[Biocheistry_97]

I am a little confused about the concept of L and D configurations to distinguish between optical isomers after seeing these diagrams. The image below shows the L and D configurations of Threose and Erythrose as Fischer Projections.





Please could somebody explain why, in a Fischer projection, D-erythrose has both -OH groups on the right-hand side and E-erythrose has both -OH groups on the left-hand side whereas D-threose has only one of the -OH groups on the right-hand side, despite both carbons which are bonded to -OH groups being chiral.

Thanks

[Dan]

Your questions is a little unclear, do you mean:

For threose, why is "left, right" D and and "right, left" L (rather than the other way around)?

If this is the question, the answer is that the D/L descriptor is assigned on the basis of the configuration of the last stereocentre (the stereocentre closest to the bottom of the Fisher projection). In the case of erythrose/threose, this is C3. For a pentose, you would assign D/L based on C4, for a hexose C5 and so on.

If the last stereocentre points right in the Fisher projection, it's a D-sugar, if it points left, it's an L-sugar. It doesn't matter what the other OHs are doing, only the last stereocentre is used to assign D/L. This is simply the definition of D/L, it is an arbitrary (but consistent) convention.

[Biocheistry_97]

Your questions is a little unclear, do you mean:

For threose, why is "left, right" D and and "right, left" L (rather than the other way around)?

If this is the question, the answer is that the D/L descriptor is assigned on the basis of the configuration of the last stereocentre (the stereocentre closest to the bottom of the Fisher projection). In the case of erythrose/threose, this is C3. For a pentose, you would assign D/L based on C4, for a hexose C5 and so on.

If the last stereocentre points right in the Fisher projection, it's a D-sugar, if it points left, it's an L-sugar. It doesn't matter what the other OHs are doing, only the last stereocentre is used to assign D/L. This is simply the definition of D/L, it is an arbitrary (but consistent) convention.

Thank you for your reply Dan. The position of the -OH facing to the left on both C2 and C3, like in the Fischer projection, for L-Erythrose (and both to the right for D-Erythrose) whereas C2 OH is on the opposite side of C3 OH for the other.

[Dan]

I meant that the position of the -OH facing to the left on both C2 and C3 like in the Fischer projection for L-Erythrose (and both to the right for D-Erythrose) whereas C2 OH is on the opposite side of C3 OH for the other.

I don't understand the question. Both pointing in the same direction in the Fischer projection is called erythro and pointing opposite directions is called threo - that is just the definition of the erythro/threo stereodescriptors.

[Yggdrasil]

Remember that L-erythrose and D-erythrose are enantiomers, meaning that they are mirror images.  Thus, the stereochemistry at every single stereocenter will be flipped.  If you flip the configuration only at one sterocenter, you get a different molecule, not an enantiomer.

[Biocheistry_97]

Remember that L-erythrose and D-erythrose are enantiomers, meaning that they are mirror images.  Thus, the stereochemistry at every single stereocenter will be flipped.  If you flip the configuration only at one sterocenter, you get a different molecule, not an enantiomer.

I understand now. Thank you kindly.