[soul76]

Stereochemistry question?

Hi, I am wondering what the difference is between the parameters of purity and optical purity in a peptide aldehyde structure?

ie. If a lab states a minimum of 98% purity but states that it is not optical purity, what does this mean exactly?

I know that Optical rotation (optical activity) is a means of measurement used in chemistry to characterize substances in solutions.  Optical activity is the ability of a chiral molecule to rotate the plane of plane-polairsed light. I understand that it is measured using a polarimeter, which consists of a light source, polarising lens, sample tube and analysing lens.  

If only one enantiomer is present a sample is considered to be optically pure, correct?  When a sample consists of a mixture of enantiomers, the effect of each enantiomer cancels out, molecule for molecule.  

So what does it mean if a lab states a minimum of 98% purity but states that it is not optical purity?  Would it just be because the chemical/peptide aldehyde is non chiral?  How exactly does it affect  the overall purity level of a peptide aldehyde if it is stated to be 98% pure but not optical pure.

Any insight or help is greatly appreciated.

The peptide aldehyde I am referring to is as follows:

Peptide Aldehyde/Chemical Structure (examples)

[discodermolide]

I would suggest that the 98% purity refers to the chemical purity determined by some suitable method, i.e. HPLC.
The optical purity refers to the excess of one enantiomer over the other (or in this case diastereoisomers).

[orgopete]

Given the structure, I could think that it retains a stereochemical configuration at the aldehyde alpha carbon. Yet, I could also think it might more easily racemize. Since the lab is stating 98% purity, but not optical purity, you should ask them what they mean. (Call them up, they'll tell you.) I agree with the opinion of discodermolide, the purity probably means the purity as measured by some test, i.e, HPLC. I am also guessing they didn't measure optical activity.

[fledarmus]

Exactly - the proteosome inhibitor was probably synthesized using standard techniques from commercially available amino acids. The amino acids may have been 98-99% of the desired enantiomer, and the coupling reactions to combine them may have been 98-99% stereospecific, but when you look at a protection reaction and three coupling reactions, plus the various protections and deprotections in between, with four stereocenters you may have small amounts of 15 different diastereomers in addition to the one that is shown on the bottle. The purity is only looking at all the compounds having the same molecular weight and structure; all 16 of the possible products together would account for the 98%.