Thread: Left-handedness of the molecules of life - chirality

Please correct me where I'm wrong :wink:

I heard that all molecules of living organisms (or all molecules of a certain type?) are left-handed on earth, and that this probably came about by coincidence (it could've been right-handed as well). What I wonder is: would life have been different if by chance the molecules of living organisms had been right-handed rather than left-handed? Or is that like asking "would life have been different if organisms evolved to breath helium rather than oxygen?", with the obvious answer "there'd be no life" ? :wink:

yeh i think you're on the right lines, the optical isomers of a compound can have/do have different properties from eachother and it would make a big difference to life.

im no biologist but isn't there something to do with optical isomerism in photosynthesis?

Originally Posted by organic god

yeh i think you're on the right lines, the optical isomers of a compound can have/do have different properties from eachother and it would make a big difference to life.

im no biologist but isn't there something to do with optical isomerism in photosynthesis?

Usually for simple left/right chirality there is no difference in the chemical properties of the molecules, unless they are interacting with other chiral molecules. So you could easily switch everything to run on R isomers instead of S isomers, and so far as I know everything would work the same.

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Scifor is correct. Our terrestrial organisms evolved with l-amino acids and d-glucose, but an early coin flip would have easily sent everything the other way. There would be no difference in the final product. We'd all arrive at this point looking and behaving exactly as we do now.

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Thinking with my stomach: would these organisms be tasty, or like old rubber bands and pennies, to us?

Originally Posted by SteveF

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Scifor is correct. Our terrestrial organisms evolved with l-amino acids and d-glucose, but an early coin flip would have easily sent everything the other way. There would be no difference in the final product. We'd all arrive at this point looking and behaving exactly as we do now.

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On the other hand, surely some proteins and enzymes etc are specific enough in their activity to require specific chirality?

All I know about this I got from Chirs McManus' Left Hand Right Hand, always a souce of fascination for sinistrals like me, and with a lovely tale of tell of asymmetry coming up through the universe all the way from the weak nuclear force...

scifor is partially right
u mentioned d-glucose as an example
glucose is taken by the cell and oxidized to produce energy , So there will be no difference d or l
but compound wich interact with enzymes or receptors must have specific configuration e.g : Insulin , dopamine.

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On the other hand, surely some proteins and enzymes etc are specific enough in their activity to require specific chirality?

Also correct, Lefty. But the entire biological system would be mirror-imaged -- enzymes, substrate, and all. Same results, and neither us nor them would know until we checked some solutions for optical activity... or figured out why their Big Macs are indigestible to us and vice versa.

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Originally Posted by SteveF

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Also correct, Lefty.
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Heh.

Ta.

Originally Posted by raed

scifor is partially right
u mentioned d-glucose as an example
glucose is taken by the cell and oxidized to produce energy , So there will be no difference d or l
but compound wich interact with enzymes or receptors must have specific configuration e.g : Insulin , dopamine.

That's why I said that there is no difference unless they are interacting with other chiral molecules.

their Big Macs are indigestible to us and vice versa.

It's war then. We meet E.T. first thing I'll ask is "left or right?" Can we shake hands? Which side are you on?

Thanks for the info! I can't really contribute to the discussion, but I'm watching and learning :wink:

There is a hypothesis that the handedness is a result of some side effect of the weak nuclear force. It seems not to be a popular explanation, but it has not yet been refuted. I have only the dimmest recollection of this and no references.

Others have tried to explain the handedness that actually exists as preferential formation on crystals or clay particles. There have been other, exotic explanations as well. I am never comfortable with explanations that run 'that's just the way it is'. They may be correct, but if we accept that position blindly then we may miss something significant.

Originally Posted by Ophiolite

There is a hypothesis that the handedness is a result of some side effect of the weak nuclear force. It seems not to be a popular explanation, but it has not yet been refuted. I have only the dimmest recollection of this and no references.

Others have tried to explain the handedness that actually exists as preferential formation on crystals or clay particles. There have been other, exotic explanations as well. I am never comfortable with explanations that run 'that's just the way it is'. They may be correct, but if we accept that position blindly then we may miss something significant.

I Cant actually understand ur words , Handedness or chirality is simply due to probability of positions of bonds , and that what make compounds chiral and others are achiral

In case of symmetric compounds , if u draw the probabilities of positions of bonds we will obtain the same molecule configuration

But in chiral molecules , if u do so will have 2 mirror image like ur 2 hands.

I should have written that the preference in biological molecules for a particular handedness may not be a matter of chance, but could have been determined by some property of the environment (surface chemistry for example) or of the constants/forces of nature (for example the role of the weak force).

Yea
I get it now

Originally Posted by Ophiolite

There is a hypothesis that the handedness is a result of some side effect of the weak nuclear force. It seems not to be a popular explanation, but it has not yet been refuted. I have only the dimmest recollection of this and no references.

Others have tried to explain the handedness that actually exists as preferential formation on crystals or clay particles. There have been other, exotic explanations as well. I am never comfortable with explanations that run 'that's just the way it is'. They may be correct, but if we accept that position blindly then we may miss something significant.

I like your instincts. I believe it is those baldfacedly obvious things we don't question, that hold the keys.

Originally Posted by Ophiolite

I should have written that the preference in biological molecules for a particular handedness may not be a matter of chance, but could have been determined by some property of the environment (surface chemistry for example) or of the constants/forces of nature (for example the role of the weak force).

It's precisely what Chris McManus explores in the book to which I linked.

Originally Posted by raed

glucose is taken by the cell and oxidized to produce energy , So there will be no difference d or l

Not true I believe, glucose is oxidised by glucose oxidase, which specifically binds to D-glucose... L-glucose won't bind in the enzyme and so won't be oxidised.

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