Questions dealing with amino acids

  1. The Strecker synthesis of amino acids involves treating aldehydes with a mixture of NH4Cl and KCN followed by hydrolysis. The first step involves a nucleophilic attack by NH3.
    1. If ethanal is the starting material, what is the structure of the compound formed after ethanal reacts with the NH4Cl/KCN mixture?
    2. Write mechanisms for the reactions which lead to the formation of this compound.

  2. Amino acid analysis of angiotensin II, a blood-pressure-regulating hormone in blood plasma, shows the composition: Arg, Asp, His, Ile, Phe, Pro,Tyr, Val. An N-terminal analysis using the Sanger or Edman methods shows that the N-terminal residue is aspartic acid. Using carboxypeptidase, the C-terminal residue is found to be phenylalanine. Partial hydrolysis of angiotensin II with dilute HCl yields the following fragments: Ile, Tyr, Val Arg, Asp, Val Phe, Pro His, Ile, Pro Arg, Tyr, Val Propose a structure for angiotensin II.

    3.  

    Asp-Arg-Val-Tyr-Ile-His-Pro-Phe

     

  3. Betaine, C5H11O2N, occurs in beet sugar molasses. It is a water-soluble solid that melts with decomposition at 300C. It is unaffected by base but reacts with HCl to form a crystalline product, C5H12O2NCl. It can be made in either of two ways: treatment of glycine with methyl iodide, or treatment of chloroacetic acid with trimethylamine. Draw a structure for betaine that accounts for its properties. 

  4. A hexapeptide with the composition Arg, Gly, Leu and 3 Pro residues is found to have proline at both the C- and N- terminal positions. Partial hydrolysis gives the following peptides: Gly-Pro-Arg, Arg-Pro, and Pro-Leu-Gly. What is the structure of the hexapeptide? 

    5. Pro-Leu-Gly-Pro-Arg-Pro

     

  5. The isoelectric point of alanine is 6.0. Draw the structure of its predominant form when present in a buffer of pH 7.0.

  6. Of the 20 L-amino acid structures presented in this course, only cysteine has an R configuration. Why?

    7.  

      NH2 has 1st priority then CH2SH as S has higher atomic number than O 

  7. A mixture of the following three peptides , Gly-Lys, Gly-Asp, and Gly-Ala, is spotted on a strip of paper which is moistened by an aqueous buffer solution of pH 6.0. The left side of the paper is attached to the positive electrodes in an electrophoresis experiment (and the right to the negative electrode). After applying a potential and allowing a time period, one spot is near the positive electrode, another is close to the negative electrode and the other hasn’t moved. Locate each peptide.

    8.  Principal forms at pH 6:

    Lysine alanine aspartic acid

     Gly-Lys moves to negative electrode, Gly-Ala doesn’t move, Gly-Asp moves to positive electrode

     

  8. Propose a synthesis of phenylalanine from benzene.

    9.  

     

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    This page is maintained by Dr. Ed Blackburn (Ed.Blackburn@UAlberta.CA), course instructor.

    Updated August 23, 2002