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Author Topic: Why do optical isomers have different biological effects?  (Read 4145 times)

Dr. Cox

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Dr. Cox  asked the Naked Scientists:

How do enantiomers perform differently in biological systems even though they have the same physical and chemical properties. My guess is that they have to be the correct way round to work?

What do you think?


 

Offline RD

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Why do optical isomers have different biological effects?
« Reply #1 on: 19/10/2008 13:32:26 »
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The origin of this homochirality in biology is the subject of much debate. Most scientists believe that Earth life's choice of chirality was purely random, and that if carbon-based life forms exist elsewhere in the universe, their chemistry could theoretically have opposite chirality.
http://en.wikipedia.org/wiki/Optical_isomerism

The origin of homochirality may be "out of this world" ...

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Sunday, May 04, 2008
The amino acid chirality mystery

Meteorites Delivered The 'Seeds' Of Earth's Left-hand Life, Experts Argue

In a report at the 235th national meeting of the American Chemical Society, Ronald Breslow, Ph.D., University Professor, Columbia University, and former ACS President, described how our amino acid signature came from outer space.

Chains of amino acids make up the protein found in people, plants, and all other forms of life on Earth. There are two orientations of amino acids, left and right, which mirror each other in the same way your hands do. This is known as "chirality." In order for life to arise, proteins must contain only one chiral form of amino acids, left or right, Breslow noted.

"If you mix up chirality, a protein's properties change enormously. Life couldn't operate with just random mixtures of stuff," he said...

So the mystery is reduced to that of why at some point in time there were many more L amino acids than the R form. It has been shown that amino acids can form spontaneously from inorganic materias under some conditions (the Miller-Urey experiments demonstrated this.) However, one would expect equal amounts of R and L amino acids under such circumstances.

But there's another way out, because we know that in fact amino acids can form in interstellar space, since they were found in parts of the Murchison meteorite (and later others) that were uncontaminated with Earthly material. Furthermore, there's one definite way that amino acids which existed originally in an equal mixture of L and R forms on a chunk of rock hurtling through space could have their proportion tilted in one direction or the other:

These amino acids "seeds" formed in interstellar space, possibly on asteroids as they careened through space. At the outset, they have equal amounts of left and right-handed amino acids. But as these rocks soar past neutron stars, their light rays trigger the selective destruction of one form of amino acid. The stars emit circularly polarized light--in one direction, its rays are polarized to the right. 180 degrees in the other direction, the star emits left-polarized light.

All earthbound meteors catch an excess of one of the two polarized rays. Breslow said that previous experiments confirmed that circularly polarized light selectively destroys one chiral form of amino acids over the other. The end result is a five to ten percent excess of one form, in this case, L-amino acids. Evidence of this left-handed excess was found on the surfaces of these meteorites, which have crashed into Earth even within the last hundred years, landing in Australia and Tennessee.

So, one asks, is it possible that this imbalance of R and L amino acids was transferred from a meteorite to prebiotic Earth? In a series of experiments Breslow confirmed that this could happen...
http://scienceandreason.blogspot.com/2008/05/amino-acid-chirality-mystery.html
« Last Edit: 19/10/2008 13:44:04 by RD »
 

Offline Bored chemist

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Why do optical isomers have different biological effects?
« Reply #2 on: 19/10/2008 14:58:46 »
A right hand glove has to be the correct shape to fit a right hand. In much the same way ( though on a smaller scale) molecules have to be the correct shape to fit with proteins, enzymes etc.
 

Offline Chemistry4me

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Why do optical isomers have different biological effects?
« Reply #3 on: 20/11/2008 06:22:51 »
Different stereoisomers rotates/bends plane polarised light in different directions
 

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Why do optical isomers have different biological effects?
« Reply #3 on: 20/11/2008 06:22:51 »

 

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