L/D and R/S configurations

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MDtoBe777

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Okay, we have L amino acids and D sugars. L amino acids rotate light in the levorotatory direction (-). Also, its happens that L amino acids are also S sterochem configuration. However, not all S configuration chiral centers are (-). I just want to make sure. So if you know a compound is R you cannot say anything about the way it rotates plane polarized light? Right?
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Right! The absolute configuration of an enantiomer has no bearing on its direction of rotation of plane-polarized light. A common MCAT trap answer will be one that gives you a chiral molecule and then tries to get you to decide which direction it will rotate plane polarized light. The answer is usually something that says 'cannot be determined.'

Good luck.
 
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The R and S configurations are particular to the stereochemistry of one, and only one, chiral carbon. In compounds containing several chiral carbons (such as long chain carbohydrates) each chiral carbon (every vertex in the Fisher projection) will typically have a different stereochemical configuration (ie R or S). As a general rule, the sterochem of the last chiral carbon in a sugar (the last vertex in the Fisher projection) is designated as the stereochem for the entire sugar compound. Hence, all D sugars have terminal chiral carbons with D (R) configuration.

The D and L configurations have nothing to do with which way the compound bends light though, so you are right. That has to be determined experimentally.

Dont fall for tricks though: if it is given that a D-Glucose bends light +11.7 degrees, you can't assume that L-Glucose bends light -11.7 degrees. Only enantiomers bend light in the same magniude but opposite direction, and L-Glucose is *not* an enantiomer of D-Glucose.
 
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So L glucose would be a diastereomer (specifically, an epimer) of D glucose. So this is why you cannot predict the rotation of D glucose knowing the rotation of L glucose?
 
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MDtoBe777 said:
So L glucose would be a diastereomer (specifically, an epimer) of D glucose. So this is why you cannot predict the rotation of D glucose knowing the rotation of L glucose?
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would glucose be considered as an ANOMER here?
Does the L/D form for sugar only pertain to the straight chain form or also to the ring form?

clarify this for me someone?
 
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Wait...no, I think anomer is when you have a ring form of let's say glucose and at the anomeric carbon the groups are switch...like one has an OH up...and the other down. So one would be beta and one alpha. I'm pretty sure this is right, but I may be off.
 
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MDtoBe777 said:
Wait...no, I think anomer is when you have a ring form of let's say glucose and at the anomeric carbon the groups are switch...like one has an OH up...and the other down. So one would be beta and one alpha. I'm pretty sure this is right, but I may be off.
Click to expand...

yea that's what i'm thinking too. Damn these mumble jumble, are we really gonna use this **** in practicing? NOOOOOOOOOOO
 
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But, I'm still wondering if you can designate ring forms of sugars as L and D. I don't think you can. But I'm not sure. You can figure out the chirality of the anomeric carbon though. haha..okay, I'm going around in circles. Does anyone know?
 
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btw which carbon is the chiral one again? the one on the bottom? someone link me to a picture!!!!
 
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wait.. I believe that L and D glucoses would be enantiomers?

they dont differ at just one chiral center, ALL of them are reversed..

which would make them mirror images.. and thus enantiomers
 
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AxlxA said:
btw which carbon is the chiral one again? the one on the bottom? someone link me to a picture!!!!
Click to expand...

in glucose, there are multiple chiral centers... but when they say D or L, it is referring to the highest numbered chiral center.
 
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But you determine L/D based on the chiral carbon furthest from the most oxidized carbon. So, by changing this one configuration you are changing an L sugar to a D sugar and vice versa. This is why I think they are epimers.
 
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bonez318ti said:
in glucose, there are multiple chiral centers... but when they say D or L, it is referring to the highest numbered chiral center.
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highest? what does that mean?

i went tomy notes, says C5.
 
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i just looked this up in McMurry to be sure.

here's the deal: D and L glucose are enantiomers, all the chiral centers are switched. the D form is the one that has the OH group on the chiral carbon farthest from the carbonyl carbon (carbonyl carbon = carbon #1, lowest chiral carbon = carbon #5) on the right in the Fischer projection (the carbonyl is on the top).

the anomeric carbon is carbon #1 (the carbonyl, or what used to be the carbonyl), and it's only important for the ring structure. when a ring forms, the OH group on C #5 performs a nucleophilic attack on the carbonyl C. this gives you two possible outcomes:
1. the OH on carbon #1 and the CH2OH on C # 5 are trans to each other --> alpha form
2. they're cis to each other --> beta form.

hth :)
 
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junebuguf said:
The R and S configurations are particular to the stereochemistry of one, and only one, chiral carbon. In compounds containing several chiral carbons (such as long chain carbohydrates) each chiral carbon (every vertex in the Fisher projection) will typically have a different stereochemical configuration (ie R or S). As a general rule, the sterochem of the last chiral carbon in a sugar (the last vertex in the Fisher projection) is designated as the stereochem for the entire sugar compound. Hence, all D sugars have terminal chiral carbons with D (R) configuration.

The D and L configurations have nothing to do with which way the compound bends light though, so you are right. That has to be determined experimentally.

Dont fall for tricks though: if it is given that a D-Glucose bends light +11.7 degrees, you can't assume that L-Glucose bends light -11.7 degrees. Only enantiomers bend light in the same magniude but opposite direction, and L-Glucose is *not* an enantiomer of D-Glucose.
Click to expand...

Okay...so D and L glucose are enantiomers...therefore, if you know the direction that one rotates the plane polarized light, you know the direction of the other one. Thanks so much for all of your responses.
 
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junebuguf said:
I thought that L and D sugars differed only in the chirality of the last sugar, not the whole structure...
Click to expand...
chiral_sugar.gif
 
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