Dear Professor Roe,
Your explanation seemed complicated to me, even more so you first take
as a reference carbon (C5) for one example (GCU) and then carbon (C4)
for the others (XYS and XYP).
A more direct and simple way to determine the type of anomer is the
following: Orient or draw the carbohydrates in the C4/1 form (ie C4 up
and C1 down), then the alpha anomers will have the anomeric oxygen
axial, and the beta anomers will have the equatorial anomeric oxygen
equatorial.
I hope this mnemonic rule would be useful to you.
Cheers,
Hector.
El 2021-10-07 20:09, Daniel Roe escribió:
> Hi All,
>
> I'm hoping someone can shed some light on an issue that's been bugging
> me for a while about determining alpha/beta forms of carbohydrates in
> cyclic form.
>
> So I get that there is the anomeric carbon (carbon bearing
> hemiacetal/acetal, typically C1/C2), the anomeric reference carbon
> (which I think is the highest stereocenter in the ring) and the
> configurational carbon (highest numbered stereocenter). The
> configuration around the configurational carbon determines D/L, and as
> I understand it _the relative orientation of the oxygen on the
> anomeric carbon to the oxygen on the anomeric reference carbon_
> determines alpha/beta (see e.g.
> https://www.masterorganicchemistry.com/2018/02/19/the-big-damn-post-of-sugar-nomenclature/#anomericcarbon).
> Specifically, if they are on the same side that's beta, and opposite
> is alpha.
>
> This seems to hold for most sugars. Take for example
> alpha-D-glucopyranuronic acid (https://www.rcsb.org/ligand/GCU). Here
> the orientation of the O at the anomeric carbon (C1) and the anomeric
> reference carbon (C5) are clearly opposite, hence alpha. However, take
> as another example alpha-D-xylopyranose
> (https://www.rcsb.org/ligand/XYS). Now, assuming I'm correct in that
> the anomeric carbon is C1 and the anomeric reference is C4 (which
> appears to be the highest numbered stereocenter), the oxygens bound
> are clearly pointing in the same direction. And for the beta
> (https://www.rcsb.org/ligand/XYP) they are on opposite sides.
>
> Now I'm sure I'm just not understanding the nomenclature correctly (or
> making some other obvious mistake), but I just can't seem to wrap my
> head around why XYS is alpha and XYP viceversa, when for almost every
> other sugar I've looked at it's been the other way (opposite=alpha,
> same=beta). Can anyone explain this?
>
> Thanks, and sorry for the long post!
>
> -Dan
>
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--
--------------------------------------
Dr. Hector A. Baldoni
Profesor Titular (FQByF-UNSL)
Investigador Adjunto (IMASL-CONICET)
Area de Quimica General e Inorganica
Universidad Nacional de San Luis
Chacabuco 917 (D5700BWS)
San Luis - Argentina
hbaldoni at unsl dot edu dot ar
Tel.:+54-(0)266-4520300 ext. 6157
--------------------------------------
First things first. But not necessarily in such order.
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Received on Thu Oct 07 2021 - 20:00:02 PDT
