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From: werner@aecom.UUCP
Newsgroups: sci.bio
Subject: Re: Human asymmetry
Message-ID: <1025@aecom.UUCP>
Date: Sat, 11-Apr-87 15:57:11 EST
Article-I.D.: aecom.1025
Posted: Sat Apr 11 15:57:11 1987
Date-Received: Sun, 12-Apr-87 17:44:51 EST
References: <1003@aecom.UUCP> <586@batcomputer.tn.cornell.edu> <3432@udenva.UUCP>
Distribution: sci
Organization: Albert Einstein Coll. of Med., NY
Lines: 36
Keywords: chirality
Summary: Prochiral versus chiral

In article <3432@udenva.UUCP>, agranok@udenva.UUCP (Alexander Granok) writes:
> In article <1013@aecom.UUCP> werner@aecom.UUCP (Craig Werner) writes:
> >
> >                                 H
> >	A prochiral center such as XCH has 2 Hydrogens that are identical.
> >				    Y
> >	With a three dimensional model, it is easy to point to one in
> >the pro-R and another in the pro-S... 
> 
> Can you make assignments such as R and S in this case?  It all depends upon


	I see that non-biochemists are troubled by the concept of
prochirality.  By definition, a prochiral molecule is a symmetric
(i.e. achiral) molecule that would be made chiral if one of the identical
atoms (Hydrogen in the above example) were replaced by a third atom,
Z.  
	It is true that the identity of the pro-R and pro-S hydrogen
is absolutely ambiguous unless one knows the priority of the Z group in
relation to X and Y, but for any given reaction where the products and
reactants are known, it will be unambiguous.
	Where it is simple hydrogenation/dehydrogenation, the assignment
is made by replacing one of the Hydrogens with Deuterium (first done
experimentally by Frank Westheimer), usually as a Gedanken experiment.

	Finally, when I said that chemists cannot avoid a mixture of
enantiomers during a chemical synthesis, while enzymes can be stereochemically
specific, I did not preclude the possibility of chemists using and/or
designing enzymes -- but I thought that went without saying.  Obviously,
to the nitpickers on the net, nothing goes without saying.... 

-- 
			      Craig Werner (MD/PhD '91)
				!philabs!aecom!werner
              (1935-14E Eastchester Rd., Bronx NY 10461, 212-931-2517)
   "If you've heard this story before, don't stop me. I want to hear it again."