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carbo question

Discussion in 'DAT Discussions' started by allstardentist, Aug 11, 2006.

  1. allstardentist

    allstardentist All-Star
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    is it possible to determine whether it's D or L of a glucose molecule from the chair conformation? If so, how? All i can figure out is whether it is alpha or beta from it.
     
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  3. Djapprentice

    Djapprentice Member
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    Allstardentist, my main man, how's the studies going? Ok, if you remember the up & down designations in the chair conformation, you can answer this question. Let's look at the Haworth projection for a minute. The rule for assigning up or down directions for Carbs, are the D hydroxyl's, the OH's being on the right side is down & the L OH's on the left side are placed up, so that's how it would be oriented in the chair conformation. So in a nutshell, as kaplan would say, lol:

    D hydroxyl = right, = down
    L hydroxyl = left = up

    hope that helps, let me know if you need anymore help or further clarification
     
  4. allstardentist

    allstardentist All-Star
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    the thing is I can tell whether it's L or D using that rule from looking at the fischer projection. However, the -OH group we use to designate L or D reacts with the aldehyde group when forming the ring as in haworth projection. So it is impossible to tell L from D in haworth projection. This is my logic. Tell me what is wrong with this.

    Btw, why am I you main man?? hahah. my studying is going OK. feel like getting burned out with a month to go.
     
  5. Notoriousjae

    Notoriousjae DAT Rockstar
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    do they even have L and D designations in the haworth form? If so i never came across it and the only way you would be able to tell is if you knew the designations before you convert to haworth in my opinion but if anyone else knows more please post
     
  6. allstardentist

    allstardentist All-Star
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    yeah, Im thinking exactly what ure thinking. but if u look in kaplan, they designated the chair conformation of a glucose molecule as beta - D - glucose. I couldnt figure out how they got the D. but, it might be an error or just assume that it came from a D-glucose molecule.
     
  7. Notoriousjae

    Notoriousjae DAT Rockstar
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    i think thats becuz they already knew it was D glucose molecule in the fischer projection to begin with. Other than that i dont see how one would be able to tell without converting the haworth form back to a chair conformation and then back to a fischer to figure that one out.
     
  8. allstardentist

    allstardentist All-Star
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    but in ester formation they start with beta-D-glucose in chair conformation to begin with.. but whatever, i think we are right.
     
  9. Djapprentice

    Djapprentice Member
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    I think so too :)
     
  10. Notoriousjae

    Notoriousjae DAT Rockstar
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    If you're talking about page 459 of the kaplan blue book: ester formation, you can easily tell its a D glucose in the chair conformation without even knowing what the fischer projection looks like. If you start counting the carbons in a chair conformation starting from the carbon that determines whether it is alpha or beta towards the left of the conformation, when you get to the 4th carbon in the conformation, that 4th carbon is synonmous with the 5th carbon on a D glucose molecule in fischer projection (you start counting the carbons starting from the carbonyl carbon as the first carbon and work your way down the molecule in a fischer projection) and if the OH group is facing down in the conformation then it is facing right in a fischer projection and if it is facing up then it facing left in a fischer projection. thats how you would be able to tell by just being given the chair conformation first. I think this is what you were asking.

    EDIT: however haworth projections do not have D or L designations because they would be cyclic so they would be termed pyranose or furanose rings etc

    If you have a orgo textbook by Paula Yurkanis Bruice, theres a good explanation in the carbohydrates chapter.
     
  11. allstardentist

    allstardentist All-Star
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    interestingly, according to this website I found, D sugar has CH2OH group above and L sugar has CH2OH group below.
    http://webpages.marshall.edu/~castella/chm204/projection.pdf#search='conversion%20fischer%20to%20haworth'

    BTW the 4th carbon from haworth projection is not synonmous with the 5th carbon(this is the carbon which its OH group reacts with the C=O) in fischer projection.
     
  12. Notoriousjae

    Notoriousjae DAT Rockstar
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    I was referring to the 4th carbon on a chair conformation, not a haworth projection and the 5th carbon on a fischer projection, because in your first post you asked how to be able to tell designations when you are onnly given a chair conformation
    the carbon which its OH group reacts with the C=O is the most chiral carbon and depending on whether the OH lies on the right or left gives it its respective designations

    that a good website. if you're concerned about drawing a haworth from a fischer projection, just keep in mind that everything on the right of a fischer falls down on a haworth ring and everything on the left of a fischer faces up. It gives a good explanation.
     
  13. allstardentist

    allstardentist All-Star
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    4th carbon on a chair conformation is the synonmous with 4th carbon on haworth but NOT synonmous with 5th carbon on the fischer projection.
     
  14. Notoriousjae

    Notoriousjae DAT Rockstar
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    I'm sorry. you're right its the 4th carbon on the fischer projection as well as the haworth. i miscounted hehe
     
  15. allstardentist

    allstardentist All-Star
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