Chirality??!

[Karnage]

:confused:

 

Hi guys I dunno if yall remember me but I took a good long break from SFN but I'm back cuz I love science (lol) Anyway I also need your help. Um it would be helpful if someone explains how chiral center is a type of stereocenter. I know a stereocenter is an atom that when you interchange two groups it produces a stereoisomer but how does a chiral center fit that description?? My text does a bad job of explaining

[RyanJ]

I assume that you know a chiral molecule is a molecule that is asymmetrical with its mirror image. basically put the two opposing isomers aren't superimposable.

 

Lets take an example of bromochlorofluoromethane (CHBrClF)

 

Stereoisomers are isometric molecules who's atomic connectivity are the same but their groups are arranged differently in space.

 

So in the above example we have two enantiomers, the positive and negative rotational isomers (don't worry about which is which). As you can see the above description fits perfectly. As the molecules have a chiral center there can be more than one form of the molecule, each being asymmetric to the other. now is you look at the description for a stereoisomer above you'll see that this fits perfectly - we have two groups with identical atomic bonding internally but their spacial configurations are different.

 

I hope that description works for you and if not I suggest you give Wikipedia a try

[Karnage]

ahhhh good now i see thnx a lot bud! \m/

[pioneer]

Different chiral centers will rotate the plane of polarize light in different directions. This is what characterizes left and right handed molecules. Light is like a sine wave (wavelength) giving off an alternating electric field. The two chiral centers have different atomic arrangements in 3-D space, which interact with the same electric field, but will create opposite affects within the field.

 

What is interesting, it doesn't matter whether these molecules are right side up, up-side down or on their side, they still cause the same affect. The only logical way to explain this, it has to with apparent negative and positive charges. One acts like it displays a negative field deflection and the other a positive field deflection. This simple explanation would allow us to flip the molecule anyway and still get the same deflection, since these charge fields will not change with spatial orientation.

 

In other words, the original energy is being generated by electrons to make the light. This is then polarized so all the waves light up in one directions. One chiral center is pushing the field one way. The other is pushing the field the other way. But both chiral centers have the same atoms and don't show any apparent differences in charge. This creates a paradox with respect to the observation. How do you get what appears to be postive deflection without any extra positive charge and a negative deflection without any extra negative charge?

 

One way to explain this paradox is using the EM force. The EM force is not just electro-static but also magnetic. It is one force with two components. That would suggest that the two chiral centers have different magnetic fields that are having an significant impact on final EM force fields. The result is that the EM field of one looks positive and other looks negative.

 

In the living state, nature tends to use one of the two chiral centers, exclusively. Based on the observation of the hydrogen bonding that occurs throughout the materials of the cell, this would suggest nature uses the one that will complement the apparent postive potential within H-protons. This may be an example of how signicant H-potential is. It will requires all the biomaterials become orientated, into one of two stereo-isomers.

[John Cuthber]

"Different chiral centers will rotate the plane of polarize light in different directions."

Correct

"This is what characterizes left and right handed molecules"

Not really, they are different and that's what characterises them as different.

They sometimes have different crystal structures- in fact that's how Pasteur did the first experment to separate a pair of chiral molecules.

 

If this "What is interesting, it doesn't matter whether these molecules are right side up, up-side down or on their side, they still cause the same affect." were true then the effect on polarised light of a crystal would be independent of the crystal's orientation. It isn't.

The reason that it deosn't seem to matter for a bulk sample is that there are so many molecules at all sorts of angles and the effect you see is the average rotation.

 

I think it's probably fair to say that the explanation or optical activity given here

http://en.wikipedia.org/wiki/Optical_activity

is more widely accepted than the rest of Pioneer's post.

[pioneer]

Thanks John. I have a tendency to theorize in situ instead of checking the literature to see what is has been done. I like reinventing the wheel for practice. Your reference gave me another reference that says what I was trying to say. It discusses the refraction of light as passes into media such as chiral centers. Optical rotation uses two indexes at the same time.

 

From http://en.wikipedia.org/wiki/Refractive_index

 

At the microscale, an electromagnetic wave's phase velocity is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the permittivity of the medium. The charges will, in general, oscillate slightly out of phase with respect to the driving electric field. The charges thus radiate their own electromagnetic wave that is at the same frequency but with a phase delay. The macroscopic sum of all such contributions in the material is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase velocity. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity.

 

I am thinking out loud. The polarize light has two circular polarizations that cancel, to get one one polarized plane. Each optical isomer essentially refracts or adds more phase delay, to one of the two circular polarizations. The refraction itself is due to occilation slightly out of phase with the driving electric field. If we put the two isomers side by side, the electrons causing the refraction are 180 degrees out of phase, so that each will occilate with only one aspect of the circular polarization. If one was positive and the other negative they would be in phase and would still cause the same type of affect. But they are not positive and negative. Both could be negative and out of phase if the magnetic field directions were opposite.. Opposite spin electrons have oppisite magnetic fields, but that is way too selective. Maybe they are mirror images even with respecr to the magnetic field directions around the chiral center. The positive-negative analogy, being in phase, does not preclude the cell selectively using the stereo isomer that is in phase with the occilation of the positive charge of the H-bonding hydrogen, since when the +charge of the H is most exposed, the negative charge of the electron, at the other side of the dipole, are also most exposed.