From: jdh64@cas.org (Dave Huddle)
Subject: Re: Left-handed sugar
Reply-To: jdh64@cas.org
Organization: What, Me Organized?
Date: Thu, 31 Dec 1992 14:52:40 GMT


OK, It's time for me to jump in here on this discussion of Carbohydrate
stereo/nomenclature!


In article <dexter.725655833@aries>, dexter@aries.scs.uiuc.edu (The Gecko)
writes:
|> winalski@adserv.enet.dec.com (Paul S. Winalski) writes:
|> 
|> >|>By using
|> >|>the term "isomer" loosely, without defining whether you mean an anomer
|> (alpha
|> >|>or beta diastereomer, eg. alpha-D-glucopyranose vs.
|> beta-D-glucopyranose),
|> an
|> >|>epimer (eg. D-glucose vs. D-mannose, or separately, D-glucose vs.
|> L-glucose) 
|> >|>or an enantiomer, you confuse the argument.  
|> 
|> >BTW, I thought "epimer" referred to stereoisomers that differ around the
|> >configuration of one carbon only, in which case D-glucose and L-glucose,
|> >which are mirror images and thus differ at all 4 asymmetric carbons, cannot
|> >be epimers.

True,  epimers differ at ONE chiral center.   D-glucose and L-glucose are
ENANTIOMERS.

|> 
|> 	I'm pretty sure that the enantiomer of D-glucose is L-idose, ie. that
|> the named designation of a monosaccharide is absolute, rather than relative 
|> to the D/L carbon.  This would make D- and L-glucose epimers at C-5, all the
|> other stereocentres being fixed.


NO!  D-glucose and L-idose are NOT ENANTIOMERS,  they are EPIMERS at C-5.  The
terms "allo, altro, galacto, gluco, gulo, ido, manno, and talo"  designate the
RELATIVE stereo of aldohexoses having 4 chiral centers.   The D/L designations
denote the configuration of the highest numbered chiral center, i.e. C-5 for
hexoses.   In the D series, the 5-OH is on the RIGHT in a FISCHER projection. 
The 5-OH is on the left in a FISCHER projection for the L series.  The
following
figures are shorthand for the 16 isomeric aldohexoses.

  o    o      o    o      o    o      o    o
  |-  -|     -|    |-     |-  -|      |-  -|
  |-  -|      |-  -|     -|    |-     |-  -|
  |-  -|      |-  -|      |-  -|     -|    |-
  |-  -|      |-  -|      |-  -|      |-  -|
  |    |      |    |      |    |      |    |

  D    L      D    L      D    L      D    L

  allose     altrose     glucose      gulose


  o    o      o    o      o    o      o    o
 -|    |-    -|    |-     |-  -|     -|    |-
 -|    |-     |-  -|     -|    |-    -|    |-
  |-  -|     -|    |-    -|    |-    -|    |-
  |-  -|      |-  -|      |-  -|      |-  -|
  |    |      |    |      |    |      |    |

  D    L      D    L      D    L      D    L

 mannose      idose     galactose     talose

Thus D-allose is an epimer of D-altrose, D-glucose, D-gulose and L-talose;
and the enantiomer of L-allose.

In the pyranose forms, the ALPHA and BETA designations denote the relative
stereo between the C-5 chiral center and the anomeric center.  So 
alpha-D-allopyranose and alpha-L-allopyranose are enantiomers, but
alpha-D-allopyranose and beta-D-allopyranose are epimers.

	CH2OH                                             CH2OH
	 |                                                |
	 C---O                    C---O                   C---O
	/     \               HO /|    \ OH              /     \ OH
       /       \               |/ CH2OH \|              /       \|
      C         C              C         C             C         C
      |\       /|               \ OH HO /              |\       /
     HO \     / OH               \|   |/              HO \     /
	 C---C                    C---C                   C---C
	 |   |                                            |   |
	 OH  OH                                           OH  OH

 alpha-D-allopyranose      alpha-L-allopyranose      beta-D-allopyranose

|> 
|> 
|> ie.			H-C=O			H-C=O
|> 			  |			  |
|> 			H-C-OH			H-C-OH
|> 			  |			  |
|> 		       HO-C-H		       HO-C-H
|> 			  |			  |
|> 			H-C-OH			H-C-OH
|> 			  |			  |
|> 			H-C-OH		       HO-C-H
|> 			  |			  |
|> 			H-CH-OH			H-CH-OH
|> 
|> 			D-glucose		L-glucose


			        	This is L-idose  (NOT L-glucose)


	-Dave Huddle	jdh64@cas.org
 



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