mixture of diastereoisomers, while in some cases we make 100% of a single diastereoisomer (so-called stereospecific reactions). There are also other cases where we selectively make a biased ratio of stereoisomers - something like a 90:10 mixture (so-called stereoselective reactions).


3) In chemistry, there are also some pretty ingenious ways to purify mixtures of enantiomers that do not involve the complex use of enzymes or anything like that. For that, we could use the properties of stereoisomers that I have mentioned earlier - diastereoisomers. While enantiomers have identical chemical and physical properties, most diastereoisomers actually have different properties, such as their boiling point or solubility in various solvents. If we take a mixture of enantiomers and react it with a different molecule that is 100% enantiomerically pure, we would get a mixture of diastereoisomers corresponding to our starting materials. We could then take that mixture of diastereoisomers, purify it using their unique properties, and get 100% (or nearly 100%) pure diastereoisomer. By using a reverse reaction of what we did at the start, we could then turn that diastereoisomer back into the starting material, giving a 100% (or nearly 100%) pure enantiomer.

[celineguler]

i love how you ask the question for us and answer it because the teachers be like
T:...so plants always make right sugar.
Me: wait but wh...
T:yeah sorry no questions we out of time

[billmcdonald]

Arthur C. Clarke wrote a short story back in 1950 called 'The Reversed Man' later re-titled to 'Technical Error' in which this very phenomenon was a crucial part of the plot.