Quiz 1, BICH 410 (MWF3-3:50), Friday, Sept. 17, 1999

Write your name on each page. Clearly circle your response to multiple-choice questions. No partial credit for multiple-choice questions. For other questions, show your work in order to receive partial credit where applicable.
gas constant R 8.315 J/mol-K

1. Structure and ionization of a dipeptide.
A) (7 pts) Draw the structure of the dipeptide, aspartame (asp-phe) that contains a methyl ester on the C-terminus. Do not worry about the correct stereochemistry, but show the ionization of charged groups at pH 7.

Answer: See key for structure. To correctly show the ionization and to answer part B), your answer needs to show i) a protonated (+) amino group on the N-terminus, ii) a deprotonated carboxylate group (-) on the sidechain of asp (aspartic acid), and iii) a methyl ester linkage that blocks the C-terminus and is not ionizable.

B) (4 pts) What is the net charge on aspartame at pH 2? pH 12? You must show your reasoning to obtain full credit for this problem.

Answer: At pH 2 all ionizable groups are protonated because it is below their pK's. Hence the amino terminus contains a + charge and the asp sidechain carboxyl is uncharged. The net charge is +1.
At pH 12 all ionizable groups in this molecule are deprotonated because it is above both pK's. Hence the amino terminus is uncharged and the carboxylate on the asp sidechain is -1. The net charge is -1.

2. (3 pts) Which amino acid from the choices below has the highest isoelectric point?
Answer: D) arginine; From this list arg is the only amino acid that contains a highly basic sidechain (the guanidino group). Hence, the isoelectric point will be shifted to a high pH.

3. (3 pts) The pH of 0.1 mM NaOH is:
Answer: B) 10; The concentration of hydroxide ion is 10-4 M. Therefore pOH = -log[OH-] = - log (10-4) = 4; Since pH + pOH = 14, pH = 14 - pOH = 14 - 4 = 10

4. (3 pts) Which amino acid from the choices below does not contain a sidechain that can participate in a hydrogen bond?
Answer: C) leucine; thr contains a hydroxyl group, tyr contains a phenolic hydroxyl group, gln contains an amide group, and his contains an imidazole group, all of which can participate in hydrogen bonds. The sidechain of leu is an aliphatic hydrocarbon, containing only carbons and hydrogens that cannot participate in hydrogen bonds.