SSimonPractice106[1]

SSimonPractice106[1] - 1.-4. For each of the following...

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1.-4. For each of the following pairs of amino acids or peptides, indicate which will emerge FIRST from a column of the specificed solid phase support, using the specificed elution solvent: 1. ALA and PHE from a column of unmodified polystyrene beads, eluted with pH 7 aqueous buffer. A. ALA B. PHE C. both emerge simultaneously D. neither can be eluted The best answer is A – ala and phe are both hydrophobic and “bind” to the beads via the hydrophobic effect, but because phe is bigger, it binds more strongly. Both will eventually come off the column. 2. GLU and HIS from a column of amine-modified polystyrene beads, eluted with an aqueous buffer of progressively decreasing pH. A. GLU B. HIS C. both emerge simultaneously D. neither can be eluted The best answer is B – the negatively charged gamma-carboxyl group on glu makes it bind tightly to an anion exchange column. 3. GLU-VAL and ASP-GLY from a column of amine-modified cellulose, eluted with a butanol- acetic acid-water buffer of progressively decreasing pH A. GLU-VAL B. ASP-GLY C. both emerge simultaneously D. neither can be eluted The polar hydroxyl groups of cellulose don’t bind more hydrophobic amino acids or peptides, and so both glu-val and asp-gly are bound only by their charged groups to the anion exchange solid phase. They come off together as the pH falls. 4. GLU and ALA-ASP-VAL from a column of sulfonic acid-modified polystyrene, eluted with an aqueous buffer of progressively increasing pH. A. GLU B. ALA-ASP-VAL C. both emerge simultaneously D. neither can be eluted The hydrophobic interactions between the polystyrene and the hydrophobic amino acids in the tripeptide tend to retard its elution. It will come off the column later than the acidic amino acid glu. For the next questions, remember that when amino acids in a peptide are listed with commas between them (e.g. W,X,Y,Z) the sequence is not specified, but when they are listed with dashes between them the sequence is listed from the amino-terminal to the C-terminal end (e.g. W-X-Y- Z).
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5. A hexapeptide has amino acid composition (ALA,ASP,GLU,GLY,MET,LYS). If the peptide is treated with boiling 6 N HCl for 24 hours and the reaction mixture is then titrated fully with NaOH, how many equivalents of base are consumed per mole of starting hexapeptide? A. 2 B. 6 C. 12 D. 14 E. 18 Sorry, there was a typo on this one – each amino acid has an alpha amino and alpha carboxyl group which are liberated after hydrolysis of the peptide to free amino acids, and three of the amino acids also have titratable side chains – 15 titratable groups in toto. 6.-8. The same starting hexapeptide (ALA,ASP,GLU,GLY,MET,LYS) is instead treated with cyanogen bromide, yielding two tripeptides, one of which contains homoserine lactone. Another aliquot of the same hexapeptide is digested with trypsin, yielding a lysine-containing dipeptide and a tetrapeptide not containing lysine. If yet another aliquot of the same starting hexapeptide is treated with dansyl chloride and then with 6 N HCl for 24 hours, DNS-GLY is found. 6. The pI of the starting hexapeptide is:
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SSimonPractice106[1] - 1.-4. For each of the following...

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