[speed[ph]]

Hi all,

Not sure if this was the right place to post, but as a fellow graduate student (albeit in the less rigorous, faux-department we call Biochem) I figured I'd give it a shot.

Been working with newly synthesized enantiomers of a compound we got from a collaborator. I'm being sent off to learn a bit about how the chemistry and stuff behind it works-- unfortunately I stink at chemistry. I've been reading up a bit on enatiomer isolation from manufacturer manuals but I was wondering if any of you chemists that are in the field can explain, in simple terms, how chiral column chromotography works. I'm just a bit confused that the stationary phase in the chiral columns they're using (beta-cyclodextrin) can differentiate between enantiomers when physical properties between enantiomers are so similar and the structure of beta-cyclodextrin, at least to me, appears to have no real way of being able to selectively retain one enatiomer or the other. It's extremely likely I'm just not understanding how the chemical interactions and such behind this all works. I'm quite stupid so please keep that in mind-- simple explanations and/or easier to understand reference would be lovely.

Thanks to all you smart chemists ahead of time.

[Honclbrif]

Let's say you've got analyte enantiomers R and S, and the cyclodextrin media which exists only as the single stereoisomer* we'll call D. R and S can each interact with D to create the complexes R-D or S-D. Its these complexes which are actually diasteromers and have different physical properties.

As for how the analyte molecules actually interact with the cyclodextrin: cyclodextrin is a ring of glucose molecules which present a hydrophilic face to the environment, but form a hydrophobic core. Analyte molecules can interact with this hydrophobic core, but since it is lined with chiral residues, the strength of the interaction of one enantiomer will be stronger than the strength of the interaction of the other enantiomer due to the diastereomeric complex formation discussed above.

*it actually has a LOT of stereocenters, but every cyclodextrin should be identical and therefore only one stereoisomer.

[fledarmus]

Chromatography works by providing two environments for analytes to interact with. One environment is stationary and the other is mobile. The difference in the interaction of the analyte with the two environments determines how fast the analyte will move, and if two analytes have differing interactions with the two environments, they will move at different speeds. Since they are put on the column at the same time, they will come off at different times, and thus be separated.

In the case of chiral chromatography, your two environments are a specific chiral environment (in your case, cyclodextrin) and an achiral environment (your solvent system). Being enantiomers, your two components will have identical interactions with the achiral environment. However, if you have chosen an appropriate chiral environment, they will have subtly different interactions with that environment. One enantiomer will interact better with the cyclodextrin and be held on the column longer, while the other will not interact as well and will leave the column sooner. Imagine walking through a hall full of people where everybody is holding out their dominant hand for you to shake with your dominant hand - if you are right-handed, you're going to be shaking a lot of hands and moving slowly through the hall, while if you are left-handed, you'll only find a few hands to shake and will be able to move much faster.

Although you are not using a Pirkle column, you might want to read some of William Pirkle's early work on developing those columns. He did an excellent job of describing the interactions between the chiral stationary phase and the chiral analytes he thought were important in separating mixtures, and showing how that directed his synthesis of chiral stationary phases.