All right, so I know that
(a) cyclodextrins require the analyte to have an aromatic function (except steroids/terpene alcohols);
(b) Protein phases such as AGP require a ring close to the chiral center, at least one H-bonding site and the distance between the two to be no greater than 3 atoms;
(c) Pirkle phases require analytes with a pi-donor aromatic group or a pi-acceptor group (like dinitrobenzoyl)
(d) Polysaccharides don't necessarily require one specific function.
What about (e) Macrocyclic Antibiotics?
Does the pi-acceptor group for Pirkle phases need to be aromatic as well?
Does the ring in protein phases have to be a certain size?
And am I correct with the polysaccharides assumption?
I tried finding more details on the wbe and in a couple of recent CSPs books but all was left rather unclear...
I've suggested defining your requirements and talking to the column manufactuers, and I strongly reiterate that suggestion, as they can offer suggestions of specific columns..
a. Cyclodextrins can operate in three modes, normal phase, reverse phase and polar organic mode, and can also have pi-bases like naphthylethyl added, which creates a much wider range of possible interactions, so molecules without aromatic groups can be separated.
Stable, high capacity, poor efficiency.
b. Proteins. From Chiral Separations by Chromatography " ...the nature of the chiral recognition processes employed is still not very clear .." There are plenty of exceptions to your rule, as different proteins ( eg BSA, HSA, alpha-1 acid glycoprotein, ovalbumin ) have been used.
Poor mechanical stability, high efficiency, low capacity, low flowrates.
c. These are the only columns where your rule is probably appropriate Regis Technologies Catalog " The Pirkle-Concept Chiral Stationary Phases generally fall into three classes: π-electron acceptor/π-electron donors, the π-electron acceptors and the π-electron donors. With Pirkle Phases, chiral recognition occurs at binding sites.
Major binding sites are classified as π-basic or π-acidic aromatic rings, acidic sites, basic sites, and steric interaction sites. Aromatic rings are potential sites for π-π interactions. Acidic sites supply hydrogens for potential intermolecular hydrogen bonds-the hydrogen is often an amido proton (N-H) from an amide, carbamate, urea, or amine. Basic sites, such as π-electrons, sulfinyl or phosphinyl oxygens, and hydroxy or ether oxygens, may also be involved in hydrogen bond formation. Steric interactions may also occur between large groups. "
Stable, good solvent resistance, high efficiency, high capacity, limited applications, predictable behaviour.
d. Polysaccharides. There are plenty of derivatives available, so many column choices results in wide range of applications. separations are difficult to predict, but can be narrowed down by general guides. eg OT and OP for molecules with aromatic group; WH, WM, WE for DL amino acid or derivatives; OA, OB, OC, OD, OK for compounds withs aromatic group, carbonyl group, hydroxyl group, nitro group, sulfinyl group, cyano group etc.
Reasonable efficiency, good capacity, poor stability, poor solvent resistance, sensitive to high pressure and flows.
e. Macrocylic glycopeptides ( eg vancomycin from Astec ). The separation mechanisms combine the interactions used in protein and cellulose phases, eg very strong anionic or cationic bonding, strong pi-pi interaction, strong hydrogen bonding, medium strong dipole stacking,
weak steric interaction, weak inclusion. Usinh some imagination, they could be considered as an extension of the inclusion ( cyclodextrin ). columns, with the same advantages and limitations.
Wide range of modes ( normal phase, reverse phase, polar organic - but not as good as cyclodextrins, polar ionic ), pH can adjust selectivity -best that analytes are not ionised. Newer versions have good capacity.
All of the above are general characterisations, which will be incorrect for many applications and columns.
Most of your questions should be answered by reading Satinder Ahuja's 1999 book "Chiral Separations by Chromatography " which is specifically written to help people select chiral separation systems. Then talk to the manufacturers.....
Bruce Hamilton
