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Author Topic: Amino acids - left handed and right handed  (Read 22026 times)

Offline Quantumcat

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Re: Amino acids - left handed and right handed
« Reply #25 on: 16/01/2004 07:21:59 »
oops, I didn't read the post and I didn't know you were talking about amino acids. I feel so stupid.

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Offline Ylide

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Re: Amino acids - left handed and right handed
« Reply #26 on: 16/01/2004 10:01:45 »
C-guy, welcome to the forums

Left or right handedness has to do the with chirality of the central carbon of the amino acid.  (the one bonded to the nitrogen and carboxyl group)  If you've never taken organic chemistry before, carbon forms a tetrahedron with its 4 bonds, and the order in which the atoms lie in this tetrahedron are it's chirality, or "handedness."  A chiral molecule is not superimposable on its own mirror image.  Putting the least dominant point of the tetrahedron in back (usually hydrogen) then counting the other 3 atoms in order of most dominant to least dominant, you'll either go clockwise or counter clockwise around the central carbon.  

I think with biomolecules, clockwise is D (or right-handed) and counterclockwise is L (or lefthanded)  Don't confuse that with Dextrorotaty and Levorotary which have to do with the optical activity of the molecule, something chiral molecules do but arent necessarily a function of the direction in which the atoms are arranged.  

Most non-biological organic molecules are designated as either S or R (from the latin rectus and sinistare, i.e. right and left)

ignore the periods in the following crude ASCII illustrations, the board wont let me put more than one space character in a row =/

......H
H2N C CH3
......COOH

......H
H2N C COOH
......CH3

Given the dominance of NH2, then COOH, then CH3 (i wont go into the rules of that) you can see that, ignoring the hydrogen, the first molecule has a counterclockwise rotation and the second has a clockwise rotation.  Since there are 2 distinct stereoisomers, the molecule is chiral.  If they were mirror images of each other, they would be called enantiomers.  They are NOT mirror images of each other, so they are called diastereomers.  If you could rotate either molecule in any way without breaking any bonds and make the atoms arrange the same way as in the other, they would be the same molecule.  (Note how in my example, you cannot do this)

Thus ends my lecture on sterochemistry.  I'll send you a cool link if you want to know more.





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Offline C-guy

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Re: Amino acids - left handed and right handed
« Reply #27 on: 17/01/2004 04:00:53 »
Is dominance determined my the urge to reduce/oxidize?
 

Offline C-guy

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Re: Amino acids - left handed and right handed
« Reply #28 on: 17/01/2004 08:48:23 »
Sorry, the question should be:

Is dominance determined by the urge to reduce/oxidize?
 
 

Offline Ylide

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Re: Amino acids - left handed and right handed
« Reply #29 on: 17/01/2004 10:30:26 »
No, it's simpler than that.  It's based on atomic number of directly attached atoms, and in the case of identical directly attached ones it falls back on secondarily attached.  Higher atomic number = more dominant.  

For instance

CH3 and COOH, COOH is more dominant than CH3 because even though both are atomic #12, COOH has oxygen attached which is #16 as opposed to hydrogen on CH3 which is #1.  NH2 beats both because N is atomic #14, higher than both carbons.  Therefore, the order of revolution about the chiral carbon is NH2, COOH, CH3.  You always place the least dominant of the 4 substituents into the plane of rotation so it is not counted.

You'll never run into a situation of equal dominance because for that to happen, 2 substituents of the chiral carbon would have to be identical, which means the molecule is not chiral so determining R/S or D/L designation is not appropriate.



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Re: Amino acids - left handed and right handed
« Reply #29 on: 17/01/2004 10:30:26 »

 

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