Why are all the OH opposite to each other in D and L glucose?

sodium.dioxid

I thought D and L referred to the direction of the OH on the LAST chirality center. So why are all the other OH also switched up?

http://upload.wikimedia.org/wikipedi...lucose.svg.png

Ygggdrasil

Because D- and L-glucose are enantiomers (i.e. mirror images), their configuration is switched at all stereocenters of the molecule.

sodium.dioxid

This makes sense. But then why do we care about the last chiral carbon?

Ygggdrasil

We don't. Like many thing in biology, the choice on how to name the different stereoisomers of glucose is somewhat arbitrary. It just turns out that, if you draw all of the naturally occuring aldohexoses in a Fisher projection, almost all of them have the same configuration about C5, so biochemists chose that position to use as the basis for the D/L nomenclature system.

sodium.dioxid

You are too kind, sir.