# Chirality: Life's building blocks are left handed.



## Ray McCarthy (Jun 15, 2016)

> All the amino acids in living creatures are left-handed, which is the bit that interests astrophysics: why did Earth end up with those molecules and not others?


I'd say it interests biologists.
The wrong handed sugars (only exist synthetically) have no sweetness as the molecules are the wrong shape. Sugar solution affects polarised light, the other handedness affects it in the opposite direction (See Kerr cells, an early technology for projection TV in 1950s and modulating lasers in late 1960s / early 1970s, which have been made with sugar)

So starlight passing through a cloud can reveal chirality of organic molecules. The right-handed form has been found for  the first time.
Astroboffins' discovery gives search for early life a left hand. Or right
There are at least two mistakes in the article.
1) It's not a molecule used in biological processes, but it is an organic chemistry molecule
2) Different chirality (Left handed or right handed version) almost always affects polarised light differently.


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## Ray McCarthy (Jun 15, 2016)

Ray McCarthy said:


> There are at least two mistakes in the article.


It seems they might be fixing the not "organic molecule" blunder.

If there is a "first" life originating "thing" on a planet, that might be a 50:50 (equal probability, but we don't know) chance. Afterwards everything will have the same chirality. 
Perhaps spectroscopic analysis of starlight passing through a planet's atmosphere will reveal if all life is "left-handed" or if it's half and half. If it turns out that "left-handed" is more common (as on bits of stuff fallen to Earth), or the only kind of life, then we need new theories about organic chemistry to explain it.
It's odd that this is (as far as I can tell from a search) the first detection of right-handed molecules at distant stars. Anyone find another reference?


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## hopewrites (Jun 15, 2016)

Wait wait, I'm confusing myself here.
If left is first, then right is wrong. So right handed sugars are not sweet and mostly synthetic in origin? Or wrong handed as currently viewed by our right hand obsessed society meaning left handed sugars are?

Sorry to fixate on a minor detail in a mindboggeling fascinating branch of scientific research and discovery.


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## Ray McCarthy (Jun 15, 2016)

Viable Synthetic sweeteners, are Left-handed like natural sugars, or they wouldn't be any use! 
Right-Handed sugars don't exist in nature and as far as I know have been synthesised only to prove they have no sweetness (as they are wrong shape for your taste buds). 

People long ago used to be mostly lefthanded, but really the Left  & right "hands" of organic chemistry is nothing to do with actual hands, it's just a convention to describe the 3D shape. The "Right handed" versions are a mirror image.


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## hopewrites (Jun 15, 2016)

Oh! Thank you.
So handedness of shapes is based on which side they are more heavily weighted or attenuated or something? 
And although mirror images, the other handed versions are similar but different. The same way in Tetris there are the  L shaped and the mirror L's that no matter how you spin them don't fit exactly the same as the L shaped ones. Also those straightened out Z's and S's...

Thanks for helping me understand better


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## Dave (Jun 15, 2016)

Yes, Hope, this has nothing to do with actual real "hands" but exactly what you describe.



Ray McCarthy said:


> If there is a "first" life originating "thing" on a planet, that might be a 50:50 (equal probability, but we don't know) chance. Afterwards everything will have the same chirality.


This 50:50 chance is what I believe - but you could be correct - maybe polarised light or some catalyst prefers one over the other.

So, if it were 50:50 and every organic molecule in the Universe was only left-handed, would that not be evidence of "Intelligent Design" and therefore, the fact that it isn't is evidence of the contrary?

However, a Solar System could be seeded with one or the other, to make all the planets the same.

There is also several science fiction stories here, surely. Aliens arrive but they are made of right-handed molecules and cannot survive. Aliens arrive and attempt to change our chemistry from left to right to make it suitable for colonisation. Aliens attempt to do this subversively, before they arrive for colonisation.


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## Ray McCarthy (Jun 15, 2016)

Dave said:


> Aliens arrive but they are made of right-handed molecules and cannot survive.


They'd have to have their own ecosystem.
They couldn't adapt ours. Only replacement would work.
Scorched Earth. So unless they can organise a solar Flare, then they'd keep looking for easier pickings.

But even without that, "left Chirality aliens" would get malnutrition. Amino acids and vitamin needs vary both in what produces them and which ones a species needs.
Different species here have needs for different vitamins. Basic "left Chirality"  fat, carbohydrate, sugars, starch, protein are probably nutritious for any "left Chirality" alien or for us on any alien world. We'd probably not be able to digest the "right Chirality" versions if they exist.

We only know of simple "right Chirality" organics not arising from biological processes.



Dave said:


> every organic molecule in the Universe was only left-handed, would that not be evidence of "Intelligent Design"


Not really (that would seem too obvious, I suspect that if God is real the universe is arranged so you can prove it!). Likely there would be some reason than only non-biologic organics occurring by some non-biologic process.  Playing Devil's advocate? 



Dave said:


> However, a Solar System could be seeded with one or the other, to make all the planets the same.


I don't subscribe to the "seeding" from comets / asteroids idea, only makes origin "harder". I think if life can "arise" because the conditions suit it, then a planet is where it starts, each one separately.  We only have a sample of one to examine right now.


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## Mirannan (Jun 15, 2016)

Just a nitpick; not all life molecules are chiral. Most triglyceride fats aren't, for a start; also glycine. For chirality, you need at least one carbon atom with four different things hanging off it. There are some rare synthetic exceptions, in which large chunks of the molecule have chiral asymmetry; but for life molecules...

Also, some odd biomolecules use opposite-chirality amino acids in their structure - some antibiotics for example.

As for sugars, I'm not sure you're right. It would appear that sweetness is conferred by multiple hydroxyl groups; common examples are glycerol and ethylene glycol (to use the common names, not IUPAC  ) That being so, I suspect that opposite-chiral sugars would have some sweetness.

Later add: Yup, thought so. L-glucose is indistinguishable in taste from the normal D form. Wikipedia: L-Glucose - Wikipedia, the free encyclopedia

Incidentally, D-amino acids and L-glucose can be metabolised by some bacteria. Bacteria are good at that sort of thing.


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## Ray McCarthy (Jun 16, 2016)

Mirannan said:


> not all life molecules are chiral. Most triglyceride fats aren't, for a start


That's handy for aliens of the wrong chirality to know.

I should treat my memories of organic chemistry with more suspicion and stick to engineering (Electronics, Communications, Computers).

In any case I've none of this "chirality" stuff in my SF, just recently researched stuff about nutrition and need for supplements.


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