MCB 354 - Exam I part II - Spring 2008

MCB 354 - Exam I part II - Spring 2008 - NAME _ Short...

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NAME ________________________________________ 7 Short Answer (125 points total) Answer the following questions in the space provided. Please write legibly! 26. (25 points) The titration curve of alanine shows the ionization of two functional groups with pK a values of 2.34 and 9.69, corresponding to the ionization of the carboxyl and the protonated amino groups, respectively. The titration of di-, tri- and larger oligopeptides with alanine on the N- and C-termini also show ionization of only two functional groups, although the experimental pK a values differ. The trend in pK a values is summarized in the table below: Amino Acid or Peptide pK a1 pK a2 Ala 2.34 9.69 Ala-Ala 3.12 8.30 Ala-Ile-Ala 3.39 8.03 Ala-(Ile-Ala)n-Ala 3.42 7.94 (a) Draw the structure of Ala-Ile-Ala at pH 6.0. Indicate the bonds whose angles are designated φ and ψ . Identify the functional groups associated with pK a1 and pK a2 . (b) Why does the value of pK a1 increase with each addition of a residue to the oligopeptide? As the length of Ala(X)Ala increases, the two terminal groups are moved farther apart, separated by an intervening sequence with an “insulating” nonpolar structure. The carboxyl group becomes a weaker acid, as reflected in its higher pKa, because the electrostatic repulsion between the carboxyl proton and the positive charge on the NH 3 + group diminishes as the groups become more distant Part (c) continued on next page!
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NAME ________________________________________ 8 (c ) Why does the value of pK a2 decrease with each addition of a residue to the oligopeptide? The negative charge on the terminal carboxyl group has a stabilizing effect on the
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This note was uploaded on 02/09/2009 for the course MCB 354 taught by Professor Spees during the Spring '09 term at University of Illinois, Urbana Champaign.

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MCB 354 - Exam I part II - Spring 2008 - NAME _ Short...

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