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Homework Help: Chromatography - Net Charge

  1. Oct 19, 2007 #1
    1. The problem statement, all variables and given/known data
    What procedures would you use to obtain gram quantities of each of the following compounds, free of each of the other compounds?
    Glycine - monoamino, monocarboxylic amino acid
    Glucose - non-ionic monosaccharide
    Starch - non-ionic large polysaccharide
    Glucose - 5-phosphate - anionic monosaccharide phosphate
    mRNA - polyanionic nucleic acid



    Compound||||||||||||||||MW
    Glycine|||||||||||||||||||75
    Glucose||||||||||||||||||180
    starch||||||||||||||||||||>>30,000
    glucose-5-phosphate||||||258
    mRNA|||||||||||||||||||||>>30,000

    To answer this question, you must make use of the ionic character of each compound. For example, fructose is non-ionic monomer whereas glycogen is also non-ionic but a polymer. So gel filtration would be useful in this case.

    Note that ribose-5-phosphate has three negative charges whereas tRNA has many negative charges and the MW is many folds larger than ribose-5-phosphate.

    Alanine is an amino acid with either positive, or neutral, or negative charge depending on the pH of the buffer used.

    (when it says ribose I think it means glucose, same for tRNA and mRNA... that's what you get for copy-paste!)

    2. Relevant equations
    I think we're only allowed to use three columns, DEAE, CM and Gel.
    DEAE=anion exchanger (negative will stick)
    CM= cation exchanger (positive will stick)
    Gel= filters by size of the molecule...


    3. The attempt at a solution
    Gel filtration will be good for the non-ionic ones:glucose and starch but if it's used first, it will just give 2 solutions filled with junk, so it has to be used last.
    mRNA is negative, glucose-5 is negative, but, mRNA is more negative because of polyanionic.
    glycine is... some website said it was non-ionic??

    k so nothing is positive?

    DEAE-> sticks the mRNA, gluc-5... glucose-5 will come out first followed by mRNA. 2 Solutions down, 3 to go...

    solution with: glycine, starch, glucose
    CM-> does nothing unless glycine is positive; then you're left with starch and glucose
    Gel->starch first, then glucose
    done!
    But... IS GLYCINE POSITIVE?!?!?!? otherwise it wouldn't work...
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Oct 19, 2007 #2

    chemisttree

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    What is the pKb of the glycine (amine part) and the pKa of the carboxylic acid part? Can you find a pH where the ammonium will not be protonated? What will happen to the carboxylic acid at that pH? Is there a pH where the ammonium will be protonated and the carboxylic acid remain in the acid form?
     
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