1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    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
    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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook